Method for renovating a flat roof

Abstract

The present invention relates to a method for renovating a flat roof (10) comprising a panel (16) made of ribbed steel, the upper face (18) of the panel (16) being provided with a first layer of thermal insulating material (24) and with a first sealing membrane (26), the renovation method comprising the following steps: - removing the first layer of thermal insulating material (24) and the first sealing membrane (26), - laying a second layer of thermal insulating material (28) on the upper face (18) of the panel (16), - laying a metal sheet (30) on the second layer of thermal insulating material (28), - attaching the metal sheet (30) to the panel (16) by means of a plurality of fastening members (36) which connect the metal sheet (30) to the panel (16), - laying a second sealing membrane (42) on top of the metal sheet (30).

Description

[0001] TITLE: METHOD FOR RENOVATING A FLAT ROOF

[0002] The present invention relates to a method for renovating a roof, in particular a flat roof comprising corrugated steel sheets (TAN).

[0003] Currently, many buildings, especially technical buildings such as hangars, are equipped with flat roofs, also called dry roofs or steel deck roofs or dry roofs, made using corrugated steel sheet panels placed on a load-bearing structure such as a frame.

[0004] Such corrugated steel sheets can also be referred to as "waterproofing substrates" since they are designed to be fitted with a waterproofing membrane. These corrugated steel sheets have specific structural characteristics that distinguish them from steel sheets used for composite floor profiles, i.e., mixed steel-concrete floors.

[0005] The technical characteristics of these corrugated steel sheets, or sealing supports, are defined in the normative documents published by AFNOR, in particular the NF DTU 43.3 P1-1 / A1 documents of 1 erDecember 2017: "Building works - Implementation of corrugated steel sheet roofs with waterproofing coating - Part 1-1: Standard technical specifications (CCT)" and NF DTU 43.3 P1-2 of April 2008: "Building works - Implementation of corrugated steel sheet roofs with waterproofing coating - Part 1-2: General criteria for the choice of materials (CGM)".

[0006] There is a strong demand for the renovation of flat roofs to install photovoltaic power plants and simultaneously improve existing thermal insulation. Legislation is planned to make such renovations mandatory.

[0007] One of the difficulties encountered is that the corrugated steel sheets generally used for this type of flat roof do not have sufficient mechanical performance to, on the one hand, take up the new permanent loads related to the installation of new elements on the flat roof, for example photovoltaic modules or a new layer of thermal insulation, and on the other hand, accept loads that will no longer be uniformly distributed, for example when the photovoltaic modules rest on pads.

[0008] One aim of the invention is therefore to propose a method for renovating a flat roof that overcomes the technical difficulties mentioned above.

[0009] To this end, the invention relates to a method for renovating a flat roof, said flat roof comprising a load-bearing structure on which a corrugated steel sheet is fixed, the corrugated steel sheet being formed of a panel having, on its upper face, parallel valleys separating platforms, the width of each valley being less than two-thirds of the platform width, the upper face of the sheet being provided with a first layer of thermal insulation material and a first waterproofing membrane, the renovation method comprising the following steps:

[0010] - Removal of the first layer of thermal insulation material and the first sealing membrane, - Installation of a second layer of thermal insulation material on the upper surface of the sheet metal, - Installation of a metal sheet on the second layer of thermal insulation material.

[0011] - fixing the metal sheet to the sheet metal by means of several fixing devices which connect the metal sheet to the sheet metal, - laying a second sealing membrane on top of the metal sheet.

[0012] The process according to the invention makes it possible to facilitate the renovation of existing flat roofs by minimizing handling operations and maintaining the continuity of operation of the building thus equipped.

[0013] The solution proposed here is particularly economical.

[0014] The assembly of the metal sheet with the corrugated steel sheet and the second layer of insulating material provides optimal mechanical performance, particularly in terms of rigidity. This allows it to support significant loads, especially those from photovoltaic installations, and ensures good load distribution across the flat roof surface.

