Rafter Length & Timber Cut Calculator
True timber lengths, plumb cuts, seat cuts, and birdsmouth allowances
Master Rafter Formula: The true geometric line length of a common timber rafter is calculated by dividing the effective horizontal run by the cosine of the pitch angle: Rafter Line Length = Run ÷ cos(θ). For an 8.0-metre building span (half-span run = 4.0m) with a 38mm ridge board at a 35° pitch, the effective run is 3.981 metres, producing a true line length of 3.981 ÷ cos(35°) = 4.860 metres (plus eaves overhang allowance).
Anatomy of a Traditional Cut Roof Rafter
In traditional British cut roofing (as governed by NHBC Standards Chapter 7.2 and BS EN 1995-1-1 Eurocode 5), every common rafter requires precise geometric cuts to transfer vertical dead and live loads safely into the structural masonry wall plates:
- Apex Plumb Cut: The vertical mitre cut at the top of the rafter that butts tightly against the central timber ridge board. The plumb cut angle equals
90° - Pitch Angle. - Birdsmouth Joint: The triangular notch cut into the lower section of the rafter that seats securely onto the 100×50mm or 100×75mm timber wall plate. The birdsmouth consists of:
- Seat Cut (Level Cut): The horizontal face of the notch bearing flat on the wall plate. The depth of the seat cut must never exceed 1/3 of the total rafter depth to avoid severe structural shear weakening.
- Plumb Cut (Heel Cut): The vertical face of the notch bearing against the outer edge of the wall plate.
- Overhang Tail (Eaves Projection): The extension of the rafter past the exterior wall plate that forms the eaves overhang for soffit and fascia board attachment. The diagonal sloped length of the tail equals
Horizontal Overhang ÷ cos(θ). - Eaves Plumb Cut: The vertical cut at the tail of the rafter where the timber fascia board is nailed.
Rafter Timber Sizing & Strength Classes (C16 vs C24)
| Clear Rafter Span (m) | Rafter Spacing: 400mm c/c | Rafter Spacing: 600mm c/c | Recommended Timber Grade |
|---|---|---|---|
| Up to 2.40 m | 47 × 100 mm | 47 × 125 mm | Grade C16 Kiln-Dried Regularised Softwood |
| 2.40 m to 3.20 m | 47 × 125 mm | 47 × 150 mm | Grade C16 / C24 |
| 3.20 m to 4.00 m | 47 × 150 mm | 47 × 175 mm / 47 × 200 mm | Grade C24 Structural Softwood |
| 4.00 m to 4.80 m | 47 × 200 mm / 47 × 225 mm | Purlin support required | Grade C24 Structural Softwood |
Worked Timber Cut Calculation: 7.2m Span at 35° Pitch
Step-by-Step Carpenter Cutting List
Given a building with an external brickwork span of 7.20 metres, a 38 mm thick treated ridge board, an eaves overhang of 0.35 metres, and a pitch angle of 35.0°:
- Step 1: Total Half-Span:
7.20m ÷ 2 = 3.600 metres - Step 2: Effective Run:
3.600m - (0.038m ÷ 2) = 3.581 metres - Step 3: Rafter True Line Length:
3.581m ÷ cos(35°) = 4.372 metres - Step 4: Overhang Sloped Tail:
0.35m ÷ cos(35°) = 0.427 metres - Step 5: Total Timber Stock Length Required:
4.372m + 0.427m = 4.799 metres(Specify standard 4.8m timber length). - Step 6: Plumb Cut Angle (Top & Heel):
90° - 35° = 55.0° - Step 7: Seat Cut Angle (Horizontal):
35.0°
Frequently Asked Questions on Rafter Calculations
What is the standard spacing for roof rafters in the UK?
In the UK, roof rafters are typically spaced at either 400 mm centres (c/c) or 600 mm centres (c/c). 400mm centres are standard for heavier concrete tiles and slates to reduce timber deflection, while 600mm centres are commonly used with engineered trussed rafters or lightweight sheet metal roofs.
How deep should a birdsmouth cut be?
Under UK Building Regulations (Approved Document A), the maximum vertical depth of a birdsmouth cut must not exceed one-third (1/3) of the rafter depth. For a 150mm deep rafter, the notch must not exceed 50mm. Cutting deeper severely weakens the rafter at its critical shear support point.
What is the difference between rafter span and rafter length?
Rafter Span is the horizontal clear distance between structural supports (e.g. wall plate to wall plate, or wall plate to internal purlin). Rafter Length is the true diagonal physical length of the timber piece cut along its sloped pitch.
📚 Primary Technical Sources & British Standards
- BS 5534:2014+A2:2018: Code of practice for slating and tiling for pitched roofs and vertical cladding. British Standards Institution.
- BS 5250:2021: Management of moisture in buildings — Code of practice. British Standards Institution.
- BS 6229:2018: Flat roofs with continuously supported flexible waterproof coverings — Code of practice.
- NHBC Standards Chapter 7.2: Pitched Roofs — Minimum pitches, rafter timber sizing, batten gauges and fixing specifications.
- UK Manufacturer Technical Literature: Marley Technical Handbooks, BMI Redland Fixing Guides, CUPA Pizarras Slate Technical Manuals.
Last technical audit date: 17 August 2026.