Board for the formation of a continuously smooth permanent drywall

Abstract

The present invention concerns a board (1) configured to form a portion of a continuously smooth permanent drywall. The board comprises a planar portion (1f) parallel to and separated from a rear surface (1r) by a nominal thickness, T ± δ. The board comprises a tapered portion of taper width (Lt), separating the planar portion (1f) from the vertical edges of the board. A groove (1g) extends along the two vertical edges and is separated from the planar portion (1f) by a distance (Tg), controlled within a tolerance, ε < δ. Because ε < δ, the taper width (Lt) can be reduced to not more than 28 mm, yet respecting the conditions to yield a visually smooth surface at the joint portions. A permanent drywall can be built having a visually smooth surface by aligning side-by-side two or more boards, coupled to one another by beams comprising a first flange inserted in the grooves of two adjacent boards.

Description

BOARD FOR THE FORMATION OF A CONTINUOUSLY SMOOTH PERMANENT DRYWALLTECHNICAL FIELD

[0001] The present invention concerns a permanent partition wall or permanent wall lining, collectively referred to as permanent drywalls, of the type comprising a series of boards to be coupled side-by-side in a row to a frame made of beams. The boards and beams of the present invention are designed for an easy assembly allowing a smooth drywall to be built easily and repeatedly, with reduced offsets between the front surfaces of adjacent boards. The reduced offset allows redesigning the plates to simplify application of a smooth joint composition in the gaps separating adjacent boards. The present invention facilitates installation of permanent partition walls or wall linings by amateur home carpenters or do-it-yourselfers (DIYer) rather than relying on professional installers.BACKGROUND OF THE INVENTION

[0002] Refurbishing an interior of a building can comprise knocking down existing walls and building new partition walls and wall linings. A partition wall is a non-load bearing wall which divides a space into two sub-spaces separated from one another.

[0003] Partition walls generally have two visible surfaces and may comprise insulation material, piping, or cables hidden from view. Wall linings are used for cladding an existing wall, e.g., to enhance the appearance of the existing wall, for insulating the existing wall, or for hiding piping or cables running along the existing wall. Wall linings can also include ceilings and, in some cases, floors. Wall linings only have one visible surface. Partition walls and wall linings are commonly referred to as drywalls without distinction. Permanent drywalls are to be distinguished from relocatable partition walls which can be removed easily, as commonly used in modulable offices for rent. An example of relocatable drywall is described in US4329820. Permanent drywalls are designed to remain in place by opposition to relocatable drywalls. In particular, the term “permanent” imposes that the drywall complies with minimum mechanical requirements defined by national laws, such as NF_DTU25.41 P1-1 (2012) chapter 5.2 in France. Because relocatable drywalls must be easily dismounted, the gaps separating adjacent boards are not filled with a joint composition to yield a smooth, homogeneous surface but are instead either left open or closed with a removable T-shaped plastic strip.

[0004] The present invention does not address relocatable drywalls and concerns permanent drywalls only, whose gaps between adjacent boards must be filled with a joint composition to yield a smooth and visually planar and homogeneous surface.

[0005] Without using heavy materials, such as concrete, bricks, blockworks, or the like, partition walls and wall linings are generally formed by a support frame supporting boards made of e.g., plaster (= calcium sulphate dehydrate), cement, wood, plywood, metal, or the like. The rear surfaces of the boards are fixed with screws against a frame generally made of metallic or wooden beams. Because of wood shortage and higher fire resistance, however, metallic beams are often preferred.

[0006] The metallic beams generally comprise top U-profile tracks fixed to a top position, such as a ceiling, and a bottom U-profile track fixed to a bottom position, generally a floor. In conventional drywall systems, studs (e.g., U-studs) cut to a length corresponding to a height of the partition wall can be screwed in place to the top and bottom U-profile tracks to form a frame. As shown in Figures 3a and 3b representative of conventional drywall systems, the rear surfaces of the boards are fixed against the studs with screws. The frame can be designed to incorporate openings for windows and doors, requiring more cutting to size and screwing in place of the metallic studs. The boards can then be fixed side-by-side in a row to the frame with screws, thus securing the boards to the top and bottom U-profile tracks and to the vertical U-stud. In partition walls, boards are screwed on both sides of the frame, whilst in wall linings and ceilings, boards are screwed to one side only of the frame (cf. Figures 10a (= wall lining) and 10b & 10c (= partition wall)). The numerous screws required to fix the boards to the frame must be hidden with a gap filling composition, a tedious and not so easy operation. The vertical studs may comprise openings in the webs thereof to allow the passage of piping and cables along a length of the partition wall or wall lining.

[0007] Once the boards are screwed in place on one or both sides of the frame, the joints between adjacent boards are sealed with a joint composition and optionally covered by an adhesive paper tape to smoothen the surface of the thus built drywall.

[0008] Although building a drywall is considered by professional builders as a relatively simple operation, only well-trained amateur home carpenters dare building a permanent drywall on their own. To simplify the building of a permanent drywall, several solutions were proposed in the art. For example, US4231205 describes a steel edged gypsum wall panel engageable by flanged studs. Steel edges with a specific design are coupled to the edges of a panel. Flanged studs can be engaged into the steel edges to fix the panels and form the drywall. The steel edges considerably increase the cost and weight of the boards and are not designed to yield a visually smooth and homogeneous surface between adjacent boards. EP4039903 and WO 2024 / 028320 describe panels provided with grooves on their vertical edges dimensioned to snugly fit the flanges of a stud having a T- or i-profile requiring the use of little to no screws. The vertical edges of the boards are tapered to allow offsets between the front surfaces of adjacent panels to be smoothened with the joint composition.

[0009] A recurrent challenge when building a drywall is the offset or step formed between adjacent boards. Indeed, the boards which are produced by continuously moulding on a conveyor as shown in Figures 6a and 6b, such as plasterboards (= boards comprising a core comprising calcium sulphate dihydrate), and boards comprising at least 50 wt% of calcium silicate, or made of fibre-cement, have a thickness, T, with a rather poor tolerance, 8. For example, NF-EN520+A1 (2009) (= EN520) defines on p.15, §4.9.1 .3 that plasterboards have a thickness, T, with a tolerance of 8 = +0.6 mm. Such tolerance yields a maximum offset or step of height, Ts = 2|<5| = 1 .2 mm between the planar portions of the front surfaces of two boards aligned on their rear surfaces, as shown in Figures 3a, 3b, 4a, and 5a. To smoothen the surface with a joint composition applied between two boards with such high offset, EN520 requires on p.16, §4.9.2.5, the edges of the boards to be tapered with a taper width, Lt = 40 to 80 mmas illustrated in Figures 4a and 4c. Not only gaps between boards with such large taper widths, Lt, must be filled with a larger amount of joint composition, but such operation on large taper widths, Lt, requires great skill and is hardly attainable by a mere DIYer.

[0010] Another norm, NF DTU 25.41 P1-1 (= DTU25.41) defines a smoothness or planarity test directly measurable on the worksite. It consists of applying a ruler of length, Lr = 200 mm with one end pressed against any point of the drywall and the other end must be separated from the drywall by a distance, Tm < 1 mm. Figure 5a illustrates how to apply this method at the level of the gap separating two adjacent boards with an offset, Ts. It is clear that, for a given offset, Ts, the distance, Tm, decreases with increasing taper width, Lt. Indeed, for a given value of Ts, the slope, tan a = Ts I Lt of the visible surface of the joint composition joining the planar surfaces of two adjacent boards decreases with increasing taper width, Lt, thus giving a more homogeneous visual aspect. Figure 5b shows that with a thickness tolerance, 6 = +0.6 mm, the taper width, Lt, must be more than 50 mm (i.e., Lt > 50 mm) to satisfy this test for a maximum offset, Ts = 26 = 1 .2 mm.

[0011] Smoothening offsets of up to 1.2 mm between adjacent boards is a tricky job and is not rendered easier by the broad tapered region, of joint width, Lj = 2Lt + L11 of the order of 85 to 165 mm, around the gap of gap width, L11 , separating two adjacent boards. In general, it can be stated that the gap width, L11 , is of the order of 5 mm. Such high values of the maximum offset, Ts, are due to the poor tolerance on the board thickness of the order of 6 = +0.6 mm, which can, however, not be reduced without unduly increasing production costs.

