What size purlin do I need for my roof?

Purlin size depends on the span between supports and the loading from the rafters. For typical UK domestic roofs: 47×150mm spans up to 2.0m, 47×175mm up to 2.4m, 47×200mm up to 2.8m, and 47×225mm up to 3.2m. These are approximate — always check with structural tables or an engineer for your specific loading conditions.

Where should purlins be positioned on rafters?

Purlins should be positioned to limit the unsupported rafter span to about 1.8-2.0m. For a 4m rafter run (from wall plate to ridge), one purlin at approximately 1.8m from the wall plate is typical. For longer rafters, add a second purlin. Purlins sit on the face of the rafters and are notched into or bolted to them.

What is a purlin strut?

A purlin strut is a diagonal timber that supports the purlin from below, transferring the roof load down to a load-bearing wall or beam. Struts typically run at 45° from the purlin to the supporting wall. They must land on a load-bearing element — never on a non-load-bearing partition. Struts are typically 100×50mm C16 timber.

Can I use steel purlins instead of timber?

Yes, steel purlins (Z-purlins or C-purlins) are common in commercial and agricultural buildings. They span further than timber for the same depth. In domestic construction, timber purlins are more common and easier to work with. Steel purlins suit wide spans and heavy cladding systems.

What grade timber is needed for purlins?

Purlins must be strength-graded structural timber — C16 is the minimum for most applications, C24 for longer spans or heavier loads. The timber must be stamped or certificated as graded. Green (unseasoned) timber can be used but expect some shrinkage. Regularised (planed) timber gives more accurate dimensions for fitting.

TradeCalculator

Roof Purlin Calculator

Calculate purlin quantities from span tables, size selection from 47x150 to 63x200, and strut positions for cut roof construction.

Roof Length (along ridge)

Building length

Rafter Span (wall to ridge)

Half-span horizontal distance

Purlin Size

Larger = longer span capability

Rafter Centres

Affects loading on purlin

Price Per Metre (optional)

How We Calculate This

This calculator determines the number of purlins needed based on rafter span and spacing, selects an appropriate purlin size from span tables, and calculates strut quantities and lengths.

The formula

Purlins per side = Rafter run ÷ Purlin spacing on rafter
Total purlins = Purlins per side × 2 (both sides)
Purlin length = Building length
Struts = 1 per purlin (at each support point)

Span table (approx)

47×150mm C16: up to 2.0m span
47×175mm C16: up to 2.4m span
47×200mm C16: up to 2.8m span
47×225mm C24: up to 3.2m span

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.