Which comes first, purlin or rafter?

Rafters or trusses go up first to set the slope. Purlins are then fixed across them, and sheets go on the purlins.

Can a purlin be used without rafters?

In small lean to sheds, purlins sometimes span directly from wall to wall. For wider roofs you need rafters or trusses to support them.

What is the best section for purlins?

C or Z purlins, MS channels, angles, or rectangular hollow sections, sized by the span and load. Ask your fabricator or engineer for the size.

Purlins vs rafters: what is the difference in a roof?

Updated 31 October 2026

In this guide

Rafters
Purlins
Side by side
How to get them right

Rafters

A rafter is a sloping member that runs from the ridge (the top of the roof) down to the eaves or wall. Rafters set the roof slope and carry the load from purlins down to the walls, columns or trusses. In a steel shed, the sloping top member of a truss, or a sloping portal frame beam, plays the role of the rafter.

Purlins

A purlin is a horizontal member running along the length of the roof, across the rafters or trusses. The roofing sheets are screwed directly to the purlins. Purlins collect load from the sheets (their own weight, wind, a person walking during maintenance) and pass it to the rafters.

Side by side

Purlins vs rafters
Point	Rafter	Purlin
Direction	Down the slope, ridge to eaves	Across the slope, along the roof length
Supports	Purlins	Roofing sheets
Supported by	Walls, columns or truss joints	Rafters or trusses
Spacing	Wider, set by truss or frame spacing	Closer, set by sheet thickness and wind
Common steel sections	ISMB, channels, pipes, truss angles	C or Z purlins, channels, angles, square or rectangular pipes

How to get them right

Purlin spacing depends on the sheet: thinner sheets need purlins closer together. See our purlin spacing guide for typical numbers and roof slopes.
Rafter or truss spacing decides purlin span, which decides the purlin size.
Fix purlins with cleats so they do not roll or twist on the rafter.
Paint all steel members with primer and enamel, especially in dairy and poultry sheds where ammonia speeds up rust.

Frequently asked questions

Which comes first, purlin or rafter?: Rafters or trusses go up first to set the slope. Purlins are then fixed across them, and sheets go on the purlins.
Can a purlin be used without rafters?: In small lean to sheds, purlins sometimes span directly from wall to wall. For wider roofs you need rafters or trusses to support them.
What is the best section for purlins?: C or Z purlins, MS channels, angles, or rectangular hollow sections, sized by the span and load. Ask your fabricator or engineer for the size.