[0015] According to other advantageous aspects of the invention, the method for renovating a flat roof comprises one or more of the following features, taken individually or in all technically possible combinations:

[0016] - the height of each valley is less than or equal to 80 mm and / or the width of each valley is less than or equal to 70 mm;

[0017] - the distance between two valleys is between 200 and 350 mm;

[0018] - the metal sheet is a profile which has parallel ribs whose height is between 6 and 15 mm, preferably between 6 and 9 mm, and whose pitch between two adjacent ribs is less than or equal to 150 mm;

[0019] - the metal sheet is placed on the second layer of thermal insulating material so that the general direction of the ribs of the metal sheet is substantially orthogonal to the general direction of the valleys of the sheet;

[0020] - the thickness of the material forming the metal sheet is between 0.70 and 1 mm, preferably greater than or equal to 0.75 mm; - the second layer of thermal insulating material has a minimum compressibility class C, as defined by CSTB document no. 2662 V2 - July 2010 and / or the second layer of thermal insulating material has a compressive stress for a 10% crush greater than or equal to 200 kPa, determined according to standard NF EN 826 - May 2013;

[0021] - the thickness of the second layer of thermal insulation material is between 30 mm and 65 mm;

[0022] - the second layer of thermal insulation material consists of expanded perlite panels;

[0023] - the process includes a step of laying a third layer of thermal insulating material on the metal sheet, the second sealing membrane being placed on the third layer of thermal insulating material;

[0024] - the process includes a step of installing at least one photovoltaic module on the second waterproofing membrane, each photovoltaic module having several distinct support areas which are distributed on the second waterproofing membrane;

[0025] - The fasteners are installed with a density of between 4 and 5.5 fasteners per m 2 ;

[0026] - the fixing devices are fixing screws, preferably spacer screws, i.e. having two distinct threads separated by a section forming a spacer.

[0027] The invention also proposes a flat roof comprising a load-bearing structure on which a corrugated steel sheet is fixed, the corrugated steel sheet being formed of a panel having, on its upper face, parallel valleys separating platforms, the width of each valley being less than two-thirds of the platform width, the flat roof being characterized in that it comprises:

[0028] - a second layer of thermal insulating material on the upper face of the sheet metal; - a metal sheet which is arranged on the second layer of thermal insulating material and which is fixed to the sheet metal by means of several fixing devices connecting the metal sheet to the sheet metal; - a second sealing membrane above the metal sheet;

[0029] the metal sheet being a profile which has parallel ribs whose height is between 6 and 15 mm, preferably between 6 and 9 mm, and whose pitch between two adjacent ribs is less than or equal to 150 mm.

[0030] According to other advantageous aspects of the invention, the flat roof comprises one or more of the following characteristics, taken individually or in all technically possible combinations: - the general direction of the ribs of the metal sheet is substantially orthogonal to the general direction of the valleys of the sheet;

[0031] - the height of each valley is less than or equal to 80 mm and / or the width of each valley is less than or equal to 70 mm;

[0032] - the thickness of the material forming the metal sheet is between 0.70 and 1 mm, preferably greater than or equal to 0.75 mm;

[0033] - the second layer of thermal insulation material consists of expanded perlite panels.

[0034] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0035] [Fig. 1] Figure 1 is a cross-sectional view which schematically represents a flat roof with a ribbed steel sheet conforming to the prior art, before renovation;

[0036] [Fig. 2] Figure 2 is a view similar to that of Figure 1 which schematically represents the flat roof of Figure 1 after implementation of the renovation process according to the invention;

[0037] [Fig. 3] Figure 3 is a longitudinal cross-sectional view which schematically represents a metal sheet used in the renovation process as implemented on the flat roof of Figure 2;

[0038] [Fig. 4] Figure 4 is a block diagram that illustrates the renovation process according to the invention.

