Disclosed is a roof
assembly for a modular
building unit that comprises a roofing sheet that is welded to the frame of the modular
building unit to form a unitary, continuous sealed weld between the roofing sheet and the generally horizontal structural frame of the modular
building unit, and that is pitched to downwardly direct water toward one end of the modular building unit. The welded roof
assembly prevents water infiltration into the modular building unit, and such watertight structure may then receive any gutter configuration the user wishes to implement without risking water infiltration into the building unit. The welded roofing sheet avoids the use of screws or other fasteners, and the associated possibility of
water leakage around the screws and into the building unit. The welded roof also is able to employ a single slope all of the way through the full span of the roof, thus avoiding the need for a centrally pitched roof
assembly, and its unitary construction avoids the tiling effect that results from the use of multiple, overlapping roofing sheets. The welded roof also avoids the need for
silicon or other filler agents between the unitary roofing sheet and the frame of the modular building unit.