[0012] There therefore remains a need for a system for easily and reproducibly building a permanent drywall with offsets of reduced height, Ts, between adjacent boards and allowing an easier application of lower amounts of joint compositions to smoothen the drywall front surface. The present invention proposes a solution for substantially simplifying the building of a permanent drywall, reducing the offset between adjacent boards and facilitating the filling of the gaps between adjacent boards to obtain a smooth surface, so that amateur DIYers can now easily build a smooth and visually planar permanent drywall. These and other advantages of the present invention are presented in continuation.SUMMARY OF THE INVENTION

[0013] The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a

[0014] The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a board configured to form a portion of a continuously smooth permanent drywall, wherein the board has a rectangular geometry, comprising, a front surface comprising a planar portion parallel to a wall plane (X, Z) and separated from a rear surface parallel to the wall plane (X, Z) by a nominal thickness, T + 6, measured along aY-axis normal to the wall plane (X, Z), wherein 6 is a tolerance on the nominal thickness,• a peripheral edge defining the rectangular geometry and comprising first and second longitudinal edges of length, L1 , extending along the X-axis, normal to and separated from one another by first and second vertical edges of board height, H1 , extending along the Z-axis, and wherein• a groove extends parallel to the Z-axis along each of the first and second vertical edges, each groove having a groove width, w, measured normal to the wall plane (X, Z), and a depth, d, measured parallel to the X-axis, and each groove being formed by machining, preferably by milling at a distance, Tg + s, from the planar portion of the front surface measured along the Y-axis, wherein s is a tolerance on the distance, Tg,• wherein each of the first and second vertical edges has an edge thickness, Te < T, measured along the Y-axis and is coupled to the planar portion (1 f) of the front surface by a tapered portion (1t) of taper width, Lt, measured along the X-axis

[0015] The board of the present invention differs from the boards of the prior art in that the tolerance, s, on the distance, Tg, is smaller than the tolerance, 8, on the nominal thickness, T (i.e., s < 8). The tolerances, s, 8, are determined as described in in continuation in relation with Figure 11 c. This allows the taper width, Lt, to be not more than 28 mm, preferably not more than 25 mm, more preferably not more than 20 mm, wherein the taper width, Lt, is measured as defined on p.21 , §5.6.1 .3 and Figures 11 and 12 of NF-EN520+A1 (2009), reproduced in present Figure 11 a. A taper portion having a lowertaper width, Lt, is advantageous in terms of lower volumes of joint composition required to fill the joint portion formed by the gap between, and taper portions of two adjacent boards and is at the same time easier to apply by an average DIYer, than a broader joint portion.

[0016] The board of the present invention preferably comprises a core comprising either at least 50 wt.% of calcium sulphate dihydrate relative to the core weight thus defining a plasterboard, preferably the plasterboard comprises at least 80 wt.% of calcium sulphate dihydrate, or the core comprises at least 50 wt.% of calcium silicate, or is made of fibre-cement. The board is preferably a plasterboard. For example, the plasterboard can comprise at least 70 wt.% gypsum, preferably at least 80 wt.% of gypsum relative to the total weight of the plasterboard. The plasterboard can have a density comprised between 0.5 and 1 .2 g / cm3, preferably between 0.6 and 0.8 g I cm3.

[0017] The board and, in particular, the plasterboard can have a nominal thickness, T, measured between the planar portion of the front surface and the rear surface comprised between 12 and 50 mm, preferably between 18 and 40 mm, more preferably between 20 and 30 mm, a preferred nominal thickness is comprised between 18 and 25 mm. The board can have an edge thickness, Te, comprised between 8 and 45 mm, preferably between 12 and 35 mm, more preferably between 14 and 30 mm, a preferred edge thickness is comprised between 15 and 22 mm. The distance, Tg, separating the groove from the planar portion of the front surface measured along the Y-axis can be comprised between 30and 70% of the nominal thickness, T (i.e., Te = 0.3 T to 0.7 T). The groove width, w, can be comprised between 1 and 7 mm, preferably between 1 .5 and 5 mm, more preferably between 2 and 4 mm. The foregoing preferred values of the nominal thickness, T, the edge thickness, Te, the distance, Tg, and the groove width, w, can be considered independently from one another or can be considered in any combination thereof.

[0018] In a preferred embodiment, the tolerance, s, on the distance, Tg, is not more than 70% of the tolerance, 8, on the nominal thickness, preferably not more than 50% of 8, more preferably of not more than 20% of 8, and more preferably of not more than 10% of 8 (i.e., | s | < 0.7 | 8 |, preferably | s | < 0.5 | 8 |, preferably | s | < 0.2 | 8 |, and preferably | s | < 0.1 | 8 |). The advantages of having a tolerance, | s | < | 8 |, allows yielding a lower offset of the planar portions of two adjacent boards coupled to one another by a beam of the kit-of-parts according to the present invention, as is explained in continuation.

[0019] The present invention also concerns a kit-of-parts for forming a portion of a continuously smooth permanent drywall. The kit-of-parts comprises,• first and second boards as defined supra,• a beam comprising a web normal to and centred on a first flange and optionally to a second flange forming a T-profile or a i-profile, wherein the first flange extends along the X-axis and comprises a first half-flange extending out of a first side of the web along the X-axis and configured to snugly fit in the groove of the first board and comprises a second half-flange extending out of a second side of the web along the X-axis opposite the first side and configured to snugly fit in the groove of the second boards (1), and• preferably a joint composition (6) configured to fill in the gap between two adjacent boards.

[0020] The elements of the kit-of-parts of the present invention can be assembled as follows to build a portion of a continuously smooth permanent drywall,• the first half-flange of a first beam is inserted into the groove of the first board,• the second half-flange (3f2) of the first beam is inserted into the groove of the second board, such that the first and second boards are held side by side with a vertical edge of the first board (1) being parallel to and separated from a longitudinal edge of the second board by a by a gap defining a distance, L11 > 0 and such that the planar portion of the first board is substantially parallel to the planar portion of the second board. The planar portions of the first and second boards are offset from one another by a misalignment step of height, Ts, measured along the Y-axis of not more than 2s. As defined supra, s is the tolerance of the groove distance, Tg, separating the grooves from the planar portions of each of the first and second boards. The tolerance | s | < 0.5 mm, preferably | s | < 0.4 mm, more preferably | s | < 0.2 mm, yielding offsets; Ts < 1.0 mm, preferably Ts < 0.8 mm, more preferably, Ts < 0.4 mm. The joint portion of the portion of permanent drywall comprising the gap and tapered portions of the first and second boards can then be filled with a joint composition, giving a smooth visualaspect over a whole area of the continuously smooth permanent drywall.

[0021] The offset, Ts, between adjacent boards can be such that it fulfils one or both of the following criteria. A first criterion is defined in EN520 which states that for a tolerance, | 8 | = 0.6 mm, considered as representative of a plasterboard, the taper width, Lt > 40 mm. Assuming a gap between two boards, L11 = 5 mm, these values define a slope, tan a = Ts I Lj = 28 I (2Lt + L11) < 1 .4%, relative to the planar portion of a straight line joining the planar portions of two adjacent boards, wherein Ts < 28, and Lj = 2Lt + L11 . A slope, tan a < 1 .4% is therefore considered by EN520 as yielding joint portions having a smooth visual appearance when filled with a joint composition. To satisfy this criterion, and in spite of a substantially narrower taper width, Lt < 28 mm, the joint portion of a portion of a continuously smooth permanent drywall according to the present invention is also characterized by a slope, tan a < 1.4%, preferably tan a < 1 .3%, more preferably tan a < 1 .25%.

[0022] A second criterion is defined in NF DTU 25.41 P1-1 and requires a second end of a ruler of length Lr = 200 mm to be separated from the planar portion of the second board by a distance Tm < 1 mm measured along the Y-axis, wherein a first end is laid at a distance Lt from the vertical edge of the first board measured along the X-axis, at a junction where the tapered portion joins the planar portion and resting at a junction of the second board where the tapered portion joins the planar portion at a distance Lt from the vertical edge of the second board measured along the X-axis, as illustrated in Figure 5a. To satisfy this criterion, and in spite of quite a narrow taper width, Lt < 28 mm, the joint portion of a portion of a continuously smooth permanent drywall according to the present invention is also characterized by a distance Tm < 1 mm between the ruler’s second end and the planar portion of the second board.

[0023] The present application also concerns a process for producing a plasterboard as defined supra; The process comprises,• feeding a gypsum slurry comprising stucco, water, and additives onto a bottom facer laid on a conveyor in motion,• applying a top facer onto a free surface of the gypsum slurry to form a sandwich structure with a core made of the gypsum slurry sandwiched between the top and bottom facer),• setting by hydration the gypsum slurry) forming the core to form a setting board on the conveyor,• cutting the setting board to desired dimensions to yield cut boards,• drying the cut boards in a drying station (11) to yield the boards.

