# Purlins and Girts: The Secondary Steel of a PEB

> Purlins are the horizontal members spanning between roof frames that carry the roof sheeting; girts are their equivalent on the walls, carrying wall cladding. Together they are the secondary framing that transfers cladding loads into the main frames.

## Why it matters

Roof and wall cladding is only as secure as the steel it is fixed to. Purlins
and girts carry wind suction, maintenance loads and the weight of the cladding
into the frames, and in storms they are often where failures begin.

## How it works in practice

| Member | Location | Typical section |
| --- | --- | --- |
| Purlin | Roof, between frames | Cold-formed Z or C |
| Girt | Wall, between columns | Cold-formed Z or C |
| Eave strut | Roof-to-wall edge | Cold-formed C |
| Sag rod | Between purlins | Restrains them sideways |

## Common mistakes

- **Uniform spacing across the roof.** Edge and corner zones see higher uplift.
- **Omitting sag rods or bracing.** Thin sections twist without restraint.
- **Hanging loads without checking.** Services, cable trays and solar panels add
  load purlins were not designed for.

## Standards referenced

Cold-formed members under **IS 801**. Wind loads under **IS 875 (Part 3)**.

## Frequently asked questions

### Why are purlins usually Z or C sections?

They are cold-formed from thin galvanised strip, which makes them light and economical. Z sections can be lapped over supports to act continuously and nest for transport; C sections are common for simple spans, eave struts and trimming around openings.

### What decides purlin spacing?

The roof loads — including wind suction and any roof-mounted equipment — and the span capability of the chosen roof sheet. Wind uplift is often the governing case, particularly at roof edges and corners.

## Sources

- [IS 801](https://standards.bis.gov.in/website/published-standards/department-wise) — Use of cold-formed light gauge steel structural members
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads on buildings

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Source: https://hagerstone.com/glossary/purlins-and-girts