Free Gambrel Roof Calculator — Rafters & Area (2026)

How to Use This Calculator

1. 1Enter the building length and width in feet.
2. 2Enter the lower (steep) pitch — typically 18/12 to 24/12 for the barn-style walls.
3. 3Enter the upper (shallow) pitch — typically 4/12 to 8/12 for the cap.
4. 4Select rafter spacing and click Calculate.

About This Material

A gambrel roof — commonly called a barn roof — features two distinct slopes on each side: a steep lower section (typically 60–70 degrees) and a shallower upper section (typically 20–30 degrees). This dual-slope design maximizes usable space inside the upper story, making it the classic choice for barns, agricultural buildings, and Colonial-style houses. The steep lower slope functions almost like a wall, providing full-height usable space in the loft, while the shallow upper slope provides a conventional roof cap. Structurally, the transition between the two slopes (the gambrel break) requires a horizontal purlin plate that acts as a structural ledge to support the upper rafters and transfer loads to the lower rafters. The lower rafters are typically 2x8 or 2x10, while the upper rafters can be 2x6 or 2x8, depending on span. Gambrel trusses can also be factory-built as a single engineered unit. The gambrel design uses about 20–30% less material than adding a full second story with a conventional gable roof, making it economical for maximizing interior volume. Common lower pitches range from 18/12 to 24/12 (56° to 63°), while upper pitches range from 4/12 to 8/12 (18° to 33°). Material costs for a 30x40 ft gambrel frame run $4,000 to $8,000 for lumber. One drawback is wind resistance — the steep lower slope catches wind, so gambrel roofs are not recommended for hurricane zones.

Installation Tips

• •Build gambrel trusses on a flat surface (jig) to ensure consistency across all trusses.
• •The purlin plate at the gambrel break is structural — use an adequate beam or doubled 2x stock.
• •Install knee braces at the lower-to-upper transition for added rigidity.
• •Sheathe across the gambrel break carefully — the angle change requires precise cuts.
• •Use collar ties or ceiling joists in the upper section to prevent outward thrust.

Common Mistakes to Avoid

• ⚠Undersizing the purlin plate at the slope transition — this carries significant structural loads.
• ⚠Not bracing the lower-to-upper transition — the break point is the weakest part of the gambrel.
• ⚠Using the same pitch on both sections — the lower must be significantly steeper than the upper to gain usable space.
• ⚠Ignoring wind exposure — gambrel roofs perform poorly in high-wind areas due to the steep lower face.

Related Calculators