By Zencalcy Editorial TeamLast reviewed About us
A gambrel roof is two pitches per side: a steep lower slope that creates usable attic space and a shallower upper slope that sheds water. The break point is usually halfway up the half-span, so each side has equal runs and the pitches decide the heights.
Dimensions and area for a gambrel (barn) roof.
run = width ÷ 4 (each segment) rise = run × pitch ÷ 12 rafter = run ÷ cos(atan(pitch ÷ 12))
Both segments take half of the half-span, so the runs are width ÷ 4. A classic gambrel is 30° lower and 20° upper, close to 7/12 and 4.5/12. Area is twice the sum of both rafter lengths times the building length.
| Symbol | Meaning |
|---|---|
| W | Building width. |
| p₁, p₂ | Lower and upper pitches. |
| A | Sloped roof area. |
Each run is 7.5 ft; the lower rafter is 8.65 ft and the upper 7.87 ft, giving a 7.7 ft rise and about 990 sq ft of roof for a 30 ft long barn.
Continue with the Roof Pitch Calculator, Rafter Length Calculator, Roof Truss Calculator, or the Roofing Calculator — or browse more roofing and all construction calculators.
Around 7/12 lower and 4/12 upper, which gives the classic barn profile and usable attic height.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026