Timber & Framing
Rafter Set-Out Calculator
Common rafter length, ridge deduction, plumb and seat cuts
Updated 9 July 2026 · Live
What this tool does
From roof span, pitch and eaves overhang, set out the common rafter: run, rise, line length, ridge deduction, net cut length and the plumb + birdsmouth angles.
Rafter Set-Out Calculator — from span, pitch and overhang, returns the common rafter length to cut plus the plumb and seat (birdsmouth) cut angles.
People also use
Formula Used
How the rafter set-out calculator works
Enter the roof span measured across the wall plates, the roof pitch, the eaves overhang and the ridge board thickness. The calculator treats a common rafter as the hypotenuse of a right-angled triangle whose base is half the span (the run) and whose height is the rise. The ridge-to-wall line length is the run divided by the cosine of the pitch; from that it deducts half the ridge board thickness along the slope and adds the overhang to give the net length to cut for each rafter.
Run, rise and line length
The run is half the clear span — the horizontal distance from the wall plate to the centre of the roof. The rise is how far the roof climbs over that run, equal to the run multiplied by the tangent of the pitch. The line length is the true sloping distance from the ridge to the outer face of the wall plate, before the ridge deduction and before the overhang are applied.
Plumb and seat (birdsmouth) cuts
Every common rafter needs two set-out angles. The plumb cut, where the rafter meets the ridge, is cut at an angle equal to the pitch measured from vertical. The seat cut, which forms the horizontal bearing of the birdsmouth on the wall plate, is cut at ninety degrees minus the pitch. Marking both from a framing square with the pitch set gives a clean, repeatable birdsmouth.
The ridge deduction
A pair of rafters bears against the two faces of the ridge board rather than meeting on its centreline, so half the ridge thickness — measured along the rafter slope — is removed from each rafter. It is a small figure, but on a shallow pitch it is enough to matter, and forgetting it pushes the walls apart by the full ridge thickness.
Scope and safety
This tool sets out the common rafters of a straightforward duo-pitch (gable) roof. Hip, valley and jack rafters are longer and use compound angles, and any trussed roof is an engineered component designed by the truss manufacturer. Rafter sizes, permissible spans and connections are structural matters governed by the Building Regulations — confirm them against a recognised span table for your timber grade and loading, or with a chartered structural engineer.
Using this alongside other BuildMetricLab tools
Pair the set-out with our rafter length, roof pitch and roof area calculators when you are planning materials and cutting lists. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent anywhere, and every formula shown so you can audit the maths.
Sources & methodology
Common-rafter set-out from right-angle trigonometry: run is half the span, line length is the run divided by the cosine of the pitch, the ridge board deduction is half its thickness along the slope, and the plumb/seat cut angles follow directly from the pitch. Every figure is computed from your inputs and all inputs are editable.
Frequently asked questions
Does the result include the overhang?
Yes. The primary figure is the net length to cut per rafter: the ridge-to-wall line length, minus half the ridge board thickness (measured along the slope), plus the eaves overhang you enter. Set the overhang to zero if you want the bare structural length.
What are the plumb and seat cuts?
The plumb cut is the vertical cut where the rafter meets the ridge — its angle from vertical equals the roof pitch. The seat (or birdsmouth) cut is the horizontal cut that sits on the wall plate — its angle equals 90° minus the pitch. Together they form the birdsmouth notch.
Why deduct half the ridge thickness?
A pair of common rafters meet on either face of a central ridge board, not on its centreline. Deducting half the ridge board thickness (measured along the rafter slope) gives each rafter its true bearing length against the ridge.
Is this only for a simple gable roof?
The common-rafter set-out applies to the main slopes of a straightforward duo-pitch (gable) roof. Hip, valley and jack rafters use different (longer) lengths and compound angles, and trussed roofs are designed as engineered components — treat those separately.
Spotted something off?
Calculations or display — let us know.