Rafter lengths and cut angles for a straightforward gable or hip roof: common rafter, hip rafter, valley rafter, plumb cut, seat cut, birdsmouth and ridge height. Enter the pitch in degrees or as a 6:12 ratio — both work, and every result is shown in all three notations. You get a fully dimensioned section drawing and a cut card you can take to the saw. Everything runs in your browser; nothing is uploaded and nothing is stored.
Geometry only. This tool works out lengths and angles. It says nothing about
whether a rafter is deep enough, whether a ridge needs to be a beam rather than a board, or how
far a member can span. Member sizing, connections and load paths are the job of a qualified
engineer or building designer for your project and location. Do not use these numbers as a
substitute for that.
Roof
Span is measured over the wall plates, wall face to wall face. Overhang is the horizontal projection past the wall, not the sloping length.
Rafter & bearing
HAP is the plumb distance from the seat cut up to the top edge of the rafter. It sets how deep the birdsmouth is: the seat cut length follows from it. Leave the intersecting span equal to the main span for a simple L or T of the same width.
Notes
Is the hip run really the span times 1.414?
No — and this is the single most common mistake in rafter arithmetic. The hip runs diagonally
across a square corner whose sides are each one common rafter run, so the hip's plan run is
the common run × √2, which is half the span × √2 (less the ridge deduction). Using the
full span doubles the hip. As a sanity check on any tool: on a 6:12 roof a common rafter measures
13.4164 units of length for every 12 units of run, and the hip measures exactly 18.0000 for the same
12 units of common run. Those two numbers have been in framing tables for a century. If a
calculator disagrees with them, it is wrong.
Where exactly are the lengths measured from and to?
Along the top edge of the rafter, from the ridge plumb cut to the plumb line at the outside face
of the wall — that is the "line length". Half the ridge board thickness has already been taken off
the run, so the length given is the piece you actually cut, not a theoretical centreline. The
overall length adds the tail, measured to a plumb fascia cut. Rise is measured from the top edge at
the wall line, which is why HAP is added to it to get the ridge height above the plate.
Why doesn't my birdsmouth land on the plate the way I expected?
Because HAP and the seat cut length are two ends of the same stick. Once you fix HAP, the seat
cut length is (rafter plumb depth − HAP) ÷ tan(pitch), and it may be longer or shorter than your
plate. The tool tells you when the seat does not match the bearing width, and the "set HAP for full
bearing" button gives you the HAP that makes them agree. Deep rafters on shallow pitches produce
surprisingly long seat cuts.
What this tool does not do
It does not size anything: no span tables, no load cases, no snow, wind or point loads, no
nailing or strap schedules, and no advice on ridge beams versus ridge boards. It assumes both roof
planes have the same pitch and that hips and valleys sit at 45° in plan — irregular or "bastard"
hips, unequal pitches, curved and swept eaves are outside its scope. Valley lengths for an
intersecting roof narrower than the main roof are given to the theoretical intersection point;
what you deduct at the top depends on whether it lands on the main ridge, a header or the
sheathing plane. Cut one rafter, offer it up, and check it before you cut thirty.