Cutting hip rafters for a 6/12 hip roof with the commons already up.

Hip Rafter Length:

1. Plumb cut (peak): Same as common rafter plumb cut (6/12 = 19.4°).
2. Level cut (seat/heel): Same angle as common rafter seat cut.
3. Length: Measure from peak plumb cut to tail plumb cut, same as common rafter length. The hip is longer diagonally—add 1.5x the common rafter's overhang.

Dropping the Hip: The hip rafter sits lower than the common rafters because it's at a 45° angle to the ridge. To make the roof planes meet properly, you must drop the hip by:

• Drop amount = (hip rafter width ÷ 2) × tan(half the roof angle)
• For 6/12: Roof angle = 26.57°, half = 13.28°, tan = 0.236
• If using 2x material (3.5" wide): Drop = (3.5 ÷ 2) × 0.236 = 0.41"

Cut List:

• Plumb cuts: 19.4° (same as commons)
• Level cuts: 90° to plumb
• Birdsmouth: Same as common rafter (1.5" deep max for 2x)
• Tail cut: Match common rafter tail angle

Safety Note: Use proper PPE and ensure the work area is secure. If unsure about load-bearing calculations, consult a structural engineer.

IRC Section R802.7.1 covers hip roof framing requirements.

Hip Length & Cuts (Commons Up Method)

Since commons are set, measure run directly: hook tape on the outside corner of the wall plates, run diagonally to the ridge centerline intersection. That’s your hip run.

Hip Length = Hip Run × 1.5 (for 6/12). Derivation: √(6²+12²)/12 = √180/12 ≈ 1.5. Cut from the long point of the cheek cuts.

Cheek Cuts (Side Cuts): Set saw to 19.47° (arctan(6/17) ≈ 19.47°). Layout: On the hip’s wide face, mark a 17/6 slope (17″ run, 6″ rise) from the long point. Cut both sides. This matches the common’s 6/12 slope projected on the hip’s 45° plan angle.

Plumb Cut (Top): Same as common: 26.57° (6/12). Cut plumb from the long point after cheek cuts.

Tail Cut (Birdsmouth): Plumb cut at wall line = 26.57°. Level cut = seat depth per code (IRC R802.5.1, min 1½″ bearing).

Dropping the Hip Because the hip is rotated 45°, its wide face sits higher than the commons at the same plane. If you don’t drop it, the sheathing hits the hip’s top edge, creating a hump.

Drop Amount = (Hip Thickness ÷ 2) × tan(Hip Slope) Hip Slope = arctan(6/16.97) ≈ 19.47° For 1½″ hip: Drop = 0.75″ × tan(19.47°) ≈ 0.265″ (≈ 17/64″).

How: Rip the drop off the top edge of the hip before cutting cheeks, or notch the birdsmouth deeper by that amount. Easiest: rip a 17/64″ taper on the top edge from ridge to tail.

Code: IRC R802.3 (framing details), R802.5.1 (bearing). Verify ridge beam sizing per R802.3.1 if hip bears on it.

Pro Tip: Cut a test scrap first. Nail commons, dry-fit hip, check sheathing plane with a straightedge.