Roof Pitch and Rafter Basics

Rafter length is a hypotenuse, not a subtraction, and the birdsmouth is the cut that decides whether the roof stands up. Both are simple geometry.

Pitch as a ratio

Pitch is rise over run, written as X:12. A 6:12 pitch rises 6 inches for every 12 inches horizontally — 26.6 degrees. It does not change with building size; a 16 foot span and a 40 foot span at 6:12 use the same angle and the same rafter size for the same spacing.

Pitch determines what materials you can use. Asphalt shingles need at least 2:12; standard three-tab needs 4:12; metal can go to 1:12; below 2:12 you need a roll membrane.

Rafter length

The rafter is the hypotenuse of half the span and the rise:

• Half span = building span ÷ 2
• Rise = half span × pitch ÷ 12
• Rafter length = √(half span² + rise²)

For a 16 foot span at 6:12: half span 8 ft, rise 4 ft, rafter = √(64+16) = 8.94 feet. Add your tail overhang — typically 0.5 to 1 foot — for the cut length.

Rafter count and ridge

At 16 inches on center, a rafter every 16 inches plus one at each end. A 16 foot span is 192 inches, giving 13 rafters per slope, 26 total, plus a ridge board the full length of the building.

The ridge board is usually one inch deeper than the rafters so the rafters butt into its sides and cannot roll off the peak.

The birdsmouth

The birdsmouth is the L-shaped notch where the rafter sits on the wall plate. It has two cuts:

• Seat cut (horizontal) — sits flat on the top plate.
• Plumb cut (diagonal) — the front edge, which should be in line with the centre of the rafter tail or the rafter will hang past the wall.

The 47.5 degree rule is a memory aid for a 6:12 pitch, not a formula. For an 8:12, the plumb cut is about 34 degrees. Set the saw to half the roof pitch angle.

The depth of the notch matters: cut too deep and the rafter has no bearing and fails. Too shallow and the rafter sits too high. The seat should bear on the wall plate, and the notch should not extend into the rafter's bottom third.

Details that matter more than the math

• Rafter ties on every rafter pair in a conventional roof. Without them the walls spread.
• Ridge board depth — one inch deeper than the rafters.
• Birdsmouth sealant — a triangular membrane in the notch prevents wind-driven rain from blowing through.
• Rafter clips or straps at the ridge where wind uplift matters.
• Overhead bracing on longer spans, in an X or W pattern between the top of the rafters.

Rafters and floors are not interchangeable. Roof rafter tables use different loads from floor joist tables — the snow load on your roof depends on where you live. Check the table that matches your snow load, not the general one.

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