Gambrel Roof Framing Guide and Calculator
Birdsmouth cuts, purlin placement, gusset reinforcement, heel height, and ridge sizing for regular and custom gambrels.
Regular vs Custom Gambrel
Regular gambrels follow the half-circle method and use only one design input: the sweep angle. The upper pitch, knuckle height, total roof height, and rafter lengths are all derived. Custom gambrels let you choose any combination of lower and upper pitch. Pick custom only when a specific upstairs headroom target requires breaking from the 45° rule. Otherwise stick with regular, because every part is easier to source and lay out.
Purlin Joint vs Raised Joint
A purlin joint runs a 2×8 or 2×10 horizontally under the knuckle, the full length of the building. Every rafter pair lands on the purlin, and the purlin transfers thrust longitudinally to the gable end walls. A raised joint relies on plywood gussets fastened to each truss and on collar ties near the ridge to manage thrust per-truss. Purlin joints are stronger for any building longer than 20 ft. Raised joints make sense for short outbuildings, pre-built trusses, or where ceiling space underneath must stay open.
Heel Height and Fascia
Heel height controls fascia depth at the eave. Standard fascia depth is 5½ in (1× nominal). If the calculator returns a heel under 5½ in for your pitch and rafter, rip down a 1×8 fascia or use a 1×6. For heels over 8 in, use a 1×10 fascia or fur the soffit down. The reference birdsmouth calculator gives the exact value.
heel = rafter_depth / cos(pitch) − plate · tan(pitch)IRC Span Quick Reference
| Lumber | Max Span (2×8 #2 SPF, 16 in oc) | Max Span (2×10 #2 SPF, 16 in oc) |
|---|---|---|
| Lower rafter, 60° pitch, 30 psf snow | 14 ft 2 in | 17 ft 4 in |
| Lower rafter, 60° pitch, 50 psf snow | 11 ft 8 in | 14 ft 4 in |
| Upper rafter, 25° pitch, 30 psf snow | 13 ft 6 in | 16 ft 8 in |