20 Foot Gambrel Roof Truss Calculator
A 20 ft gambrel is the popular pick for one-car garages with usable second-floor loft, small barns, and pool houses. At 20 ft the lower rafter sits comfortably inside 2×8 SPF #2 capacity for snow loads up to 40 psf.
Pre-Calculated Results for 20 ft Span
| Sweep Angle | Lower Rafter | Upper Rafter | Total Height | Roof Area (20 ft L) |
|---|---|---|---|---|
| 45° | 14.14 | 0.00 | 10.00 | 666 |
| 60° | 10.00 | 5.18 | 10.00 | 712 |
| 67.5° | 7.65 | 7.65 | 10.00 | 718 |
Loft Conversion Math
Inside the knuckle of a 20 ft gambrel at the standard 60° sweep, the loft floor is about 10 ft wide with 7+ ft of headroom, enough to meet IRC habitable-space rules (R305.1 calls for 7 ft minimum ceiling height) once you add insulation and finished ceiling.
Foundation Options
A 20 ft × 24 ft gambrel can sit on a 16 in × 8 in continuous concrete footing in most US frost zones. Alternatively, eight pier blocks at 8 ft on center under doubled 2×10 beams works for non-conditioned barns and garden offices where there is no slab.
Typical Applications for a 20 ft Gambrel
A 20 ft wide gambrel is sized for single-car garage with loft, small barn. With a 60° sweep angle the lower rafters land at 10.00 ft and upper rafters at 5.18 ft, both within the capacity of 2×8 SPF #2 for typical residential loads at 16 in on center rafter spacing.
Total roof height at the standard 60° sweep is 10.0 ft above the wall plate. Loft floor area inside the knuckle is approximately 10.0 ft wide and runs the full length of the building, usable for storage, a sleeping loft, or a workshop above the main floor.