Roof Pitch & Rafter Calculator
Enter building dimensions and pitch to get rafter lengths, ridge board, wall plate and total timber
Typical: 150-300 mm past the wall face
Allow 10% for standard roofs, 15% for complex shapes
Job Costing (optional)
Used for common rafters and hip rafters
UK Market Prices
Fills in the material cost fields with current UK market prices. You can review and edit each price before applying.
Results
Pitch Angle
-
degrees
Rafter Length
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metres (with eaves)
Total Rafters
-
common rafters
Roof Area
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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) -
Timber Summary
Ridge board length -
Wall plate total -
Total timber (all items) -
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Frequently Asked Questions
How do I calculate rafter length?
Rafter length = half-span ÷ cos(pitch angle). For a 4 m half-span at 35°: 4 ÷ cos(35°) = 4.88 m. Add 50-75 mm for the eaves overhang and deduct half the ridge board thickness at the top. Always order timber longer than calculated and cut to length on site.
What is roof pitch and how is it measured?
Roof pitch is expressed in degrees, as a rise:run ratio (7:12 = 7 units rise per 12 run), or as a percentage. To measure an existing pitch: hold a spirit level horizontally at the rafter, measure 300 mm along it, then measure vertically down to the rafter - arctan(vertical ÷ 300) = pitch angle. 35° is common for UK residential roofs and suits most tile types.
What size timber should I use for roof rafters?
Common UK sizes: 47 x 100 mm, 47 x 125 mm, or 47 x 150 mm C16 or C24 at 400 or 600 mm centres. Span tables in Approved Document A give maximum clear spans. Over 3.5 m: 47 x 125 mm minimum. Over 5 m: 47 x 150 mm or larger. Size depends on span, spacing, and loading.
What Building Regulations apply to roof construction?
Part A (Structure) - rafters sized for dead and imposed loads. Part C (Moisture) - underlay or breathable membrane required. Part L (Energy Efficiency) - minimum insulation U-value 0.16 W/m²K for new roofs in England. Building Regulations approval required for new dwellings and extensions.
How long should a ridge board be?
Gable roof: ridge runs the full building length. Hip roof: ridge = building length - (2 x half-span). A 10 m x 6 m hipped roof (3 m half-span) has a 4 m ridge. Ridge board is typically 47 x 150 mm or 47 x 175 mm, at least as deep as the rafter plumb cut.
What is the difference between a gable and a hip roof?
A gable roof has two sloping faces and two vertical triangular end walls - simpler to build, less timber, more loft space. A hip roof has four sloping faces meeting at a ridge with diagonal hip rafters at each corner - more complex and costly but better in high-wind areas as there are no exposed gable walls.