Trapezoidal Metal Roof Sheet — Profile and Fixing

What it is

Roll-formed metal sheet with trapezoidal ribs, fixed through the crown into purlins with self-drilling screws. The most common industrial roof in India by a wide margin, because it is fast, cheap and well understood.

Specification at a glance

Property	Typical specification
Substrate	Galvalume to IS 15961, or galvanised to IS 277
Coating mass	Stated explicitly, e.g. AZ150
Thickness	0.45mm — 0.80mm TCT
Rib depth	28mm — 45mm common; deeper for long spans
Cover width	Typically around 1,000mm
Fasteners	Self-drilling with bonded EPDM washer

The three leak paths, and the rule for each

Path	Rule
Fasteners	On the crown, washer compressed to a slight bulge, never crushed
Side laps	Sealed with butyl tape where slope is low or exposure high
End laps	Length increases as slope decreases; follow the profile table

Fastener spacing must be closer at edges, corners and ridges, where wind suction under IS 875 Part 3 is far higher than across the roof field. A uniform spacing across the whole roof is the single most common structural shortcut and the reason roofs peel from a corner in a storm.

Where projects go wrong

• Coating mass unspecified, so a thin AZ70 sheet arrives for a building meant to last decades.
• Swarf left on the roof. Drilling swarf rusts in place and pits the coating; sweep at the end of every shift.
• Screws in the valley instead of the crown, putting a hole in the water path.
• End laps too short for a shallow slope, so water siphons back.
• Walking on the sheet between purlins, denting the profile and breaking the coating.

Standards referenced

Wind loading on roof and wall cladding under IS 875 (Part 3). Coated sheet substrates under IS 15961 and IS 277.