Roof Purlin Calculator

Purlin size depends on the clear span between supports (struts, gable walls or partitions) and the load from the rafters above. As a guide from TradeCalculator's own Eurocode 5 (BS EN 1995-1-1, UK National Annex) purlin derivation for C16 timber (purlin at mid-slope supporting rafters of up to 2.5m slope span each side, dead load 0.75 kN/m² on the slope, imposed 0.75 kN/m² on plan, installed plumb, major-axis bending): 47×150mm spans up to 1.32m, 47×175mm up to 1.55m, 75×150mm up to 1.57m, 75×200mm up to 2.12m (the most common domestic purlin), and 75×225mm up to 2.40m between supports. Heavier coverings, wider rafter tributaries or snow beyond the covered allowance all need a specific check. Always confirm the section with a structural engineer for your actual covering, snow load and pitch.

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.

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.

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.

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.