[0039] In the following description, identical or similar elements will in some cases be designated by the same reference numbers.

[0040] In the following description, we will use, as a non-limiting example, an orientation according to a V, L, T reference frame, represented in the figures, corresponding to vertical V, longitudinal L, and transverse T directions.

[0041] Figure 1 shows a flat roof 10 constructed in accordance with the teachings of the prior art.

[0042] The flat roof 10 has a supporting structure 12, here schematically represented as two beams 14. A corrugated steel sheet 16, also designated as TAN for Ribbed Steel Sheet, is fixed to the supporting structure 12. The sheet 16 forms a waterproofing support.

[0043] The corrugated steel sheet 16 is presented here in the form of a panel or profile having, on its upper face 18, parallel valleys 20 separating a plurality of platforms 22, or sections. The valleys 20 extend here along a main longitudinal direction A1. The sheet 16 meets the technical criteria as defined in the normative documents mentioned at the beginning of this application (NF DTU 43.3 P1-1 / A1 and NF DTU 43.3 P1-2).

[0044] For this type of flat roof 10, the width Lv of each valley 20 is less than two-thirds of the width Lp of the plateau 22.

[0045] The valley width Lv here refers to the transverse dimension of the valley at its upper end, also called the upper rib opening (OHN) or valley opening.

[0046] Advantageously, the sheet 16 is chosen with a width Lv of each valley 20 less than half a width Lp of the plate 22. More preferably, the sheet 16 is chosen with a width Lv which is less than or equal to 70 mm.

[0047] Advantageously, the valley bottom width LF is between 15 and 45 mm. Here, the valley bottom width LF refers to the transverse dimension of the valley at its lower end, i.e., at the valley floor.

[0048] Advantageously, sheet metal 16 is chosen with a height Hv of each valley 20 which is less than or equal to 80 mm.

[0049] Advantageously, the Pv step between two adjacent valleys is between 200 and 350 mm.

[0050] As an example, the 16-gauge steel plate used here is the Hacierco® 4.214.74 model from ArcelorMittal Construction. This plate has a valley width (Lv) of 70 mm, a valley bottom width (LF) of 24 mm, and a valley height (Hv), or wave height, of 74 mm. The pitch (Pv) between two adjacent valleys is 214 mm for this model. The thickness of the 16-gauge steel plate is, for example, 0.75 mm.

[0051] Of course, the flat roof 10 can consist of several sheet metal panels 16 which are assembled and cut appropriately to cover a determined area.

[0052] The flat roof 10 of figure 1 also includes a first layer of thermal insulating material 24, which is arranged on the upper face 18 of the sheet 16. The first layer of thermal insulating material 24 thus rests on the platforms 22 of the sheet 16.

[0053] The flat roof 10 of Figure 1 also includes a first waterproofing membrane 26, which is placed on the first layer of thermal insulating material 24. This first waterproofing membrane 26 uses, for example, bitumen.

[0054] We now describe a process for renovating the flat roof 10 shown in Figure 1, taking into account in particular the illustrations in Figures 2, 3 and 4.

[0055] This renovation process includes a first step E1 of removing the first layer of insulating material 24 and the first sealing membrane 26. The removal of the first layer of insulating material 24 and the first sealing membrane 26 can be carried out by any suitable means, the objective being to expose the upper face 18 of the sheet 16.

[0056] During this first stage, the 16 corrugated steel sheets are left in place. This ensures, in particular, the continued operation of the associated building.

[0057] The renovation process then includes a second step E2 of laying a second layer of thermal insulating material 28 on the upper face 18 of the sheet 16. This second layer of thermal insulating material 28 is preferably between 30 and 65 mm.

[0058] This second layer of thermal insulation material 28 is, for example, formed from an assembly of expanded perlite panels, such as Fesco® panels produced by the company Sitek (F-67163 Wissembourg). These panels comprise expanded perlite particles mixed with recycled paper fibers, organic binders, and water repellents.

