Calculate rafter length from roof span, pitch, and overhang
Calculating rafter length accurately is essential for ordering timber, estimating costs, and ensuring your roof structure meets Building Regulations. Whether you are constructing a new roof, building an extension, or replacing damaged rafters, knowing the exact length from wall plate to ridge -- plus the additional overhang at the eaves -- avoids costly mistakes and wasted material. In UK domestic construction, rafters are typically made from C16 or C24 graded structural timber, most commonly in sizes like 47x100mm, 47x125mm, or 47x150mm for smaller spans. The rafter length depends on three key measurements: the roof span (the full width of the building from wall plate to wall plate), the pitch angle, and the desired eaves overhang. This calculator handles all the trigonometry for you, including the reduction for ridge board thickness, and gives you the precise cutting length needed for each rafter.
To calculate rafter length: 1. Enter the full roof span in metres. This is the distance from the outer face of one wall plate to the outer face of the opposite wall plate. For a terraced or semi-detached house, measure the full width of the building where the pitched roof sits. A typical UK terraced house might have a span of 5-7 metres, while a detached house could be 7-10 metres. 2. Enter the pitch angle in degrees. If you do not know the pitch, use our Roof Pitch Calculator to determine it from rise and run measurements. Common UK pitches are 22.5 degrees for interlocking tiles, 35 degrees for plain tiles, and 40-45 degrees for slate or steep traditional roofs. 3. Enter the eaves overhang in millimetres. This is the horizontal distance the rafter projects beyond the wall plate. A standard UK overhang is 300mm, providing adequate weather protection for the wall below. Some designs use 450mm or more for deeper fascia details or additional shelter. 4. Optionally, adjust the ridge board thickness. The default 50mm is standard for a single ridge board. If using a ridge beam or no ridge board (as in some truss designs), set this to 0. 5. Review the results. The calculator shows the rafter length to the ridge, the additional length for the overhang (measured along the slope, not horizontally), the total cutting length, the roof rise, and the half span. When ordering timber, round up to the next standard length (rafters typically come in 300mm increments from 2.4m to 6.0m).
The rafter length calculation uses trigonometry based on a right-angled triangle formed by the half span (horizontal), the rise (vertical), and the rafter (hypotenuse): Half span = (roof span / 2) - (ridge board thickness / 2000). The ridge board sits between the two opposing rafters at the apex, so each rafter is shortened by half the board thickness. Rafter length = half span / cos(pitch angle). For an 8m span at 35 degrees with a 50mm ridge board: half span = (8/2) - (50/2000) = 3.975m. Rafter = 3.975 / cos(35) = 3.975 / 0.8192 = 4.85m. Overhang rafter length = (overhang / 1000) / cos(pitch angle). For a 300mm horizontal overhang at 35 degrees: 0.3 / cos(35) = 0.3 / 0.8192 = 0.37m. This is the additional rafter length along the slope needed to achieve the desired horizontal projection. Total rafter length = rafter length + overhang rafter length = 4.85 + 0.37 = 5.22m. Rise = half span x tan(pitch angle) = 3.975 x tan(35) = 3.975 x 0.7002 = 2.78m. This is the vertical height from the wall plate level to the underside of the ridge board. These calculations assume a simple symmetrical gable roof. For hip roofs, valleys, or asymmetrical designs, additional geometric calculations are needed, and professional input is recommended.
When cutting rafters, remember to account for the birdsmouth cut at the wall plate and the plumb cut at the ridge. The birdsmouth is a notch cut into the underside of the rafter where it sits on the wall plate, typically one-third of the rafter depth. UK Building Regulations and BS 8103-3 provide guidance on rafter sizing based on span, pitch, and spacing. For spans exceeding the standard tables, or where unusual loading conditions apply (such as solar panels, green roofs, or heavy snow areas), consult a structural engineer. Standard timber lengths in the UK go up to 6.0m; for longer rafters, consider using engineered timber joists or specifying spliced connections approved by your building control officer.