Trapezoidal Metal Roof Sheet — Profile and Fixing

Trapezoidal roof sheet is roll-formed metal sheet with trapezoidal ribs, fixed through the rib crown into purlins with self-drilling screws, and is the most common industrial roof in India. Rib depth sets how far it spans and how much water it carries, and it leaks at fasteners, side laps and end laps rather than through the sheet.

By Dhruv Agarwal · · 1 min read

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

PropertyTypical specification
SubstrateGalvalume to IS 15961, or galvanised to IS 277
Coating massStated explicitly, e.g. AZ150
Thickness0.45mm — 0.80mm TCT
Rib depth28mm — 45mm common; deeper for long spans
Cover widthTypically around 1,000mm
FastenersSelf-drilling with bonded EPDM washer

The three leak paths, and the rule for each

PathRule
FastenersOn the crown, washer compressed to a slight bulge, never crushed
Side lapsSealed with butyl tape where slope is low or exposure high
End lapsLength 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.

Standards referenced

  • IS 875 (Part 3) — Wind loads — roof and wall cladding (Bureau of Indian Standards)
  • IS 15961 — Aluminium-zinc alloy metallic coated steel sheet (Bureau of Indian Standards)
  • IS 277 — Galvanised steel sheets (Bureau of Indian Standards)

Frequently asked

Rib depth sets how far the sheet can span between purlins and how much water it can carry without overtopping. Deeper ribs span further and handle lower slopes. Choosing the profile first and then spacing purlins to suit is the wrong order: the structural grid and the design wind pressure under IS 875 Part 3 should determine the span, and the profile is selected to meet it.

At fasteners, side laps and end laps, almost never through the sheet. Over-tightened screws crush the washer, under-tightened ones never seal, side laps left unsealed admit wind-driven rain, and end laps shorter than the slope requires siphon water back up. All three are installation and detailing issues, not material defects, which is why supervision matters more than paying for a premium sheet.

Self-drilling screws with bonded EPDM washers, in a grade suited to the environment — stainless-capped for coastal and corrosive atmospheres, carbon steel with a durable coating inland. The washer is the seal, so it must be compressed correctly: just enough to bulge slightly, never crushed flat. Screws should land on the rib crown, not in the valley where water runs.

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