# 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.

## 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**.

## Frequently asked questions

### How is rib depth chosen?

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.

### Where do trapezoidal roofs actually leak?

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.

### Which fasteners should be used?

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.

## Sources

- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads — roof and wall cladding
- [IS 15961](https://standards.bis.gov.in/website/published-standards/department-wise) — Aluminium-zinc alloy metallic coated steel sheet
- [IS 277](https://standards.bis.gov.in/website/published-standards/department-wise) — Galvanised steel sheets

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Source: https://hagerstone.com/materials/trapezoidal-metal-roof-sheet