[0024] The tapered portions are formed connecting the planar portion of the front surface to the vertical edges. The process differs from prior art processes at least in that, after the setting of the gypsum slurry the grooves are formed by machining with a rotating cutting tool, preferably by milling, along each vertical edge of the setting board or of the cut boards or of the boards, by controlling the distance of the cutting tool relative to the planar portion to ensure that the groove is at a distance, Tg, from the planar portion within a predefined tolerance, E, on the distance Tg. To ensure a tight control of the tolerance,E, on the distance, Tg, it is important that the position of the groove along the Y-axis be controlled relative to the position of the planar portion of the front surface of the board and not relative to the rear surface thereof. The grooves can be formed by milling with a milling cutter whose position along the Y-axis relative to the planar portion (1f) is controlled by a measuring system.

[0025] The tapered portions can be formed either,• by feeding the gypsum slurry onto the conveyor which has a corresponding geometry, or• by machining square edges of the setting board or of the cut boards or of the boards.

[0026] The present invention also concerns a use of a board as defined supra to form a portion of a continuously smooth permanent drywall as defined supra.BRIEF DESCRIPTION OF THE FIGURES

[0027] For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:Figure 1 shows a board according to the present invention.Figures 2a to 2f show various embodiments of the tapered portion of boards.Figure 3a: shows a cross-section of two adjacent square-edged boards of the prior art screwed side by side to a beam, with the step of height, Ts, between the two boards due to the poor tolerance, 8, on the thickness, T.Figure 3b: shows a cross-section of two adjacent tapered-edged boards of the prior art screwed side by side to a beam, with the step of height, Ts, which can reach Ts = 28, between the two boards due to the poor tolerance, 8, on the nominal thickness, T. According to EN520, the taper width, Lt = 40 to 80 mm to allow a smooth enough surface to be obtained after application of the joint composition.Figure 3c: shows a cross-section of two adjacent tapered-edged boards of the invention held side by side by the flanges of a beam inserted in the grooves of the two boards. The step between the two boards can reach a height, Ts = 2s < 28, because the tolerance |s| on the distance, Tg of the groove to the planar surface is much smaller than the tolerance, 8, on the board nominal thickness, T.Figure 4a: shows two boards held side by side and aligned on their rear surfaces according to the prior art. The step formed between the two boards is necessarily controlled by the tolerance, 8, on the nominal thickness.Figure 4b: shows two boards held side by side and aligned on their grooves according to the present invention The step formed between the two boards is necessarily controlled by the tolerance, s, on the the distance, Tg, with s < 8.Figure 4c shows a graphical representation of the relationship between taper width, Lt, and tolerance, 8or E, controlling the height, Ts, of the step between adjacent boards. The white circle indicates the requirements according to EN520: 8 = 0.6 mm, Lt = 40 mm, yielding an angle, a, defined by tan a = Ts I Lj. The straight line defines Lt = f(<5, s) to yield a constant angle, a. To keep the same angle, a, with a taper width, Lt = 28 mm according to the present invention, the tolerance, s, on the distance, Tg, must be smaller than or equal to +0.4 mm.Figure 5a illustrates the ruler test according to DTU 25.41 , wherein two boards are held side-by-side and one end of a ruler of length, Lr = 200 mm is pressed against the first board at a distance, Lt, from the vertical edge (at the boundary between the tapered portion and the planar portion), and a second end is separated from the planar portion of the second board, by a distance, Tm, which cannot exceed 1 mm.Figure 5b shows a graphical representation of an approximation of the relationship (cf.

[0049] ) between taper width, Lt, and tolerance, 8 or s, controlling the distance, Tm < 1 mm, of the second end of the ruler to the planar portion of the second board. The white circle identifies the requirements of EN520: 8 = 0.6 mm, Lt = 40 mm, which do not satisfy the DTU25.41 . To ensure that the distance, Tm < 1 mm, with a taper width, Lt = 28 mm according to the present invention, the tolerance, s, on the distance Tg must be smaller than or equal to +0.22 mm.Figures 6a and 6b show two embodiments of processes for producing a board according to the invention.Figure 6c is a cut C - C of Figures 6a and 6b showing one way of moulding the tapered portions of the boards.Figure 6d is a cut D - D of Figures 6a and 6b showing one way of forming the grooves by milling with a control of the distance, Tg.Figure 7a shows an exploded view of a first board and beams forming the frame, including top and bottom C-profiles and a beam with flanges forming an i-profile.Figure 7b shows the first board fixed to the top C-profile and with the flanges of the beams being inserted into the grooves of the first board and of a second board.Figure 7c shows a portion of permanent drywall comprising first and second boards fixed to the top C-profile and coupled to one another with good alignment by means of the flanges of the beams being engaged in the corresponding grooves of the boards.Figure 8a shows a perspective view of the edge of a board, with groove and tapered portion.Figure 8b shows a perspective view of the edges of two boards according to Figure 8a coupled to one another by a beam (3) and separated from one another by a gap of width, L1 1 .Figures 9a to 9c show a beam with one or two flanges forming (a) a T-profile, (b) an i-profile, and (c)a h-profile.Figure 10a shows a top view of a wall lining hiding an existing wall and comprising boards coupled side-by-side to the one side of the frame in good alignment and forming a smooth surface after application of a joint composition.Figures 10b and 10c show top views of two embodiments of a partition wall comprising boards coupled to both sides of the frame with the boards coupled side-by-side to the frame in good alignment and forming smooth surfaces on both sides of the partition wall after application of a joint composition.Figure 11a reproduces Figures 11 and 12 of EN520 for measuring the taper width, Lt..Figures 11 b and 11c show (b) a cut of the edge portion identifying the distance, Tg, and tolerance s and (c) method for determining the values of the distance, Tg, and of the tolerance, s.DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention concerns a board (1) configured to form a portion of a continuously smooth permanent drywall which is easier to build than state-of-the-art drywalls. A drywall collectively refers to a partition wall or wall liner. The drywalls of the present invention are permanent drywalls, as defined e.g., in NF_DTU25.41 P1-1 (2012), which are not meant to be moved from one position to another as exist in modulable open spaces for forming individual offices a la carte. The board (1) of the present invention has a rectangular geometry and comprises a front surface comprising a planar portion (1f), a rear surface (1 r), and a peripheral edge (1 e) comprising vertical edges and longitudinal edges. The vertical edges are coupled to the planar portion (1 f) by a tapered portion (11) and are provided with a groove (1g). Contrary to the plasterboard described in US4231205, the edges and grooves (1g) fof the boards of the present invention are not cladded with any metal sheet.

[0029] The front surface comprises the planar portion (1 f) which is parallel to a wall plane (X, Z) and is separated from the rear surface (1 r) parallel to the wall plane (X, Z) by a nominal thickness, T + 8, measured along a Y-axis normal to the wall plane (X, Z), wherein 8 is a tolerance on the nominal thickness. The peripheral edge defines the rectangular geometry and comprises first and second longitudinal edges of length, L1 , extending along the X-axis, normal to and separated from one another by first and second vertical edges of board height, H1 , extending along the Z-axis.

[0030] The groove (1g) extends parallel to the wall plane (X, Z) along each of the first and second vertical edges. Each groove has a groove width, w, measured normal to the wall plane (X, Z), and a depth, d, measured parallel to the X-axis. The grooves are formed by machining, preferably by milling at a controlled distance, Tg + s, from the planar portion (1 f) of the front surface measured along the Y-axis, wherein s is a tolerance on the distance, Tg. Each of the first and second vertical edges has an edge thickness, Te < T, measured along the Y-axis and is coupled to the planar portion (1 f) of the front surface by a tapered portion (1t) of taper width, Lt, measured along the X-axis. A tapered portion isdefined herein as any geometry becoming gradually (not necessarily linearly) narrower orthinnertoward one end. Non exhaustive examples of tapered portion geometries are illustrated in Figures 2a to 2f.

[0031] The gist of the present invention is that the taper width, Lt, can be reduced to not more than 28 mm, preferably not more than 25 mm, more preferably not more than 20 mm, wherein the taper width, Lt, is measured as defined on p.21 , §5.6.1.3 and Figures 11 and 12 of NF-EN520+A1 (2009), readingMeasure the taper width on each edge (300 ± 50) mm from each end. Determine the taper width (AB) by applying a metal rule to the face of the board parallel to the end as shown in Figure 12 for tapered edge boards and in Figure 11 for half-rounded tapered edge board [Figures 11 and 12 of EN520 are reproduced in present Figure 11a],

[0032] . Although the taper width, Lt < 28 mm, is considerably lower than Lt = 40 to 80 mm as defined in NF-EN520+A1 (2009), and much lower than required by DTU 25.41 with Lt > 50 mm for a tolerance on the nominal thickness, 6 = +0.6 mm, the slope, tan a, of a straight line joining the planar portions (1 f) of two adjacent boards (1) remains as required by both NF-EN520+A1 (2009) and DTU25.41 , without changing the tolerance on the nominal thickness, 8 = +0.6 mm, of the boards (1). This is made possible because, as explained in detail in continuation, the boards (1) of the present invention are aligned on the positions of the grooves (1g) and not on the positions of the rear surfaces (1 r) as in the prior art. By accurately controlling the tolerance, s, on the distance, Tg, of the groove (1g) to the planar portion (1 f) during machining of the grooves (e.g., by milling), the maximum offset, Ts, between two adjacent boards is reduced to Ts < 2s instead of Ts < 26 as in the prior art, with s < 8.

