Gambrel Roof Calculator

Gambrel Roof Wind Load Reference: ASCE 7 Pressure Coefficients

Design wind speed, exposure category, and pressure coefficients for gambrel windward and leeward faces.

Velocity Pressure qz
31.3 psf
Windward Pressure
25.1 psf
Leeward Suction
31.3 psf
Net Uplift
40.7 psf
Per-Rafter Tie Load
61 lb

Why the Lower Slope Catches Wind

A 65° lower gambrel pitch is geometrically much closer to a vertical wall (90°) than to a typical roof slope (15–30°). Wind hitting the windward face produces near-wall positive pressure; wind passing over to the leeward face produces near-wall suction. Standard roof GCp coefficients understate this load on gambrels. Use the wall coefficients for the lower segment and the roof coefficients for the upper segment.

Hurricane Ties and H-Clips

In wind zones above 130 mph (most of coastal Florida, Gulf Coast, and Carolina coast), every rafter needs a hurricane tie at the wall plate. Simpson H2.5 or equivalent is the standard for residential gambrels; H10 ties go at the knuckle. Sheathing fasteners increase from 6d at 6/12 to 8d at 4/6 in wind zones. Always cross-check with the local jurisdiction's wind speed map.

Field Tip
Size the lower slope's connections using wall pressure coefficients, not roof coefficients, even though it is technically part of the roof. Treating a 65° lower slope like a low-pitch roof under-counts the windward pressure and leeward suction it actually sees. When in doubt, tie every rafter at both the wall plate and the knuckle rather than just one or the other.

Frequently Asked Questions

›Why is gambrel wind performance worse than a gable?
The steep lower slope (60–75°) behaves like a near-vertical wall and catches positive wind pressure on the windward side and suction on the leeward side. A standard gable roof at 6:12 deflects wind upward. In high-wind zones a gambrel needs heavier hurricane ties and stiffer sheathing.
›What is GCp on a gambrel roof?
GCp is the external pressure coefficient from ASCE 7. For the steep lower slope it sits around +0.8 windward and −1.0 leeward, similar to a wall, not a roof. The shallow upper slope sees lower coefficients similar to a low-slope gable.
›What hurricane ties are needed for a gambrel?
In ASCE 7 wind zones above 130 mph design speed, every rafter-to-plate connection needs a Simpson H2.5 or equivalent hurricane tie rated to at least 415 lb uplift. Knuckle connections need an H10 or equivalent.

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