Roof Pitch & Rafter Calculator

Enter building dimensions and pitch to get rafter lengths, ridge board, wall plate and total timber

Roof Type

Gable Hip Mono-pitch

Gable - two slopes, vertical ends. Hip - four slopes. Mono-pitch - single slope (lean-to / extension).

Building Width (m) - full span, wall to wall

Building Length (m)

Roof Pitch

Common pitches range from 22.5° (interlocking tiles) to 45° (plain tiles / slate)

Eaves Overhang (mm) Typical: 150-300 mm past the wall face

Rafter Centres (mm)

Timber Cutting Allowance

10%

Allow 10% for standard roofs, 15% for complex shapes

Job Costing (optional)

Rafter timber per linear metre Used for common rafters and hip rafters

Ridge board per linear metre

Wall plate per linear metre

Labour per m² of roof area

Delivery (flat fee)

Results

Pitch Angle
-
degrees

Rafter Length

metres (with eaves)

Total Rafters

common rafters

Roof Area

m² incl. eaves

Pitch Detail

Angle -

Pitch ratio (1 in X) -

Rise at ridge -

Half span (run) -

Common Rafters

Structural length (no eaves) -

Length including eaves -

Rafters per side -

Total common rafters -

Total rafter timber (with waste) -

Hip Rafters

Structural length -

Length including eaves -

Hip rafters4

Total hip rafter timber (with waste) -

Timber Summary

Ridge board length -

Wall plate total -

Total timber (all items) -

Common UK Pitches

17.5° - Minimum for most clay and concrete plain tiles
22.5° - Typical for concrete interlocking tiles
30° - Very common in UK domestic construction
35° - Common - shallower appearance, good for most tile types
40° - Steeper, traditional character
45° - Very steep - period or decorative roofs, natural slate

Measuring Tips

Building width - Full external dimension from wall face to wall face. The half-span (run) is this ÷ 2
Rise - Vertical height from wall plate level to the underside of the ridge board
Rafter length - The calculator gives you the structural length (ridge to wall plate) plus your eaves overhang
Rafter centres - 400 mm is standard for most roofs; 600 mm is common where the roof deck load allows
Add extra to rafter lengths for the birds-mouth cut at the wall plate (typically 25-50 mm)

Roof Types

A gable roof has two sloping sides and two vertical triangular ends. The ridge runs the full building length.

A hip roof has four sloping sides - the ridge is shorter than the building and four diagonal hip rafters run from each corner. This calculator gives the hip rafter length; note that jack rafters vary in length and are not included.

A mono-pitch (lean-to) has a single slope from a high wall down to a low wall. Common for extensions, porches, garages and outbuildings. No ridge board - the rafter runs the full width.

Roof Pitch & Rafter Calculator