How do I calculate rafter length from span and pitch?

The rafter length (common rafter) equals half the roof span divided by the cosine of the pitch angle. For example, an 8m span at 40° pitch: run = 4m, rafter length = 4 / cos(40°) = 5.22m. Add eaves overhang for the true cutting length.

What is a birdsmouth cut and how deep should it be?

A birdsmouth is a notch cut into the rafter where it sits on the wall plate. It consists of a seat cut (horizontal) and a plumb cut (vertical). The seat cut depth should be no more than one-third of the rafter depth to maintain structural integrity — e.g., 50mm for a 150mm deep rafter.

How do hip and valley rafter lengths differ from common rafters?

Hip and valley rafters run diagonally at 45° in plan view, so they are longer than common rafters. The hip/valley factor multiplies the common rafter run by √(1 + 1/cos²(pitch)), which accounts for the diagonal travel across the roof plan.

What rafter spacing should I use — 400mm or 600mm?

In the UK, 600mm centres are standard for most domestic roofs with tile or slate coverings. Use 400mm centres for heavier coverings like concrete interlocking tiles on lower pitches, or where building control specifies closer spacing for wind uplift resistance.

What timber size do I need for rafters?

Common domestic rafter sizes in the UK range from 47×100mm for short spans up to 47×200mm for longer spans. The required size depends on span, spacing, pitch and loading. BS 8103-3 and approved span tables (e.g., TRADA) provide the correct sizes for your specific situation.

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Rafter Length Calculator — Common, Hip & Valley Rafters

Calculate common, hip and valley rafter lengths with birdsmouth depth, plumb cut angle and timber requirements for UK roof construction.

Roof Span

Total span wall-to-wall

Roof Pitch

Angle from horizontal

Rafter TypeCommon, hip or valley rafter

Eaves Overhang

Horizontal projection beyond wall

How We Calculate This

This calculator determines rafter lengths using roof geometry and trigonometry. It supports common, hip and valley rafters, accounts for ridge board thickness and eaves overhang, and calculates the birdsmouth seat cut depth.