[0033] A reduced taper width (Lt) reduces the joint width, Lj = 2Lt + L11 , (wherein L11 is the gap width) which requires less joint composition to fill and is considerably easier to apply by DIYers to yield a smooth joint portion between two boards, invisible to the naked eye after painting or covering with wallpaper.THE BOARD (1)The Board (1) - Geometry

[0034] The board (1) of the invention has a rectangular geometry, comprising front and rear main surfaces. The front surface comprises a planar portion (1 f). The planar portion (1 f) and the rear surface (1 r) are parallel to the wall plane (X, Z) and separated from one another by a nominal thickness, T, measured along the Y-axis normal to the wall plane (X, Z).

[0035] The board preferably comprises a core (1 c) sandwiched between bottom and top facers (21 , 22). The core (1 c) can comprise at least 50 wt.% of calcium sulphate dihydrate (= gypsum) thus defining a plasterboard. Alternatively, the core (1 c) can comprise at least 50 wt.% of calcium silicate, or is madeof fibre-cement. The core (1 c) can comprise additives as conventionally used in the art. The present invention is not restricted to any particular selection of additives. The plasterboard preferably comprises at least 70 wt.%, more preferably at least 80 wt.% of calcium sulphate dihydrate relative to the total weight of the plasterboard. The plasterboards preferably have a density comprised between 0.5 and 1 .2 g / cm3, more preferably between 0.6 and 0.9 g I cm3, more preferably between 0.7 and 0.8 g I cm3. In one embodiment, the first and second boards are plasterboards comprising at least 70 wt.% gypsum, preferably at least 80 wt.% of gypsum relative to the total weight of the plasterboard, and having a density comprised between 0.5 and 1 .2 g / cm3, preferably between 0.6 and 0.8 g I cm3. The core (1 c) can comprise other components such as starch, PVA, foaming agents, reinforcing fibres, and the like. For example, a board core made of calcium sulphate dihydrate can comprise between 70 and 94 wt.% calcium dihydrate relative to the core weight. Additives such as starch, polyvinyl acetate (PVA), a foaming agent, and reinforcing fibres (e.g., glass fibres) can be added to the core composition.

[0036] The nominal thickness, T, of the boards measured between the planar portion (1f) of the front surface and the rear surface (1 r) can be comprised between 12 and 50 mm, preferably between 18 and 40 mm, more preferably between 20 and 35 mm, more preferably between 25 and 30 mm. A preferred nominal thickness, T, is comprised between 18 and 25 mm. Because of the process for producing plasterboards, the nominal thickness, T, has a tolerance, +5 = +0.6 mm according to NF-EN520+A1 (2009) (= EN520), which cannot be reduced substantially without changing the process of production of the boards, which is not the purpose of the present invention. With a tolerance, +5 = +0.6 mm, the nominal thicknesses, T; of two boards can therefore differ from one another up to 26 < 1.2 mm, regardless of the value of the nominal thickness, T.

[0037] The tolerance, 6, (shown in Figure 11 b) can be determined by measuring the nominal thickness, T, of a representative number of boards (1). In practice, as illustrated in Figure 11 c, the tolerance, 6, on the nominal thickness, T, are determined by measuring the nominal thickness, T, at 3 points (Nil to Ni3) randomly selected over the planar surface (1f) of each board (1 .j, j = 1 to 3), repeating the operation on three boards (1) to yield nine values (Nij, with i = 1 to 3 and j = 1 to 3) of the nominal thickness, T. The values (Nij) thus measured are averaged to yield the nominal thickness, T, and the standard deviation is determined and defined as the tolerance, 6.The Board (1) - Edges and Tapered Portion (11)

[0038] The peripheral edge defines the rectangular geometry of the board (1). As shown in Figure 1 , the peripheral edge comprises first and second longitudinal edges of length, L1 , extending along the X-axis, separated from one another by and normal to first and second vertical edges of board height, H1 , extending along the Z-axis, The boards (1) can be produced with a height (H1) of up to 2400 mm or up to 3050 mm and even up to 3600 mm measured along the Z-axis. The boards can later conveniently be cut to the desired height (H1) at the shop, as most DIY-shops offer such service. Theboards (1 ) can have a width of the order of L1 = 600 to 1300 mm, measured along the X-axis. Each of the first and second vertical edges has an edge thickness, Te < T, measured along the Y-axis and is coupled to the planar portion (1f) of the front surface by a tapered portion (1t) of taper width, Lt, measured along the X-axis. The expression “tapered portion” is construed herein as any geometry of gradually decreasing thickness from the nominal thickness, T, at the level of the planar portion (1f) to the edge thickness, Te, at the level of the vertical edges which is smaller than the thickness of the nominal thickness, T. Various non-exhaustive geometries of the tapered portion are illustrated in Figures 2a to 2f and 8. Figure 2b is a preferred embodiment of the tapered portion. The edge thickness, Te, of the vertical edges is preferably comprised between 8 and 45 mm, preferably between 12 and 35 mm, more preferably between 14 and 30 mm, a preferred edge thickness is comprised between 15 and 22 mm. In a preferred embodiment, the thickness of the tapered portion decreases linearly from the nominal thickness, T, to the edge thickness, Te.

[0039] The taper depth, (T - Te) measured along the Y-axis, is not particularly restricted by the present invention. The taper depth, T - Te, can be comprised between 0.5 and 6 mm, preferably between 0.3 and 4.0 mm, preferably between 1.0 and 2.5 mm, or between 1.2 and 1.5 mm. The taper depth, (T - Te), is measured as explained in §5.6.2.1 to 5.6.1 .4 of EN520, wherein §5.6.2.3 reads,Measure the taper depth on each edge (300 ± 50) mm from each end. Place the board on a flat surface. Place the measuring device on the face of the board, with the gauge 150 mm from the edge and adjust the scale to zero. Move the device towards the edge and take the reading (10 ± 1) mm from the edge, for the tapered edge board and (20 ± 1) mm from the edge for the half-rounded tapered edge board.

[0040] The boards (1) have a tapered portion (1t) to substantially reduce the visual impact caused by the offset between the front surfaces of two adjacent boards (1) of thicknesses (T) differing from one another by a thickness difference, Ts < 28. When filling the gap (11g) between two adjacent boards (1) with a joint composition, the visible surface of the joint composition forms a slope of angle, a, with the planar portions (1 f) of the boards (1). Even small thickness differences, Ts, of the order of 1 or 1 .5 mm which can be comprised within tolerance boundaries for plasterboards (made of gypsum) or fibre-cement boards, can create a step which is disturbingly visible on the final drywall. As shown in Figure 3a even after filling the gap between the two adjacent boards with a joint composition (6), the slope of the joint composition bridging the edges of the two adjacent boards (1) forms such an angle, a, that the light reflects differently onto the surface of the drywall at the level of the joint portion. By contrast, as shown in Figure 3b, if the edges are tapered, the filler can bridge the planar surfaces (1 f) of the two adjacent boards with an angle, a, much closer to zero than in the case of Figure 3a. This way, with tapered edges, the thickness difference between the two adjacent boards (1) yields a substantially less disturbing optical effect on the appearance of the final drywall, with a morehomogeneous light reflection on the drywall surface.

[0041] Mathematically, this can be expressed as the slope, tan <z = Ts / Lj. The slope can be decreased by decreasing the step height (offset), Ts, and I or by increasing the joint width, Lj, defined as Lj = 2Lt + L11 , wherein L11 is the width of the gap (1 1g) between two boards (1). A steep slope as shown in Figure 3a with boards having square edges defines a joint width, Lj = L11 , yielding a high slope, tan a , visible to the naked eye. With the same step height, Ts, the slope in Figure 3b formed by the joint composition (6) is reduced with the tapered portions, increasing the joint width, Lj = 2Lt + L11 , thus smoothening the joint portion accordingly by a corresponding reduction of the slope, tan a, formed by the joint composition (6). An inconvenient with broad joint widths, Lj, is that more joint composition is required to fill the joint portion and it is trickier to apply the joint composition into the joint portion to yield a smooth surface when the joint width, Lj, is larger.