[0059] Panels with a thickness of approximately 50 mm are advantageously chosen, which allows for a good compromise in terms of rigidity and resistance to compression.

[0060] Advantageously, the second layer of thermal insulation material has a minimum compressibility class C, as defined by CSTB document no. 2662 V2 - July 2010. By compressibility class C, it is understood that a 50 x 50 mm specimen (for a specimen thickness less than or equal to 50 mm) or a 100 x 100 mm specimen (for a specimen thickness greater than 50 mm and less than or equal to 100 mm) exhibits a crushing of less than 5% when a test load of 40 kPa is distributed over the surface of the specimen and maintained for 7 days at a test temperature of 80°C.

[0061] Preferably, the second layer of thermal insulation material has a compressibility class of D, as defined by CSTB document no. 2662 V2 - July 2010. By compressibility class D, it is understood that a 50 x 50 mm specimen (for a specimen thickness less than or equal to 50 mm) or a 100 x 100 mm specimen (for a specimen thickness greater than 50 mm and less than or equal to 100 mm) exhibits a crushing of less than 5% when a test load of 80 kPa is distributed over the surface of the specimen and maintained for 7 days at a test temperature of 80°C.

[0062] Advantageously, the second layer of thermal insulation material has a compressive stress for a 10% crush greater than or equal to 200 kPa, determined according to standard NF EN 826 - May 2013.

[0063] Other thermal insulation materials, such as rock wool, rigid foam of the PIR type for polyisocyanurate or PUR for polyurethane, are possible. Preferably, the second layer of thermal insulation material 28 covers the entire surface of the sheet 16.

[0064] During this second step, the second layer of thermal insulating material 28 can be fixed to the corrugated steel sheet 16, for example by gluing, according to the recommendations of the manufacturer of the second layer of thermal insulating material.

[0065] The renovation process then includes a third step E3 of laying a metal sheet 30 on the second layer of thermal insulating material 28.

[0066] The metal sheet 30, shown in longitudinal section in Figure 3, is preferably a profile with parallel ribs 32 whose height HN is between 6 and 15 mm, preferably between 6 and 9 mm. This rib height provides optimal mechanical strength. It also conceals the heads of the fasteners 36 described later and minimizes unwanted air gaps. Furthermore, it results in a profile with a weight almost identical to that of a flat sheet, thus facilitating handling.

[0067] The ribs 32 are separated by grooves 34, or valleys. Preferably, the width Lv of valley 34 is less than or equal to 70 mm.

[0068] Preferably, the pitch PN between two adjacent ribs 32 is less than or equal to 150 mm. This facilitates the fixing of the photovoltaic module supports 44 onto the metal sheet 30.

[0069] Preferably, the metal sheet 30 is placed on the second layer of thermal insulation material 28 so that the general direction A2 of the ribs 32 of the metal sheet 30 is substantially orthogonal to the main direction A1 of the valleys 20 of the sheet 16. Such a placement promotes a distribution of point loads transverse to the main direction A1 of the valleys 20 of the sheet 16, which contributes to increasing the mechanical performance of the roof.

[0070] Preferably, the thickness of the material forming the metal sheet 30 is between 0.70 and 1 mm, preferably greater than or equal to 0.75 mm.

[0071] The metal sheet 30 is, for example, a T rapeza® 11.100.8 model from ArcelorMittal Construction. This sheet has ribs with a height HN of 7.5 mm. The valley width Lv 34 is, for this model, equal to 45 mm. The pitch PN between two adjacent ribs is, for this model, 100 mm. The steel thickness forming the metal sheet 30 is, for example, 0.75 mm.

[0072] Of course, the flat roof 10 can comprise several metal sheets 30 which are positioned adjacently and cut appropriately to cover the entire upper surface of the second layer of thermal insulation material 28. In such cases, the metal sheets 30 are preferably stitched together to transfer point loads and negative pressure loads from one sheet to another. The stitching is preferably achieved using fixing screws evenly spaced in the overlap area between two adjacent sheets.

