How this gambrel roof calculator works
A gambrel is two pitched roofs stacked: a steep lower slope and a shallow upper one meeting at the break. This calculator puts the break at the quarter span — each slope covers half of the half-width — which is the common barn layout:
rafter = run × √(1 + (pitch/12)²) · area = 2 × (lower + upper) × length
Worked example: rafters for a 24 × 32 gambrel barn
Each slope runs 6 ft. At 18/12 the lower rafter is 10′-9 13/16″ at 56.3°; at 6/12 the upper is 6′-8 1/2″ at 26.6°; the ridge sits 12 ft above the plate. All four planes add up to 1,122 ft² — 38 bundles of shingles with 10% waste.
How much roof does a gambrel barn have?
| Barn | Roof area | Ridge above plate | Bundles |
|---|---|---|---|
| 12 × 16 shed | 280 ft² | 6 ft | 10 |
| 24 × 32 barn | 1,122 ft² | 12 ft | 38 |
| 30 × 40 barn | 1,752 ft² | 15 ft | 58 |
| 36 × 48 barn | 2,524 ft² | 18 ft | 84 |
Gambrel roof FAQ
What angles do I cut for a gambrel roof?
At the default barn pitches — 18/12 below the break, 6/12 above — the slopes run 56.3° and 26.6° from horizontal, and the calculator restates them for any pitch pair. The classic “circle method” profile lands nearby at 67.5° and 22.5°; both read as a proper barn.
How do I calculate the area of a gambrel roof?
Add the slope lengths of the lower and upper rafters, double for both sides, and multiply by the building length: the default 24 × 32 ft barn works out to 1,122 ft² of roof — noticeably more than a gable of the same footprint, which is the price of all that loft space.
How tall is a gambrel roof?
Ridge height is the two rises stacked: with the pitch break at the quarter span, the default barn rises 9 ft on the steep slope and 3 ft on the shallow one — 12 ft above the wall plate on a 24 ft width. Steepen the lower pitch and the loft grows almost foot for foot.
Why build a gambrel roof instead of a gable?
Loft volume. The near-vertical lower slope pushes usable headroom almost to the walls, which is why barns, sheds and garages with storage lofts wear this shape. The tradeoffs are more roof area to shingle and a framing package that wants gussets or purlins at the break.
How many shingles does a gambrel roof need?
38 bundles for the default barn — 11.22 squares plus 10% waste at 3 bundles per square, about $1,520 at $40 a bundle. Steep-slope staging makes the lower planes slower to shingle than the math suggests; crews price gambrels as steep work.
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