[0042] As illustrated in Figure 3c, the present invention aims to reduce the step height (or offset), Ts, and thus allowing to reduce the taper width, Lt, and joint width, Lj, with the use of grooves (1g) machined at a controlled groove distance, Tg, of the groove (1g) to the planar portion (1 f). The taper width, Lt, is measured as defined on p.21 , §5.6.1.3 and Figures 11 and 12 of EN520. The taper width, Lt, of the present invention can thus be reduced to Lt < 28 mm, preferably, Lt < 25 mm, more preferably Lt < 20 mm. For example, assuming a gap width, L11 = 5 mm, a taper width, Lt = 28 mm yields a joint width, Lj = 2Lt + T11 = 2 x 28 + 5 = 61 mm, instead of Lj = 2 x 40 + 5 = 85 mm with a taper width, Lt = 40 mm as defined in EN520.The Board (1) - the grooves (1 g)The grooves (1g), extend parallel to the wall plane (X, Z) along each of the first and second vertical edges, preferably over the whole board height (H1). The grooves have a groove width, w, measured along the Y-axis, and a groove depth, d, measured along the X-axis. The grooves (1g) preferably have a groove width, w, can be comprised between 1 and 10 mm (i.e., w = 1 to 10 mm), preferably between 1.5 and 6 mm (i.e, w = 1.5 to 6 mm), or more preferably w = 2 mm + 0.3 mm, measured along the Y-axis.

[0043] As explained in continuation, the grooves (1g) must be dimensioned such as to snugly engage the half-flanges (3f1 , 3f2) of the beams (3), preferably opposing a moderate friction against the introduction of a flange into the groove. By engaging the first half-flange (3f1) of the beam (3) into a groove (1 g) of a first panel (1) and the second half-flange (3f2) of the beam (3) into a groove (1 g) of a second panel (1), a portion of permanent drywall is formed with a reduced offset, Ts, between the planar portions (1 f) of the first and second boards (1) thanks, on the one hand, to the controlled tolerance, E, on the distance, Tg, and, on the other hand, on the alignment of the two boards along a reference plane (10R) parallel to the wall plane (X, Z) and at a same distance, Tg, from the planar portion (1 f) than the distance, Tg; separating the groove (1 g) from the same planar portion (1 f).

[0044] It is important that the grooves are not formed by moulding, but instead the grooves must be formed by machining, preferably by milling, to accurately control the distance, Tg, of the groove to the planar portion (1f) of the front surface measured along the Y-axis, within the tolerance, s, on the distance, Tg, It is important that the tolerance, s, on the distance, Tg, be smaller than the tolerance, 8, on the thickness. (i.e., s < 8). As discussed supra, 8 = 0.6 mm for plasterboards and similarly produced boards by continuous moulding, as illustrated in Figures 6a and 6b. To satisfy the requirements underlying norms EN520 and DTU25.41 on the planarity or smoothness of the permanent drywall, it is preferred that the tolerance, s, on the distance, Tg, be not more than 70% of the tolerance, 8, on the nominal thickness, preferably not more than 50% of 8, more preferably not more than 20% of 8, and more preferably not more than 10% of 8 (i.e., | s | < 0.7 | 8 |, preferably | s | < 0.5 | 8 |, preferably | s | < 0.2 | 8 |, and preferably | s | < 0.1 | 8 |).

[0045] The tolerance, s, on the distance, Tg, can be determined by measuring the distance, Tg, on a representative number of boards (1). The distance, Tg, of each board (1) is measured at a representative number of spaced apart points of both grooves (1g) of each plate (1). In practice, as illustrated in Figure 11 c, the tolerance, s, on the distance, Tg, is determined by measuring the distance, Tg, at 3 points (Mil to Mi3) of each groove (1 g) of each board (1.j, j = 1 to 3), repeating the operation on three boards (1) to yield nine values (Mij, with i = 1 to 3 and j = 1 to 3) of the distance, Tg. The values (Mij) thus measured are averaged to yield the distance, Tg, and the standard deviation is determined and defined as the tolerance,

[0046] The distance, Tg, separating the groove (1g) from the planar portion (1f) of the front surface measured along the Y-axis can be comprised between 30 and 70% of the nominal thickness, T (i.e., Te = 0.3 T to 0.7 T), preferably between 40 and 60% of the nominal thickness, T (i.e., Te = 0.4 T to 0.6 T), or Tg = 0.5 T ± 0.05 T.

[0047] The groove (1g) can have a groove width, w, comprised between 1 and 7 mm, preferably between 1 .5 and 5 mm, more preferably between 2 and 4 mm. The groove depth, d, of the groove (1g) can be comprised between 8 and 60 mm, preferably, d = 10 to 50 mm, more preferably d = 12 to 35 mm more preferably between 14 and 25 mm. For example, the groove depth can be, d = 8 to 15 mm.KIT-OF-PARTSComponents of the kit-of-parts

[0048] The present invention also concerns a kit-of-parts for forming a portion of a continuously smooth permanent drywall. The kit-of-parts comprises first and second boards (1) as defined supra, and a beam (3). As shown in Figures 9a and 9b, the beam (3) comprises a web (3w) normal to and centred on a first flange (3f) and optionally to a second flange forming a T-profile or a i-profile. The first flange (3f) extends along the X-axis and comprises a first half-flange (3f1) extending out of a first side of the web(3w) along the X-axis and is configured to snugly fit in the groove (1g) of the first board (1). The first flange (3f) comprises a second half-flange (3f2) coplanar with the first half-flange (3f1) and extending out of a second side of the web (3w) along the X-axis opposite the first side and configured to snugly fit in the groove (1 g) of the second boards (1).

[0049] The kit-of-parts also preferably comprises one or more top profile tracks (2u) and one or more bottom U-profile tracks (2d). Preferably the top U-profile tracks (2u) are identical to the bottom U-profile tracks (2d).Beams (3)

[0050] As shown in Figures 7a to 7c, the beams (3) are used as struts aligned along the Z-axis to couple two adjacent boards (1) side-by-side in a row by engaging the first and second half-flanges (3f1 , 3f2) of the first flange (3f) in the grooves (1g) of the adjacent vertical edges of the first and second boards (1). The beams (3) can be coupled to the top and I or bottom U-profile tracks (2u, 2d) but this is not even necessary. Consequently, the beam (3) can have a height (H3) measured along the Z-axis which is smaller than the distance separating the top and bottom U-profile tracks (2u, 2d). For example, the beam height, H3, can be comprised between 50 and 300 cm, preferably between 100 and 250 cm, more preferably between150 and 230 cm, depending on the height, H1 , of the boards (1). High height ratios, H3 / H1 , of the beam height, H3, to the board height, H1 , contribute to reinforcing the mechanical properties of the drywall. For example, the height ratio, H3 / H1 > 50%, preferably H3 / H1 > 70% or > 85%. The height ratio, H3 / H1 , is preferably smaller than 100%, preferably smaller than 90% to facilitate the engagement of the half-flanges (3f1 , 3f2) into the corresponding grooves (1g) of their whole flange height, H3. It is preferred that the groove width, w, and thicknesses of the first and second half-flanges (3f1 , 3f2) be so dimensioned along the Y-axis that the half-flanges can penetrate into the groove (1g) with a sufficient friction to hold the beam in place when the first half-flange (3f1) is inserted in the groove (1g) of the first board (1). The first and second half-flanges therefore have a thickness, Tr, substantially equal to, slightly lowerthan the groove width, w. To facilitate insertion of the half-flanges into the grooves, the free ends of the half-flanges can be tapered relative to the Y-axis.

[0051] The first and second half-flanges (3f1 , 3f2) are preferably identical, and the first and second grooves (1g) of all plates are also preferably identical, so that the beam can be used in any orientation, with any half-flange (3f1 , 3f2) fitting any groove (1g). The half-flange length, Lfh, measured along the X-axis is preferably smaller than or equal to the sum of the groove depth, d, and half the gap width, L11 , to ensure that it can entirely fit in the groove (1g). The half-flange (3f1 , 3f2) therefore preferably have a length, Lfh, defined as, 8 < Lfh < 64 mm, preferably, 10 < Lfh < 30 mm, more preferably 12 < Lfh < 22 mm.