[0073] The renovation process then includes a fourth step E4 of fixing the metal sheet 30 to the sheet 16 by means of several fasteners 36 such as fixing screws. The fasteners 36 are mounted through the second layer of thermal insulation material 28, and they connect the metal sheet 30 to the sheet 16.

[0074] This fastening ensures the cohesion of the assembly formed by the metal sheet 30, the second layer of thermal insulation material 28, and the sheet metal 16, thus contributing to a sandwich effect and a sufficient increase in the mechanical performance of the roof. In particular, since the sheet metal 16 has a width Lv of each valley 20 less than two-thirds of the width Lp of the platform 22, the second layer of thermal insulation material 28 is primarily in contact with the sheet metal 16, which promotes the sandwich effect.

[0075] The fasteners 36 are screwed, preferably from above, more preferably at the valleys 34 of the metal sheet 30, into the assembly formed by the metal sheet 30, the second layer of thermal insulating material 28, and the sheet 16. More preferably, the fasteners 36 are screwed into the plates 22 of the sheet 16.

[0076] Advantageously, the fasteners 36 are spacer screws. Each spacer screw has two separate threads divided by a spacer section. This type of screw allows a first thread 38, at the bottom, to be screwed into the sheet metal 16 and a second thread 39, at the top, to be screwed into the metal sheet 30. This type of screw prevents excessive compression of the second layer of thermal insulation material 28.

[0077] The fasteners 36 are preferably installed with a density of between 4 and 6 fasteners per m 2 This density represents the best compromise for the distribution of point loads over the entire 16 ribbed steel sheet.

[0078] According to a first embodiment MR1, which is illustrated by Figure 2 and Figure 4, the renovation process then includes a fifth step E5 of laying a third layer of thermal insulating material 40 on the metal sheet 30.

[0079] The third layer of thermal insulation material 40 includes, for example, rock wool, or rigid foam insulation such as PI R for polyisocyanurate or PUR for polyurethane, in foam form. This third layer of thermal insulation material increases the thermal insulation of the roof.

[0080] The renovation process then includes a sixth step E6 of laying a second sealing membrane 42 on the third layer of thermal insulating material 40.

[0081] The second waterproofing membrane 42 contains, for example, bitumen.

[0082] According to a second embodiment MR2, illustrated by figure 4, the second sealing membrane 42 can be positioned directly on the metal sheet 30, the flat roof 10 then being devoid of a third layer of thermal insulating material 40.

[0083] According to an advantageous embodiment, the renovation process includes a seventh step E7 of installing at least one photovoltaic module 44 on the second waterproofing membrane 42.

[0084] Figure 2 shows a single photovoltaic module 44 as an example. Of course, depending on the area covered by the flat roof 10, it is possible to arrange several photovoltaic modules 44.

[0085] The photovoltaic module 44 includes, for example, an inclined photovoltaic panel 46, supported by supports, such as pads 48 of different heights or rails.

[0086] The supports are placed or fixed onto the second sealing membrane 42. They can, for example, be welded to the second sealing membrane 42. The supports can also be fixed to the metal sheet 30 using fasteners, such as fixing screws. Such a fixing promotes the anchoring of the photovoltaic module.

[0087] The supports thus form several distinct support zones which are distributed over the second waterproofing membrane 42. The weight of the photovoltaic module at the level of these support zones is distributed homogeneously over the whole roof thanks to the sandwich assembly formed of the metal sheet 30, the second layer of thermal insulation material 28 and the sheet 16.

[0088] The type and thickness of the third layer of thermal insulation material 40 are chosen so as to be compatible with the type of photovoltaic module 44 installed on the flat roof 10, and so as to support the associated loads.

[0089] According to the embodiment shown in the figures, the second layer of thermal insulating material 28 is in contact, on the one hand, with all the plates of the sheet 16 and, on the other hand, with all the grooves / valleys of the metal sheet 30.

