How do I calculate rafter length?

Divide the roof into a right-angled triangle. The run is the horizontal distance from the wall plate to the ridge line. The rise is the vertical height from the wall plate to the ridge. Rafter length equals the square root of (run squared plus rise squared), or run divided by the cosine of the pitch angle. This calculator does that maths automatically.

Is the run the same as the span?

No. The span is the full width of the building from wall plate to wall plate. The run is half the span on a standard dual-pitch roof. Always enter the run, not the span, into this calculator.

Do I include overhang in rafter length?

The basic rafter length calculation covers only the slope from wall plate to ridge. If you want to include the eaves overhang, enable the overhang option in this calculator and enter the horizontal overhang distance. The tool will add the sloping overhang length to the result.

What is a birdsmouth allowance?

A birdsmouth is the notch cut into the underside of a rafter where it sits on the wall plate. The birdsmouth allowance in this calculator adds extra stock length to account for the seat and plumb cuts, giving you a more practical timber order length rather than just the theoretical slope length.

Can this calculator be used in millimetres?

Yes. Select metric units and enter the run in millimetres. The result will also be in millimetres. You can switch to metres if preferred — the pitch angle calculation works the same in either unit.

Is this enough to size the timber structurally?

No. This calculator gives you the sloping length of the rafter for planning and ordering purposes. Structural sizing — the cross-section of timber needed to carry the roof load safely — requires span table calculations or engineer sign-off. Do not use rafter length alone to specify timber size.

What pitch do I need to enter?

Enter the roof pitch in degrees. If you only know the rise and run, use the roof pitch calculator first to convert to degrees, then enter that figure here. Common UK domestic roof pitches range from about 22.5 degrees to 45 degrees.

Can I use this for hip or valley rafters?

No. This calculator is for common rafters only. Hip and valley rafters run at a diagonal across the roof and require different trigonometry. Use a dedicated hip rafter calculator for those members.

Rafter Length Calculator UK

Use this rafter length calculator to work out the sloping length of a common rafter from the roof pitch and horizontal run. It is useful when setting out cut roofs, checking timber take-offs, or pricing a small pitched roof before ordering materials. This version also allows for overhang and birdsmouth allowance, which makes it more practical for real roof work than a basic triangle-only calculator. For UK jobs, the result is useful as a planning figure, but final timber sizing and structural checking should always be done separately using the relevant span tables or professional guidance.

Inputs

Roof Type

Units

Half-span (wall plate to ridge line)

Use half the building span.

Pitch Input Mode

Pitch (degrees)

Include eaves overhang

Include birdsmouth allowance (stock length only)

Results

Embed this tool

Rafter Length (Slope)

2.749 m

Pitch:30.0°

Rafter length (slope):2.749 m

Rise:1.375 m

Cut List Summary

Ridge allowance applied:−19 mm runRun (effective):2.381 mRise:1.375 mSlope length (base):2.749 m

Total stock length:2.749 m

How to use this calculator

1.Enter the roof run, not the full building span. For a typical dual-pitch roof, the run is usually half the span.
2.Enter the roof pitch in degrees.
3.Add any eaves overhang if you want the rafter tail included.
4.Add birdsmouth allowance if you want an estimated stock length rather than just the theoretical slope length.
5.Review the result and sense-check it against the actual roof layout before cutting timber.

Formulas Used

Rise

rise = run × tan(pitch)

Base rafter length

base rafter length = run / cos(pitch)

Equivalent base-length formula

base rafter length = √(run² + rise²)

Overhang slope length

overhang slope = overhang / cos(pitch)

Birdsmouth slope allowance

birdsmouth slope = seat depth / sin(pitch)

Estimated total stock length

total stock = base rafter length + overhang slope + birdsmouth slope

Run is the horizontal distance used for one side of the roof, not the full building span. This calculator estimates common rafter length for planning only, not structural sizing.

Worked example

Take a small garage roof with a run of 2.4 m and a pitch of 35 degrees. The base rafter length works out at about 2.93 m along the slope. If you then add a 300 mm horizontal overhang, the sloping overhang adds roughly another 0.37 m, giving a total of about 3.30 m before any extra allowance for trimming or birdsmouth cutting. That is the kind of check that helps when pricing timber or deciding whether a standard stock length will do the job without waste.

Run, span, overhang and birdsmouth

The most common mistake with rafter calculations is using the full span instead of the run. On a standard dual-pitch roof, the run is usually half the span, adjusted if necessary for ridge thickness or the exact set-out detail. Overhang also matters because the rafter continues beyond the wall plate to form the eaves. Birdsmouth allowance is different again. It is not the same as the structural span or the sloping rafter length. It is simply extra stock allowance for the seat and plumb cuts at the wall plate, which the calculator adds to the total so the estimated timber length is closer to what you would actually order.

What this calculator helps with

This tool is useful when you need a quick common rafter length before ordering timber, checking a cut roof layout, or pricing a small roofing job. It gives you the slope length faster than doing the trigonometry by hand and makes it easier to compare stock lengths once overhang is included. It is especially helpful early in the estimating stage, but it should not be used on its own to size rafters structurally.

Frequently asked questions

Related tools

Floor Joist Span Calculator UK — maximum span for C16/C24 joists once the roof is done
Roof Pitch Calculator UK — convert rise and run into angle and pitch ratio
Tile Calculator — estimate tile quantities for floors and walls
Roof Tile Calculator — work out gauge, tiles per m² and total tiles for your roof

New to rafter set-out? See how to calculate rafter length for the full method and a worked example.

Disclaimer: This calculator is for planning and estimating only. Always verify roof measurements on site and use the relevant structural design guidance, span tables, or professional advice before cutting or specifying timber.