[0052] The beam (3) comprises a web (3w) and two half-flanges (3f1 , 3f2) extending on either side of the web, forming a first flange (3f) at a first end of the web and thus a T-profile as shown in Figures 3c,9a, and 10a. The beam (3) can also comprise a second flange at the other end of the web (3w). The second flange can comprise one half-flange forming a h-profile as shown in Figures 9a and 9c, or two half-flanges, forming a i-profile as shown in Figures 7a to 7c, 9b and 9c (right insert). A T-profile is suitable for building a permanent wall lining as illustrated in Figure 9a, whilst an T-profile and a h-profile can be used for both permanent wall linings and partition walls. As shown in Figures 10b and 10c, the i-profile is particularly advantageous for building partition walls with two rows of boards (1) coupled to both sides of the top and I or bottom U-profile tracks (2u, 2d), with the grooves (1g) of two adjacent boards (1) on one side of the U-profile tracks being coupled to the first flange (3f) of the same beam (3) on whose second flange is inserted in the grooves of two adjacent boards (1) on the other side of the U-profile tracks. This way, less to no screw are required to fix the boards to the top or bottom U-profile tracks (2t, 2d). As shown by comparing the two inserts of Figure 10c, the i-profile or the h-profile can be used to build a partition wall with beams (3) spaced apart from one another by a distance smaller than the longitudinal length, L1 , of the boards.Tracks (2d, 2u)

[0053] The top and bottom U-profile tracks (2u, 2d) are preferably identical to one another. For partition walls, their width measured along the Y-axis must fit the height, Tw, of the web (3w) of the i-profile beam (3) so that when boards are coupled with their grooves engaging the first half-flange (3f1) of both first and second flanges, the two boards are pressed against each side of the top and bottom U-profile tracks (2u, 2d) (cf. Figures 10b and 10c).PORTION OF A PERMANENT DRYWALL

[0054] A portion of a continuously smooth permanent drywall can be built with the kit-of-parts of the invention in the following manner. The top U-profile tracks (2u) can be fixed in a row extending along the X-axis to a top position with screws (8), preferably a ceiling, and the bottom U-profile tracks (2d) can be fixed in a row extending along the X-axis to a bottom position with screws (8), preferably a floor, in alignment along the Z-axis with the top U-profile tracks (2u), as shown in Figure 7a. The first board (1) can be fixed to the top U-profile track (2t) with screws (8) or any other manner as shown in Figure 7b. The first half flange (3f1) of the first flange (3f) of a first beam (3) is inserted into a groove (1 g) of the first board (1). The second board (1) is positioned next to the first board, with the second half-flange (3f2) of the first flange (3f) inserted in a groove (1g) of the second board (1) such that the vertical edges of the first and second boards (1) are parallel and separated from one another by a gap (11g) of gap width, L11 > 0, preferably L11 = 5 mm + 2 mm, more preferably, L11 = 5 mm + 1 mm The second board (1) can be fixed to the top U-profile tracks (2u) with screws (8) or any other manner (e.g., adhesive). The first and second boards are thus held side by side to form a portion of permanent drywall.

[0055] The planar portion (1 f) of the first board (1) is substantially parallel to the planar portion (1 f) of the second board (1), and the planar portions (1 f) are offset from one another by a misalignment stepof height, Ts, measured along the Y-axis of not more than 2s, wherein s is the tolerance of the groove distance, Tg, separating the grooves (1g) from the planar portions (1 f) of each of the first and second boards (1). The tolerance on the distance, Tg, is preferably, |s| < 0.5 mm, preferably |s| < 0.4 mm, more preferably |s| < 0.3 mm, more preferably, |s| < 0.2 mm or aven |s| < 0.1 mm.Smoothness (planarity) as defined in EN520 and DTU25.41

[0056] The norms EN520 and DTU25.41 define conditions for yielding a drywall looking homogeneously smooth at the level of the joint portions between adjacent boards (1). The terms “smoothness” and “planarity” are considered herein as synonyms, and as defining a visually smooth and flat surface. The satisfactory visual aspect of the joint portion is dictated by the slope formed by the visible surface of the joint composition between the planar portions (1 f) of the two boards. The slope is defined as tan a = Ts I Lj, wherein Ts = 26, is the height of the step (or offset) between the planar portions (1 f) of the first and second boards (1), and Lj = 2Lt + L11 , is the joint width. By rearranging this expression, the taper width, Lt, can be expressed as a function of the tolerance, 6, on the nominal thickness, as Lt = f(<5) = 6 I tan a - % L11 , which defines a straight line of gradient, dLt / d<5 = 1 / tan a.EN520

[0057] EN520 defines conditions to ensure that the joint portions comprising a gap (11 g) between two adjacent boards filled with a joint composition (6) are visually smooth or planar, in spite of potential offsets, Ts, between the positions of the planar portions (1 f) of the boards due to variations, +5, on the nominal thicknesses, T, of the boards. EN520 assessed that the tolerance, 6, on the nominal thickness of plasterboard is, +5 = +0.6 mm. To compensate a potential offset, Ts < 26 = 1.2 mm, a taper width, Lt = 40 to 80 mm is required to give the drywall a smooth aspect between two adjacent boards. These values define the maximum allowed slope, tan a = Ts I Lj, formed by the joint composition (6) to bridge the planar portions of the two boards. Defining L11 = 5 mm and introducing the values of Lt = 40 mm and Ts = 1.2 mm into the function Lt = f(<5), yields the slope, tan a = 1 .2 1 85 = 0.0141 . The corresponding linear function, Lt = f(<5), with tan a = 0.0141 is represented in Figure 4c. The white circle represents the dimensions defined in EN520 of a tolerance, 6 = 0.6 mm and a corresponding taper width, Lt = 40 mm. To maintain the same slope formed by the joint composition between the two boards with taper widths, Lt, smaller than 40 mm, the step height (or offset), Ts = 26, must be reduced accordingly. The present invention proposes a solution to reduce the value of the step height, Ts, with boards having the same tolerance, 6, on the nominal thickness, T. This is achieved as explained below.

[0058] As shown in Figure 4a (prior art), the boards being screwed to a beam (3) pressing the rear surfaces (1 r) against the beam. The rear surfaces of the boards are therefore level on a reference plane (10R) (= dashed line), whilst the planar portions (1 f) of the front surfaces are offset by a step of height, Ts < 26 = 1 .2 mm. By comparison, the boards (1) illustrated in Figure 4b have the same dimensions as the ones of Figure 4a, but instead of aligning them over their rear surfaces, the boards in Figure 4b arealigned along a reference plane (10R) defining the position of their grooves (1g) (compare the positions of the dashed lines (10R) in Figures 4a and 4b). Since the tolerance on the distance, Tg, of the groove to the planar portion is,s, the step height, Ts < 2s < 28.

[0059] Referring to Figure 4c, reducing the value of, s, allows reducing the taper width, Lt, while maintaining the same slope, tan a, of the joint composition, which is defined in EN520 as ensuring a smooth and homogeneous visual appearance of the joint portion. The boards (1) of the present invention have a taper width, Lt < 28 mm. Referring to Figure 4c, to keep the same slope tan a, and thus satisfy the requirements of visual planarity defined in EN520, the taper width, Lt can be reduced from 40 mm down to 28 mm provided the tolerance, s, on the distance, Tg; is reduced to s < 0.4 mm (cf. shaded area in Figure 4c). Such tolerance on the distance, Tg, between the groove and the planar portion can easily be controlled when machining the grooves, e.g., by milling.DTU25.41

[0060] Figure 5a illustrates the test to be performed to obtain a visually smooth surface as defined in DTU25.41 . A ruler of length Lr = 200 mm having a first end laid at a distance Lt from the vertical edge of the first board (1) measured along the X-axis, at a junction where the tapered portion (1t) joins the planar portion (1 f) and resting at a junction of the second board (1) where the tapered portion (11) joins the planar portion (1 f) at a distance Lt from the vertical edge of the second board measured along the X-axis. The planarity test is passed if a second end of the ruler is separated from the planar portion (1 f) of the second board (1) by a distance Tm < 1 mm measured along the Y-axis.

[0061] Figure 5b plots an approximation of the relationship between the taper width, Lt, and the step height (or offset), Ts = 26 or 2s. Geometrical considerations define that sin a = Try / Lr = Tm / Lrp <=> Lrp = Tm Lr / (Ts + Tm), where the symbols are as defined in Figure 5a. For very small angles, a li-m>0 tana = sin a = Tm / Lrp = Ts / Lj <=> Lj = Ts Lr / (Ts+ Tm), with Lj = 2Lt + L11 , and Ts = 26 or 2s.Introducing the values, Tr = 200 mm and Tm =1 mm yields the curve plotted in Figure 5b.