[0090] Although the invention was described in connection with the installation of photovoltaic power plants, it is suitable for reinforcing any roof intended to support point loads. Tests were carried out to quantify the improvement in mechanical performance achieved by the invention.

[0091] Three models were tested on a test bench under pressure load, on three supports, in accordance with standard NF P 34-503.

[0092] The first model consists of two 16 mm thick Hacierco® 4.250.40 corrugated steel sheets from ArcelorMittal Construction. This model has a valley width (Lv) of 70 mm, a valley bottom width (LF) of 20 mm, and a valley height (Hv), or wave height, of 39 mm. The pitch (PN) between two adjacent ribs is 250 mm for this model. The thickness is 0.75 mm. The two sheets are placed adjacent to each other along one of their longitudinal edges so that they interlock. The two sheets are fixed to a structure and stitched together at the point where they interlock.

[0093] The second model (according to the invention) consists of the same ribbed steel sheets 16 as the first model, 50 mm thick Fesco® C insulation panels screwed to the sheets, and metal sheets 30 in the form of Trapeza® 11.100.8 profiles arranged so that the general direction of their ribs is substantially orthogonal to the main direction of the valleys in the sheet. The Trapeza profiles are stitched at their overlap with a center-to-center spacing of 750 mm. They are also fixed to the sheet with the following distribution:

[0094] Parallel to the valleys of the sheet metal: a fixing every four valleys of Trapeza® profile (i.e. a center distance of 400 mm),

[0095] Perpendicular to the valleys of the sheet metal: one fixing every two notches of the sheet metal (i.e., a center distance of 500 mm)

[0096] an offset of one sheet metal pitch alternating in the fixing lines parallel to the valleys of the sheet metal.

[0097] This distribution represents a density of 5 fixations per m 2 .

[0098] Fesco® C insulation panels have a compressibility class of D and a compressive strength for 10% crushing greater than or equal to 200 kPa.

[0099] The third model is identical to the second model except that the metal sheet 30 is not mechanically fixed to the ribbed steel sheet 16.

[0100] Compared to the first prototype, the second prototype (which is according to the invention) offers the following performance gains:

[0101] 28% increase in maximum permissible deflection (L / 200),

[0102] 6% gain in permanent deformation

[0103] A 33% reduction in ruins. Compared to the third model, the second model (which is according to the invention) provides the following performance gains:

[0104] 19% gain in maximum permissible deflection (L / 200),

[0105] 4% gain in permanent deformation

[0106] - 12% gain in ruin.

Claims

DEMANDS 1. Method for renovating a flat roof (10), said flat roof (10) comprising a load-bearing structure (12) on which is fixed a corrugated steel sheet (16), the corrugated steel sheet (16) being formed of a panel having, on the side of its upper face (18), parallel valleys (20) separating platforms (22), the width (Lv) of each valley (20) being less than two-thirds of the width of platform (22), the upper face (18) of the sheet (16) being provided with a first layer of thermal insulation material (24) and a first waterproofing membrane (26), the renovation method comprising the following steps: Removal of the first layer of thermal insulation material (24) and the first waterproofing membrane (26), Installation of a second layer of thermal insulating material (28) on the upper face (18) of the sheet metal (16), Installation of a metal sheet (30) on the second layer of thermal insulating material (28), Fixing the metal sheet (30) to the sheet (16) by means of several fixing devices (36) which connect the metal sheet (30) to the sheet (16), Installation of a second sealing membrane (42) above the metal sheet (30).

2. Renovation method according to the preceding claim, characterized in that the height (Hv) of each valley (20) is less than or equal to 80 mm and / or the width (Lv) of each valley (20) is less than or equal to 70 mm.

3. Renovation method according to claim 1 or 2, characterized in that the pitch (Pv) between two valleys (20) is between 200 and 350 mm.

