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Work out the length of a roof rafter from the span and pitch or rise. Choose a pitched or lean-to roof, the rafter size and centres, enter the sizes, and press Calculate to see the rafter length, cut angles, birdsmouth, timber to buy and number of rafters.
| Pitch | Rise | Rafter Length | Timber |
|---|
The same span and overhang at other pitches. Rafter length includes the overhang.
You get the rafter length along the edge, split into the part from wall plate to ridge and the eaves tail, the pitch and rise, the plumb and seat cut angles, the maximum birdsmouth depth, the timber to buy and the number of rafters. A section drawing shows the roof, a table compares other pitches, and you can copy the result or save it as a PDF.
A rafter is the sloping side of a right-angled triangle. The level side is the run, from the wall plate to the ridge, and the vertical side is the rise. With the pitch angle, the rafter length is:
Rafter length = run ÷ cos(pitch) + overhang ÷ cos(pitch)
For a pitched roof, the run is half the span less half the thickness of the ridge board, because each rafter stops against the face of the ridge. For a lean-to the run is the full span to the wall. If you know the rise rather than the angle, the pitch is tan⁻¹(rise ÷ run), and the length can also be found with Pythagoras: √(run² + rise²).
| Pitch | Rise Per Metre Of Run | Rafter Per Metre Of Run |
|---|---|---|
| 15° | 268 mm | 1.035 m |
| 22.5° | 414 mm | 1.082 m |
| 30° | 577 mm | 1.155 m |
| 35° | 700 mm | 1.221 m |
| 40° | 839 mm | 1.305 m |
| 45° | 1,000 mm | 1.414 m |
Multiply the run by the factor to get the rafter length. The pitch you can use depends on the roof covering: tile and slate makers set a minimum pitch for each product, often between about 15° and 30°, so check the data sheet before you settle on the angle. Planning rules may also limit the ridge height.
A roofing square or a speed square set to the pitch makes marking quick. Cut one pattern rafter, try it in place, and copy it for the rest.
Rafters in UK roofs are usually strength-graded C16 or C24 softwood, 47 mm thick and 100 to 200 mm deep, at 400 mm or 600 mm centres. The size needed depends on the span along the slope, the pitch, the roof covering and the loads, and must be taken from published span tables or worked out by a structural engineer. This calculator does not size the rafters; it sets out their length and cuts once the size is known. Deeper rafters also leave room for insulation between them; our U-value calculator checks the roof against Part L.
Divide the length of the roof by the rafter centres, round up and add one for the end rafter. A pitched roof has that number on each side:
Rafters per side = roof length ÷ centres, rounded up, + 1
An 8 m roof at 400 mm centres needs 21 rafters each side, 42 in all. Add trimmers around roof windows and chimneys. Rafter timber comes in lengths from 2.4 m upwards, usually in 300 mm steps, so the calculator picks the shortest length that fits with 100 mm spare for cutting.
A lean-to rafter runs from a wall plate on the new wall up to a timber plate fixed to the house wall. The run is the full span from the outer wall plate to the face of the house wall, and the rise is the height of the top plate above the lower one. Lean-to roofs are often shallow, so check the minimum pitch of the tiles or sheets, and fit a lead or code-approved flashing where the roof meets the wall. For the rafter strapping, wall plate fixing and any steelwork, follow the approved drawings for the extension.
A pitched roof with a 6,000 mm span at 35°, a 38 mm ridge board and 300 mm eaves overhang, with 47 × 150 mm rafters at 400 mm centres on an 8 m long roof:
The calculator takes half the span for a pitched roof, or the whole span for a lean-to, as the level distance from the wall plate to the ridge. If you enter the rise, it works out the pitch from the rise and that distance. It takes half the ridge board off the run, divides the run and the overhang by the cosine of the pitch, and adds them for the rafter length along the edge. The timber is the next stock length after adding 100 mm for cuts, and the number of rafters is the roof length divided by the centres, rounded up, plus one, doubled for a pitched roof. The birdsmouth limit is a third of the rafter depth. Rafter sizes are not checked.
Divide the run, from the wall plate to the ridge, by the cosine of the pitch, then add the overhang divided by the cosine of the pitch.
About 3.64 m from wall plate to ridge, plus about 0.37 m for a 300 mm overhang: about 4.0 m in all, so buy 4.2 m lengths.
The level distance from the wall plate to the ridge: half the span less half the ridge board for a pitched roof.
Pitch = tan⁻¹(rise ÷ run). A 2,100 mm rise over a 3,000 mm run is 35°.
As a general rule no more than a third of the rafter depth, so 50 mm on a 150 mm rafter.
90° less the pitch, measured from the rafter edge, so 55° on a 35° roof. The seat cut is at the pitch angle.
Divide the roof length by the centres, round up and add one, for each side of a pitched roof.
It depends on span, pitch, loads and timber grade. Use span tables or ask a structural engineer.
Rafter geometry from right-angled triangle trigonometry. Birdsmouth depth limited to a third of the rafter as a general rule (see the birdsmouth joint). Reviewed November 2026.