[0062] The white circle in Figure 5b is representative of the requirements of EN520, with a taper width, Lt > 40 mm for a tolerance, 6 = 0.6 mm, on the nominal thickness, T. It can be seen that the ruler test DTU25.41 is more severe than EN520 and defines a taper width, Lt > 50 mm for the tolerance, 6 = 0.6 mm instead of Lt = 40 mm according to EN520. A board (1) according to the invention has a taper width, Lt < 28 mm. To satisfy the ruler test according to DTU25.41 with the taper width, Lt < 28 mm, it can be seen in the shaded area of Figure 5b that a tolerance, s < 0.18 mm is required on the distance, Tg, between the groove and the planar portion, which can be achieved by machining the grooves, e.g., by milling, with a control of the distance, Tg.PARTITION WALL AND WALL LINING

[0063] The top and bottom U-profile tracks (2u, 2d) are separated from one another by a distance substantially equal to a height (H1) of the boards (1) measured along the Z-axis. The top U-profile tracks(2u) are fixed side by side to a top position extending along the X-axis with the U- opening facing towards a bottom position. The bottom U-profile tracks (2d) are fixed side by side to the bottom position extending along the X-axis with the U-opening facing towards the top position. In many instances, wherein a partition wall must divide a large room into two smaller rooms, extending from floor to ceiling, the top position is a ceiling of the large room and the top U-profile tracks (2u) are fixed to the ceiling. The bottom position is a floor of the large room and the bottom U-profile tracks (2d) are fixed to the floor. The top and bottom U-profile tracks (2u, 2d) are preferably aligned side by side along the X-axis defining the position of the partition wall or wall lining. In special cases the top and bottom U-profile tracks may define a broken line. But this depends on the wishes of the builder and does not restrict the present invention. The top and bottom U-profile tracks (2u, 2d) can be fixed to the ceiling and floor with screws, an adhesive, double-sided tapes, and the like.

[0064] A wall lining as illustrated in Figure 10a can be built by simply repeating the steps described supra with respect to the portion of permanent drywall comprising two boards aligned side-by-side, to align as many boards as required to complete the wall lining. For permanent wall linings, the beams (3) can be T-profiles as shown in Figure 10a. The boards can then be fixed to the top and I or bottom U-profile tracks (2u, 2d) with anyone of screws, nails, an adhesive, double-sided tapes, and the like. Alternatively, the beams (3) can be h-profiles or i-profiles, with one or two half-flanges forming the second flange at the opposite end of the web (3w) from the first flange (3f) as illustrated in Figures 9b and 9c. The half-flange(s) of the second flange can serve to fix the beams (3) with screws or nails to the wall (11) hidden by the wall lining. If the second flange of the beams (3) are fixed to the wall (11), the boards (1) need not necessarily be fixed to the top and bottom U-profile tracks.

[0065] As illustrated in Figures 10b and 10c, a partition wall comprises boards (1) aligned side-by-side on both sides of the top and bottom U-profile tracks (2u, 2d). In a first embodiment, illustrated in Figure 10b, the boards (1) at one side of the top and bottom U-profile tracks are arranged face-to-face with the boards (1) at the second side of the top and bottom U-profile tracks. As can be seen in Figure 10b, in this embodiment, a same i-profile beam (3) couples two boards side-by-side on both sides of the top and bottom U-profile tracks. This has the effect that the boards positions on both sides of the top and bottom U-profile tracks are fixed by the web length, Tw, of the web (3w) and the boards need not be extensively fixed to the top and bottom U-bottom tracks. This also has the effect that the beams (3) are separated from one another along the X-axis by a distance of about the length, L1 , of the boards (1). Care must be taken that the distance between beams (3) be adequate for the desired mechanical properties of the partition wall.

[0066] In an alternative embodiment illustrated in Figure 10c, the boards aligned at one side of the top and bottom U-profile tracks are arranged offset with respect to the boards (1) at the second side of the top and bottom U-profile tracks. Preferably, as illustrated in Figure 10c, the boards at one side are offset relative to the boards at the other side of the U-profile tracks along the X-axis by half the length, L1 / 2,of the boards (1). The beams contacting the rear surface (1 r) of a board can be coupled to one another with an adhesive or a double sided adhesive tape (8a). This embodiment allows using boards of larger dimensions, L1 , along the X-axis, while aligning enough beams (3) to yield the required mechanical properties of the permanent partition wall.PROCESS FOR PRODUCING A PLASTERBOARD

[0067] The present invention also concerns a process for producing a plasterboard as described supra. The process comprises the following steps, illustrated in Figures 6a and 6b. A gypsum slurry (1s) comprising stucco, water, and additives is fed onto a bottom facer (21) laid on a conveyor (7) in motion. The bottom facer (21) corresponds to the front surface of the plasterboard thus produced. A levelling blade (5) control the thickness of the layer of slurry deposited onto the bottom facer (21). A top facer (22) is applied onto a free surface of the gypsum slurry to form a sandwich structure with a core (1 c) made of the gypsum slurry sandwiched between the top and bottom facers (21 , 22). The top facer (22) corresponds to the rear surface (1 r) of the plasterboard thus produced. The gypsum slurry (1g) forming the core (1 c) is allowed to set by hydration to form a setting board on the conveyor. The setting board is cut at a cutting station (9) to desired dimensions to yield cut boards (1 cp) and the cut boards (1 cp) are dried in a drying station (not shown) to yield plasterboards.

[0068] The tapered portions (1t) are formed. This is advantageously achieved during the feeding of the slurry onto the bottom facer (21), with the conveyor (7) having a geometry comprising raised edges forming the tapered portions (1t). This embodiment is illustrated in Figure 6c, with prismatic inserts (7t) coupled to the conveyor (7) form the tapered portions (1t) with desired geometry as the slurry is being fed. Another option is to machine the square edges of the setting board or of the cut boards (1 cp) or of the plasterboards obtained with a flat conveyor (7) to form the tapered portions with desired geometry. This solution, however, requires an additional process stage and generates considerable amounts of dust and loss of material. Furthermore, machining the tapered portions (1t) removes the bottom facer (21) from the tapered portions (1t), exposing the gypsum core (1 c) at the tapered portions (1t) of the thus produced boards (1).

[0069] After the setting of the gypsum slurry, the grooves (1g) are formed by machining with a rotating cutting tool (40c) along each vertical edge of the setting board or of the cut boards (1 cp) or of the plasterboards. As illustrated in Figure 6d, the grooves (1g) are preferably formed by milling. The machining tool (40g) is preferably a milling tool. The machining tool (40g) comprises a motor (40m) configured to drive the rotation of the cutting tool, preferably the milling cutter. It is essential that the position of the cutting tool (40c) be controlled relative to the position of the planar portion (1f) of the front surface. A measuring system (40s) can be used to ensure that the distance, Tg, between the groove (1g) being formed by the cutting tool (40c) and the planar portion (1f) be within the preselected tolerance, E, on the distance, Tg. It is clear that if the position of the groove (1g) is controlled relative tothe rear surface (1 r), the opposite effect is obtained. Instead of reducing the maximum offset from Ts = 26 to Ts = 2s, the maximum offset between two adjacent boards is increased to Ts = 2(5 + s), which is of course not desirable.

[0070] As shown in Figure 6a, the grooves (1 g) can be formed in line by installing the machining tool for forming the grooves (1g) upstream of the cutting station (9) (upstream is defined with respect to the direction of the conveyor (7)). This embodiment has a clear advantage of efficacy. Care must be taken, however, to account for possible dimension variations upon drying the boards (1), in particular for possible variations of the distance, Tg, which can, however, be predicted with accuracy. In an alternative embodiment, illustrated in Figure 6b, the grooves (1g) are formed after the setting board was cut into individual cut boards (1 cp) and dried. This embodiment adds a separate stage to the process but has the advantage of requiring no amendments to an existing production line and of cutting the grooves on boards having their final dimensions after drying.CONCLUDING REMARKS

[0071] Permanent drywalls according to the present invention are simple to install by unexperienced DIYers. In particular, the recurrent issue of planarity of a permanent drywall is substantially simplified with the boards and kit-of-parts of the present invention. Because the maximus offset, Ts, between two adjacent boards is reduced to Ts < 2s < 26, wherein +s is the tolerance on the distance, Tg, of the groove (1g) to the planar portion (1 f) and +5 is the tolerance on the nominal thickness, T, of the boards, and s < 6. This has the effect that the taper width, Lt, required by the norms to yield a visually smooth and planar surface at the level of the joint portions can be reduced accordingly. A tapered joint portion having a lower joint width, Lj = 2Lt + L11 , is substantially easier to fill in with a joint composition (6) to yield a smooth surface than tapered joint portions of larger dimensions along the X-axis. Consequently, the building of a visually smooth and planar permanent drywall is substantially facilitated with the present invention.