4. Renovation method according to any one of the preceding claims, characterized in that the metal sheet (30) is a profile which has parallel ribs (32) whose height (HN) is between 6 and 15 mm, preferably between 6 and 9 mm, and whose pitch (PN) between two adjacent ribs (32) is less than or equal to 150 mm.

5. Renovation method according to the preceding claim, characterized in that the metal sheet (30) is placed on the second layer of thermal insulating material (28) such that the general direction (A2) of the ribs (32) of the metal sheet (30) is substantially orthogonal to the general direction (A1) of the valleys (20) of the sheet (16).

6. Renovation method according to any one of the preceding claims, characterized in that the thickness of the material forming the metal sheet (30) is between 0.70 and 1 mm, preferably greater than or equal to 0.75 mm.

7. Renovation process according to any one of the preceding claims, characterized in that the second layer of thermal insulating material (28) has a minimum compressibility class C, as defined by CSTB notebook no. 2662 V2 - July 2010 and / or the second layer of thermal insulating material (28) has a compressive stress for a 10% crush greater than or equal to 200 kPa, determined according to standard NF EN 826 - May 2013.

8. Renovation method according to any one of the preceding claims, characterized in that the thickness of the second layer of thermal insulating material (28) is between 30 mm and 65 mm.

9. Renovation method according to the preceding claim, characterized in that the second layer of thermal insulating material (28) is made up of expanded perlite panels.

10. Renovation method according to any one of the preceding claims, characterized in that it comprises a step of laying a third layer of thermal insulating material (40) on the metal sheet (30), the second sealing membrane (42) being disposed on the third layer of thermal insulating material (40).

11. Renovation method according to any one of the preceding claims, characterized in that it comprises a step of installing at least one photovoltaic module (44) on the second sealing membrane (42), each photovoltaic module (44) comprising several distinct support areas which are distributed on the second sealing membrane (42).

12. A renovation method according to any one of the preceding claims, characterized in that the fasteners (36) are installed with a density of between 4 and 5.5 fasteners per m 2 .

13. Renovation method according to any one of the preceding claims, characterized in that the fixing members (36) are fixing screws, preferably spacer screws, i.e. having two distinct threads (38, 39) separated by a section forming a spacer.

14. Flat roof (10) comprising a load-bearing structure (12) on which is fixed a corrugated steel sheet (16), the corrugated steel sheet (16) being formed of a panel having, on the side of its upper face (18), parallel valleys (20) separating platforms (22), the width (Lv) of each valley (20) being less than two-thirds of the width of platform (22), the flat roof being characterized in that it comprises: A second layer of thermal insulating material (28) on the upper face (18) of the sheet (16); A metal sheet (30) which is arranged on the second layer of thermal insulating material (28) and which is fixed to the sheet (16) by means of several fixing devices (36) connecting the metal sheet (30) to the sheet (16); A second sealing membrane (42) above the metal sheet (30); the metal sheet (30) being a profile which has parallel ribs (32) whose height is between 6 and 15 mm, preferably between 6 and 9 mm, and whose pitch between two adjacent ribs (32) is less than or equal to 150 mm.

15. Flat roof (10) according to the preceding claim, characterized in that the general direction (A2) of the ribs (32) of the metal sheet (30) is substantially orthogonal to the general direction (A1) of the valleys (20) of the sheet (16).

16. Flat roof (10) according to claim 14 or 15, characterized in that the height (Hv) of each valley (20) is less than or equal to 80 mm and / or the width (Lv) of each valley (20) is less than or equal to 70 mm.

17. Flat roof (10) according to any one of claims 14 to 16, characterized in that the thickness of the material forming the metal sheet (30) is between 0.70 and 1 mm, preferably greater than or equal to 0.75 mm.

18. Flat roof (10) according to any one of claims 14 to 17, characterized in that the second layer of thermal insulating material (28) is made of expanded perlite panels.

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