RafterBuddy Roof Rafter Calculator

Rafter length, roof pitch and birdsmouth cuts for gable and shed roofs, with every measurement you need at the saw.

Roof type
From outside of wall to the fascia cut, measured level.
Length inputs take 12' 6-1/2", 12'6, 150.5" or 12.5 ft. Span and length default to feet, overhang to inches.

Roof section

Looking at the end of the building. Blue dimensions are measured level or plumb.

Pattern rafter layout

The board as it sits on your sawhorses, crown up, drawn to scale. Every measurement runs along the top edge. Lettered marks match the steps below.

Birdsmouth detail

HAP (height above plate) is measured plumb, from the corner of the notch to the top of the rafter. Keep the HAP the same on every rafter so the roof plane stays flat.

Cutting steps

    Square and saw settings

    Rafter calculator FAQ

    How do I calculate rafter length?

    Divide the building span by 2 for a gable roof, subtract half the ridge board thickness to get the run, then multiply the run by the slope factor: the square root of (rise² + 144) divided by 12. For a 12' span at 4/12 with a 1-1/2" ridge, that's 71.25" × 1.0541 = 75-1/8" (6' 3-1/8") from the ridge cut to the birdsmouth. Add the overhang (tail) length for the total.

    What is a birdsmouth cut?

    A birdsmouth is the notch cut near the bottom of a rafter so it sits flat on the wall's top plate. It has a level seat cut that bears on the plate and a plumb heel cut against the outside of the wall. RafterBuddy gives you the seat, the heel and the HAP for your pitch and lumber.

    What is HAP (height above plate)?

    HAP is the distance measured plumb from the corner of the birdsmouth notch up to the top edge of the rafter. Keeping the HAP the same on every rafter keeps the roof plane flat and sets the ridge height.

    How deep can a birdsmouth notch be?

    A common rule is that the notch should not cut more than one third of the rafter's depth. If it does, use a deeper rafter or bear on only part of the wall plate. RafterBuddy warns you when a notch is too deep.

    How do I mark a plumb cut with a speed square?

    Hold the speed square's pivot point on the top edge of the rafter and rotate it until your pitch number (for example 4 for a 4/12 roof) on the COMMON scale lines up with the same edge. Draw along the edge of the square to get a plumb line. A level seat line is drawn square to the plumb line.

    Can I use this for a shed or lean-to roof?

    Yes. Choose "Shed on ledger" for a lean-to that hangs from a ledger on a house wall, or "Shed on 2 walls" for a freestanding shed with a low wall and a high wall. RafterBuddy gives the rafter lengths, both birdsmouth cuts and how much taller the high wall must be.

    Rafter size and spacing must suit your span, snow load and local code, so check a span table or your building department. Lengths are theoretical: they assume straight lumber and a level, square building. Always test-fit the first rafter.

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