What is a cut roof vs a truss roof?

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.

How many common rafters do I need?

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.

Where should purlins be positioned on a cut roof?

Purlins support the rafters at mid-span and are essential for spans over about 2.4m. Position the first purlin approximately 1.8m from the wall plate along the rafter. For larger spans, add a second purlin. Purlins are typically 47×150mm or 47×175mm C16/C24 graded timber, and must be supported by struts down to a load-bearing wall.

What are collar ties and when do I need them?

Collar ties are horizontal timbers connecting pairs of rafters, typically positioned in the upper third of the rafter span. They prevent the rafters from spreading apart and the ridge from dropping. Use 47×100mm timber at every rafter pair or alternate pairs. Building Regulations may require them depending on the roof span and pitch.

How do jack rafters differ from common rafters?

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 equal to the rafter spacing × tan(pitch).

TradeCalculator.co.uk

Cut Roof Framing Calculator

Calculate all cut roof components: common rafters, hip rafters, jack rafters, purlins, ridge board and collar ties for gable and hipped roofs.

Roof TypeGable or hipped configuration

Roof Span

Wall plate to wall plate

Building Length

Length of the roof along the ridge

Rise

Vertical height wall plate to ridge

Rafter Centres

400mm standard, 600mm for thicker rafters

Include collar ties

Horizontal ties connecting rafter pairs

Price Per Rafter (optional)

How We Calculate This

This calculator determines all the timber components for a cut (traditional) roof including common rafters, hip rafters, jack rafters, the ridge board, purlins and collar ties.

The formula

Common rafter pairs = (Ridge length ÷ Centres) + 1
Jack rafters per hip = (Half span ÷ Centres - 1) × 2 sides
Ridge (hipped) = Building length - Span
Purlins = Floor(Rafter run ÷ Purlin spacing) × 2 sides