Rafter calculator

Enter your roof’s span and its pitch or rise to get the common rafter length in millimetres, the cut angles, how many rafters you need and the timber lengths to order. It works for roofs with two slopes and for lean-tos.

Roof size

Roof type

Span

Outside face to outside face of the wall plates. Measured inside? Add both plate widths.

Ridge board

Its thickness. 0 if there isn’t one.

Overhang

Measured level, past the outside of the wall plate. 0 for none.

Roof length

Along the ridge, end to end.

Pitch

I know the

Pitch

The rafter’s angle from level.

Rafters and timber

Rafter centres

Use the spacing your span table or engineer’s design is based on.

Rafter depth

Only adds length for the angled end cuts. It doesn’t check strength.

Stock lengths

Many merchants stock 2.4, 3.0, 3.6, 4.2 and 4.8 m. Some also carry 3.9 and 4.5 m.

Rafter length, wall plate to ridge

3,647 mm

4,013 mm with the 300 mm overhang.

Run: 2,988 mm
Rise: 2,092 mm
Pitch: 35°
With the overhang: 4,013 mm
Overhang along the slope: 366 mm
Cut length (with end cuts): 4,118 mm
Plumb cut (ridge and tail): 35°
Seat cut (birdsmouth): 55°
Rafters per slope: 21
Rafters in total: 42
Roof area (with overhang): 64.2 m²
To order: 42 × 4.2 m

This gives geometry and quantities only. Rafter size (depth and grade) depends on span, load and timber grade: use the TRADA span tables or a structural engineer, and Building Regulations Approved Document A. Check your measurements on site before you cut.

How the rafter calculator works

Each common rafter is the sloping side of a right-angled triangle. The level side is the run, the upright side is the rise and the angle at the bottom is the pitch. Once you know two of them, the rest follows.

Run. On a roof with two slopes, the run is half the span minus half the ridge board thickness. On a lean-to, it’s the full span.
Rise. Rise = run × tan(pitch). If you enter the rise instead, the pitch is tan⁻¹(rise ÷ run).
Rafter length. Rafter length = run ÷ cos(pitch). That’s the length along the slope from the wall plate to the face of the ridge board.
Overhang. The level overhang is added to the run first: (run + overhang) ÷ cos(pitch).
End cuts. Plumb cuts at both ends offset the top and bottom edges by the rafter depth × tan(pitch), so that’s added to give the cut length.
Order length. The cut length, rounded up to the next stock length.
Rafter count. Roof length ÷ centres, rounded up, plus one, for each slope.

Where to measure the span

The calculator measures the run to the outside face of the wall plate. So measure the span from the outside face of one wall plate to the outside face of the other. If you measure between the inside faces, add the width of both plates, or every rafter comes out short by the width of one plate.

For a lean-to, measure from the outside face of the wall plate to the face of the wall or ledger board the rafters meet.

Plumb and seat cuts

The plumb cut is the upright cut at the ridge and at the tail. It’s set at the pitch, measured from a square cut across the rafter, which is how a speed square reads it. The seat cut is the level cut of the birdsmouth that sits on the wall plate, at 90° minus the pitch on the same scale. At 35°, that’s a 35° plumb cut and a 55° seat cut.

Worked example

A roof with two slopes. The span is 6,000 mm outside the wall plates, with a 25 mm ridge board, a 35° pitch and a 300 mm overhang. The roof is 8 m long, with rafters 150 mm deep at 400 mm centres.

Run: 6,000 ÷ 2 − 25 ÷ 2 = 2,987.5 mm
Rise: 2,987.5 × tan 35° (0.7002) = 2,092 mm
Rafter length: 2,987.5 ÷ cos 35° (0.8192) = 3,647 mm
With the overhang: (2,987.5 + 300) ÷ 0.8192 = 4,013 mm
End cuts: 150 × 0.7002 = 105 mm, so the cut length is 4,118 mm
Order length: 4.2 m, the next stock length up
Rafters: 8,000 ÷ 400 = 20, plus 1 = 21 per slope, 42 in all
Cuts: 35° plumb and 55° seat

So you’d order 42 lengths of 4.2 m. These are the calculator’s starting values, so change one figure at a time to see what moves.

Rafter length per metre of run

On site, multiply your run in metres by the figure for your pitch.

Rise and rafter length for 1 metre (1,000 mm) of run
Pitch	Rise per metre of run	Rafter length per metre of run	Plumb cut	Seat cut
15°	267.9 mm	1,035.3 mm	15°	75°
20°	364.0 mm	1,064.2 mm	20°	70°
22.5°	414.2 mm	1,082.4 mm	22.5°	67.5°
25°	466.3 mm	1,103.4 mm	25°	65°
30°	577.4 mm	1,154.7 mm	30°	60°
35°	700.2 mm	1,220.8 mm	35°	55°
40°	839.1 mm	1,305.4 mm	40°	50°
45°	1,000.0 mm	1,414.2 mm	45°	45°
50°	1,191.8 mm	1,555.7 mm	50°	40°

Worked out from tan and cos of each pitch. Example: a 2.9875 m run at 35° gives 2.9875 × 1,220.8 = 3,647 mm.

FAQ

Questions about rafters

How do I calculate rafter length?

Work out the run: half the span, minus half the ridge board. Then divide it by the cosine of the pitch. A 6,000 mm span with no ridge board at 35° has a 3,000 mm run, so the rafter is 3,000 ÷ 0.8192 = 3,662 mm. Know the rise instead? Use Pythagoras: the square root of run² plus rise².

How do I work out rafter length with an overhang?

If you measured the overhang level, add it to the run before dividing by the cosine of the pitch. A 300 mm level overhang at 35° adds 300 ÷ 0.8192 = 366 mm. If you measured it along the slope, just add it to the rafter length.

How do I find my roof pitch?

Measure the rise and the run, then pitch = tan⁻¹(rise ÷ run). A rise of 2,100 mm over a 3,000 mm run is tan⁻¹(0.7) = 35°. Choose “Rise” in the calculator and it does this for you. On an existing roof, an angle finder held on a rafter reads it directly.

What spacing should rafters be?

UK building control guides give rafter sizes at 400 mm and 600 mm centres. The spacing changes the size of rafter you need, so it’s part of the structural design. Use the spacing your span table or engineer’s design is based on.

What size rafters do I need?

This calculator doesn’t size rafters. The right depth, thickness and strength grade depend on the span, the spacing, the pitch, the loads on the roof and the timber grade, such as C16 or C24.

Approved Document A, in England and in Wales, points to the TRADA span tables for solid timber members in dwellings. They cover rafters and purlins in traditional pitched roofs on buildings up to three storeys. If your roof falls outside them, have a structural engineer design it.

What length timber should I order?

The cut length, rounded up to the next length your merchant stocks. Many stock carcassing timber in 600 mm steps (3.0, 3.6, 4.2, 4.8 m) and some carry in-between lengths such as 3.9 and 4.5 m, so the calculator offers both. Long lengths vary: one merchant we checked stops at 6.0 m, another at 7.2 m.