Roofing sheet length and overlap

Side lap runs along the sheet, where the edge of one sheet sits inside the corrugation of its neighbour; on a GC sheet it is counted in corrugations, one and a half of them for a roof and one for a wall, per Tata Shaktee's own guide. End lap runs across the sheet, where the tail of one sheet sits under the head of the next sheet going up the slope; it is measured in millimetre, 150 or 200 depending on how flat the roof is.

Both joints get a fastener of their own. Tata Shaktee's guide asks for a fastener along every side lap and keeps the pitch between fasteners under 375 mm across the sheet's width, with none closer than 25 mm to the sheet's own end. The free overhang at the eaves is capped at 300 mm and the side overhang held to a minimum of 100 mm, so a sheet is never left hanging further than the gutter it is meant to drain into.

On the tina sheet, Tata Shaktee's own rule is plain: 150 mm of end lap at a 1 in 3 (18 degree) pitch, and 200 mm the moment the pitch comes down to 1 in 4 (14 degrees) or flatter still. Below that, the same guide adds a sealant bead, a minimum 3 mm line of silicone along the centre of the fastener hole, because a flat joint holds water long enough to find its way through a dry lap.

Durashine's own minimum is a flat number rather than a sliding one: 5 degrees, below which Tata Steel Colors does not publish the sheet at all. Its leaflet carries no matching end lap or side lap figure the way Tata Shaktee does; the anti capillary groove built into the Roof profile does that job through the shape of the sheet itself.

IS 277:2018 is the standard behind the ISI mark on a GC sheet, entered at serial 7 of the Steel and Steel Products (Quality Control) Order, 2024, and it fixes the thickness, the zinc coating and the tolerance the sheet is rolled to. It says nothing about how far one sheet should sit under the next; that is a laying question, not a material one.

IS 3007 is sometimes named for exactly this laying question, and on a GC or colour sheet the name does not fit: Part 1 of that standard is titled Laying of Asbestos Cement Sheets, and it governs a cement sheet, not a galvanised or colour coated steel one. No BIS code of practice currently fixes an end lap or a side lap figure for a GC or colour sheet the way IS 3007 fixes one for an asbestos cement sheet. What sits above this paragraph is each mill's own installation guidance, tested against its own profile, not a government mandate.

Give us the roof's length and width, its slope, and whether the run is tina or colour, and we work the lap into the sheet count before anything is cut. A roof measured without the lap in mind comes up a sheet or two short once the first row is laid, and that shortfall shows up on the site, well after the count was agreed.

We stock the same set of fittings for both sheets, purlins, ridge, flashing and self drilling screws, and the fixings for one sheet are not sold onto the other's profile. Choosing between the two in the first place is its own question, answered on the colour coated vs GC chadra page; here we take the sheet as already picked and get the length and the lap right for it.