Claims

CLAIMS1. A board (1) configured to form a portion of a continuously smooth permanent drywall, wherein the board has a rectangular geometry, comprising,• a front surface comprising a planar portion (1f) parallel to a wall plane (X, Z) and separated from a rear surface (1 r) parallel to the wall plane (X, Z) by a nominal thickness, T + 8, measured along a Y-axis normal to the wall plane (X, Z), wherein 8 is a tolerance on the nominal thickness,• a peripheral edge defining the rectangular geometry and comprising first and second longitudinal edges of length, L1 , extending along the X-axis, normal to and separated from one another by first and second vertical edges of board height, H1 , extending along the Z-axis, and wherein• a groove (1g), extends parallel to the wall plane (X, Z) along each of the first and second vertical edges, each groove having a groove width, w, measured normal to the wall plane (X, Z), and a depth, d, measured parallel to the X-axis, and each groove being formed by machining, preferably by milling at a distance, Tg + s, from the planar portion of the front surface measured along the Y-axis, wherein s is a tolerance on the distance, Tg,• wherein each of the first and second vertical edges has an edge thickness, Te < T, measured along the Y-axis and is coupled to the planar portion (1 f) of the front surface by a tapered portion (1t) of taper width, Lt, measured along the X-axis, characterized in that, the tolerance, s, on the distance, Tg, is smaller than the tolerance, 8, on the nominal thickness, T (i.e., s < 8), in that, the taper width, Lt, is not more than 28 mm, preferably not more than 25 mm, more preferably not more than 20 mm,2. The board (1) according to claim 1 , comprising a core (1 c) comprising either at least 50 wt.% of calcium sulphate dihydrate relative to the core weight thus defining a plasterboard, preferably the plasterboard comprises at least 80 wt.% of calcium sulphate dihydrate, or the core comprises at least 50 wt.% of calcium silicate, or is made of fibre-cement.

3. The board (1) according to claim 2, being a plasterboard comprising at least 70 wt.% gypsum, preferably at least 80 wt.% of gypsum relative to the total weight of the plasterboard, and having a density comprised between 0.5 and 1 .2 g / cm3, preferably between 0.6 and 0.8 g I cm3.

4. The board (1) according to claim 2 or 3, wherein the nominal thickness, T, of the plasterboard measured between the planar portion (1f) of the front surface and the rear surface (1 r) is comprised between 12 and 50 mm, preferably between 18 and 40 mm, more preferably between 20 and 30 mm, a preferred nominal thickness is comprised between 18 and 25 mm.

5. The board (1) according to anyone of the preceding claims, wherein• the edge thickness, Te, is comprised between 8 and 45 mm, preferably between 12 and 35 mm, more preferably between 14 and 30 mm, a preferred edge thickness is comprised between 15 and 22 mm, and I or wherein• the distance, Tg, separating the groove (1g) from the planar portion of the front surface measured along the Y-axis is comprised between 30 and 70% of the nominal thickness, T (i.e., Te = 0.3 T to 0.7 T).

6. The board (1) according to anyone of the preceding claims, wherein the groove width, w, is comprised between 1 and 7 mm, preferably between 1 .5 and 5 mm, more preferably between 2 and 4 mm.

7. A kit-of-parts for forming a portion of a continuously smooth permanent drywall, comprising,• first and second boards (1) according to anyone of the preceding claims,• a beam (3) comprising a web (3w) normal to and centred on a first flange (3f) and optionally to a second flange forming a T-profile or a i-profile, wherein the first flange (3f) extends along the X-axis and comprises a first half-flange (3f1) extending out of a first side of the web (3w) along the X-axis and configured to snugly fit in the groove (1 g) of the first board and comprises a second half-flange (3f2) extending out of a second side of the web (3w) along the X-axis opposite the first side and configured to snugly fit in the groove (1g) of the second boards (1), and• preferably a joint composition (6) configured to fill in the gap (11) between two adjacent boards.

8. A portion of a continuously smooth permanent drywall comprising the elements of the kit-of-parts of claim 8 assembled as follows,• the first half-flange (3f1) is inserted into the groove (1g) of the first board (1),• the second half-flange (3f2) is inserted into the groove (1g) of the second board (1), such that the first and second boards are held side by side with a vertical edge of the first board (1) being parallel to and separated from a longitudinal edge of the second board (1) by a by a gap (11g) defining a distance, L11 > 0 and such that the planar portion (1 f) of the first board (1) is substantially parallel to the planar portion (1 f) of the second board (1), and the planar portions (1 f) are offset from one another by a misalignment step of height, Ts, measured along the Y-axis of not more than 2s, wherein s is the tolerance of the groove distance, Tg, separating the grooves (1g) from the planar portions (1 f) of each of the first and second boards (1), and wherein | s | < 0.5 mm, preferably | s | < 0.4 mm, more preferably | s | < 0.2 mm.

9. The portion of the continuously smooth permanent drywall of claim 9, wherein a joint portioncomprising the gap separating, and the tapered portions of the first and second boards is filled with a joint composition (6), giving a smooth visual aspect over a whole area of the continuously smooth permanent drywall.

10. The portion of the continuously smooth permanent drywall of claim 9 or 10, wherein• a slope, tan a = Ts I Lj, relative to the planar portion (1 f) of a straight line joining the planar portions (1 f) of two adjacent boards (1) is not more than 1 .4%, preferably not more than 1 .3%, wherein Lj = 2Lt + L11 , and I or• a ruler of length Lr = 200 mm having a first end laid at a distance Lt from the vertical edge of the first board (1) measured along the X-axis, at a junction where the tapered portion (11) joins the planar portion (1 f) and resting at a junction of the second board (1) where the tapered portion (1t) joins the planar portion (1 f) at a distance Lt from the vertical edge of the second board measured along the X-axis, and wherein a second end of the ruler is separated from the planar portion (1 f) of the second board (1) by a distance Tm < 1 mm measured along the Y-axis, as required by NF DTU 25.41 P1-1 .11 . Process for producing a plasterboard according to anyone of claims 2 to 7, comprising• feeding a gypsum slurry (1s) comprising stucco, water, and additives onto a bottom facer (21) laid on a conveyor (7) in motion,• applying a top facer (22) onto a free surface of the gypsum slurry to form a sandwich structure with a core (1 c) made of the gypsum slurry sandwiched between the top and bottom facers (21 , 22),• setting by hydration the gypsum slurry (1g) forming the core (1 c) to form a setting board on the conveyor,• cutting the setting board to desired dimensions to yield cut boards (1 cp),• drying the cut boards in a drying station (11) to yield the boards (1), and the tapered portions (11) are formed connecting the planar portion (1 f) of the front surface to the vertical edges, characterized in that, after the setting of the gypsum slurry the grooves (1g) are formed by machining with a rotating cutting tool (40c), preferably by milling, along each vertical edge of the setting board or of the cut boards (1 cp) or of the boards (1), by controlling the distance of the cutting tool (40c) relative to the planar portion (1 f) to ensure that the groove (1g) is at a distance, Tg, from the planar portion (1 f) within a predefined tolerance, E, on the distance Tg12. Process according to claim 11 , wherein Tg, is determined by measuring the distance, Tg, at 3 points (Mil to Mi3) of each groove (1 g) of each board (1 .j, j = 1 to 3), repeating the operation on three boards (1) to yield nine values (Mij, with i = 1 to 3 and j = 1 to 3) of the distance, Tg, the values (Mij) thus measured are averaged to yield the distance, Tg, and the standard deviation is determined and defined as the tolerance and wherein, the tolerance, 6, on the nominal thickness,T, are determined by measuring the nominal thickness, T, at 3 points (Nil to Ni3) randomly selected over the planar surface (1f) of each board (1.j, j = 1 to 3), repeating the operation on three boards (1) to yield nine values (Nij, with i = 1 to 3 and j = 1 to 3) of the nominal thickness, T. the values (Nij) thus measured are averaged to yield the nominal thickness, T, and the standard deviation is determined and defined as the tolerance, 6.

13. Process according to claim 11 or 12, wherein the tapered portions (1t) are formed either,• by feeding the gypsum slurry onto the conveyor (7) which has a corresponding geometry, or• by machining square edges of the setting board or of the cut boards (1cp) or of the boards (1).

14. Process according to any one of the claims 11 to 13, wherein the grooves (1g) are formed by milling with a milling cutter (40c) whose position along the Y-axis relative to the planar portion (1 f) is controlled by a measuring system (40s).

15. Use of a board according to any of the claim 1 to 7 to form a portion of a continuously smooth permanent drywall

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