Rafter Length Calculator
Enter your building span, roof pitch, and overhang to get exact rafter length and birdsmouth notch dimensions. The cross-section diagram updates as you type - print it or screenshot it for the build. Nothing uploaded.
Building Span
Roof Pitch
Overhang
mm beyond wall
Rafter Size
Ridge Board Thickness
mm (0 for no ridge board)
Results
Cross-Section Diagram
Learn more: Rafter cutting and roof geometry
Why rafter calculations need more than a tape measure
Cutting a rafter one centimetre too short means an ugly gap at the ridge or a rafter that overhangs the wall plate wrong. The birdsmouth notch is especially critical: too deep and you weaken the rafter; too shallow and it rocks on the plate. Building codes in most jurisdictions limit the seat cut depth to one-third of the rafter depth.
How to use the rafter calculator
Enter the full building span and roof pitch (as rise:run, degrees, or percent slope). The calculator divides the span by two for each rafter run, accounts for ridge board thickness and overhang, and returns exact dimensions for cutting: rafter length, birdsmouth seat depth, plumb cut depth, and a live cross-section diagram.
FAQ
How do I calculate rafter length from span and pitch?
Rafter length = horizontal run x pitch factor. The run per rafter is half the building span minus half the ridge board thickness. The pitch factor is the hypotenuse of the rise/run right triangle: sqrt(rise^2 + run^2) / run. For a 4:12 pitch the factor is 1.054, so a 3 metre run gives a 3.16m rafter before overhang.
What is a birdsmouth notch on a rafter?
A birdsmouth is a triangular notch cut into the underside of the rafter where it meets the top plate of the wall. It has two cuts: the seat cut (horizontal, resting on the plate) and the plumb cut (vertical, against the fascia or wall). Building codes typically limit the seat cut depth to one-third of the rafter depth.
Can I enter pitch as degrees or percent slope instead of rise:run?
Yes. The calculator accepts rise:run ratios (like 4:12 or 5/12), pitch angle in degrees, or percent slope. All three convert to the same underlying angle so results are identical. Engineers often use degrees, architects and builders in North America use rise:run, and European standards often use percent slope.