Cut Roof Framing Calculator

A cut roof is constructed on-site from individual timbers (rafters, purlins, ridge board) by a carpenter. A truss roof uses factory-made triangulated frames delivered to site. Cut roofs offer more flexibility for complex shapes and allow the loft space to be used, making them essential for loft rooms. They are more labour-intensive but give a traditional, high-quality result.

Common rafters are spaced at 400mm or 600mm centres on each side of the ridge. The number of pairs = (Ridge length ÷ Rafter centres) + 1. Each pair needs two rafters. For a 6m ridge at 400mm centres: (6 ÷ 0.4) + 1 = 16 pairs = 32 common rafters. Always add a few spare for the cutting out.

Purlins support the rafters at an intermediate point so each rafter spans in two shorter lengths. As a guide they are used where rafters span more than about 2.5m, with the first roughly mid-way up the slope. This calculator counts purlins by dividing the horizontal run (half the span) by the spacing you set, so a 1.8m setting on a 3.5m half-span gives one purlin per side. NHBC Standards 7.2.8 (Restraint) limits purlins used for restraint to no more than 2m centres unless a design proves a wider spacing works. Purlin number, size and grade are structural and should come from a current TRADA or NHBC span table (member sizing per NHBC 7.2.3 Design of pitched roofs, designed to Eurocode 5, BS EN 1995-1-1) or an engineer for your actual span, load and timber grade — the count here is indicative.

Collar ties are horizontal timbers connecting pairs of rafters in the upper third of the rafter, where they help resist the ridge dropping and the roof racking in wind. (Ties placed low down, at ceiling-joist level, are the ones that resist the rafters spreading at the wall plate.) A common arrangement is 47×100mm at every pair, or alternate pairs at wider centres, but this is a trade convention rather than a fixed rule — collar size, position and frequency are structural and should follow NHBC Standards 7.2.3 or an engineer's design for your span and pitch.

Jack rafters are shorter rafters that run from the wall plate to a hip rafter (or from a valley rafter to the ridge). They have a compound cut at the hip/valley end and decrease in length progressively. Each jack is shorter than the previous by a constant amount — the common difference — equal to the rafter spacing ÷ cos(pitch), i.e. spacing × sec(pitch). That is the same as the rafter line-length per unit of run. For example at a 35.5° pitch and 400mm spacing the common difference is 400 ÷ cos(35.5°) ≈ 491mm. (Spacing × tan(pitch) gives the difference in rise, not in sloping length, so do not use it for cutting jacks.)