Roof Framing and Sheathing
Roof Framing and Sheathing
This domain tests roof geometry, rafter layout, framing members, and sheathing. Expect slope calculations, birdsmouth cuts, ridge and valley framing, and sheathing nailing.
Core rules
- Roof slope is rise over run; a 6/12 roof rises 6 in per 12 in of horizontal run.
- Rafter length is the hypotenuse of the run and rise, plus overhang, minus ridge and seat adjustments.
- A birdsmouth gives a bearing seat but must not cut more than the allowed depth; check local rules.
- Ridge boards, collar ties, and rafter ties perform different jobs: collar ties resist uplift near the ridge; rafter ties resist thrust lower down.
- Sheathing must be rated for the span and nailed to the schedule; underlayment and flashing shed water, not the sheathing alone.
Worked example
A gable roof has a 12-ft run and a 6/12 slope. Rise = 12 × 6/12 = 6 ft. Rafter length from ridge to wall line = √(12² + 6²) = √180 ≈ 13.42 ft. Add the overhang and deduct for the ridge and seat cut to get the actual rafter. The common mistake is using the run as the rafter length.
Traps
Confusing slope with pitch, cutting a birdsmouth too deep, and omitting valley framing are frequent errors. Also do not rely on sheathing to correct a misaligned rafter.