Gambrel (Barn) Roof Calculator
A gambrel roof — often called a barn roof — has two pitches on each side: a shallower upper slope and a steeper lower slope. This classic style maximizes usable attic space. Because there are two distinct slopes, material calculation is slightly more involved. Enter your dimensions below and we will do the math.
How to use this calculator
- Enter your roof's length and width — measure the outer footprint of the building, not the slope length.
- Select your roof pitch (rise:12). If unsure, see the pitch reference table below or count the rise in inches for every 12 inches of horizontal run.
- Choose your shingle type and adjust the waste factor (add more for complex rooflines).
- Read your squares, bundles, and underlayment rolls from the result panel — use these numbers to get supplier quotes.
How we calculate this
A gambrel roof is calculated as two separate shed roofs stacked on top of each other. The lower slope (typically 12:12) covers roughly the outer third of the span on each side; the upper slope (typically 6:12) covers the inner two-thirds. Each zone's area = its horizontal run × length × its own pitch multiplier. Total area = lower zone + upper zone (both sides).
Pitch multiplier reference
The pitch multiplier converts your flat footprint to actual sloped roof area. A steeper roof means more material.
| Pitch | Multiplier | Description |
|---|---|---|
| 1:12 | 1.0035 | Almost flat — commercial membrane |
| 2:12 | 1.0138 | Low slope — common on additions |
| 3:12 | 1.0308 | Low pitch — walkable |
| 4:12 | 1.0541 | Moderate — ranch homes |
| 5:12 | 1.0833 | Standard |
| 6:12 | 1.118 | Standard — most common in US |
| 7:12 | 1.1577 | Moderate-steep |
| 8:12 | 1.2019 | Steep — requires toe boards |
| 9:12 | 1.25 | Very steep |
| 10:12 | 1.3017 | Very steep — safety harness required |
| 12:12 | 1.4142 | 45° — high-end residential |
Example material estimates — Gambrel (Barn) Roof at 6:12 pitch
Pre-calculated for common house footprints. Click any row to pre-fill the calculator above.
| Footprint | Roof Area | Squares | Bundles (arch.) | Underlayment | Ridge Cap |
|---|---|---|---|---|---|
| 800 sq ft (33 × 24 ft) | 964 sq ft | 11.1 | 34 bundles | 3 rolls | 1 bundles |
| 1,000 sq ft (37 × 27 ft) | 1,216 sq ft | 14 | 42 bundles | 4 rolls | 2 bundles |
| 1,200 sq ft (40 × 30 ft) | 1,460 sq ft | 16.8 | 51 bundles | 4 rolls | 2 bundles |
| 1,500 sq ft (44 × 34 ft) | 1,820 sq ft | 20.9 | 63 bundles | 5 rolls | 2 bundles |
| 1,800 sq ft (49 × 37 ft) | 2,206 sq ft | 25.4 | 77 bundles | 6 rolls | 2 bundles |
| 2,000 sq ft (51 × 39 ft) | 2,420 sq ft | 27.8 | 84 bundles | 7 rolls | 2 bundles |
| 2,400 sq ft (57 × 42 ft) | 2,913 sq ft | 33.5 | 101 bundles | 8 rolls | 2 bundles |
| 2,500 sq ft (58 × 43 ft) | 3,035 sq ft | 34.9 | 105 bundles | 8 rolls | 2 bundles |
| 3,000 sq ft (64 × 47 ft) | 3,660 sq ft | 42.1 | 127 bundles | 10 rolls | 2 bundles |
Includes 15% waste. Architectural shingles, 3 bundles/square.
What this calculator doesn't include
The default split (33% lower / 67% upper) is the traditional barn proportion. Your actual roof may differ — adjust if you have drawings showing the exact break point. Ridge cap is for the central ridge only; the break line between slopes uses a different flashing detail, not ridge cap.
Frequently asked questions
Traditional barn-style gambrel roofs use a steep lower slope of around 12:12 (45°) and a shallower upper slope of around 6:12. Modern residential gambrel roofs sometimes use 10:12 lower and 4:12 upper for a slightly flatter overall profile. The proportions affect both material quantities and attic headroom.
Yes — the additional framing complexity (the break at mid-slope), the extra cuts, and the flashing detail at the pitch change all add labour cost. Materials are similar to a gable of comparable square footage. The trade-off is significantly more usable loft space.
Yes, on both slopes as long as the upper slope meets the minimum 2:12 pitch requirement for asphalt shingles. The steep lower slope is straightforward. The junction between the two slopes requires careful flashing to prevent water infiltration.
Measure the overall footprint (length × width) and identify the pitch of each slope. If you have the actual rafter lengths, you can multiply each rafter run by the roof length and add both sides. This calculator uses the footprint and a traditional 1/3–2/3 slope split as a good estimate.
Significantly more than a comparable gable — typically 30–50% more usable floor area in the loft because the steep lower walls push the knee walls outward. This is why gambrel roofs are so common on barns and loft-style garages.