Gambrel Roof Calculator
Size a classic barn-style gambrel roof. Enter building width and length in feet plus the lower and upper pitch (rise per 12 in of run) to get rafter lengths, total roof area, squares, and shingle bundles with a waste factor.
Example: with Building width (ft) 24 · Building length (ft) 30 · Lower pitch (rise per 12 in) 24 · Upper pitch (rise per 12 in) 6 · Waste factor (%) 10 → Roof area: 1,207 sq ft of roof plane.
- Lower rafter length13.42 ft (13 ft 5 in) each
- Upper rafter length6.71 ft (6 ft 8 in) each
- Area with waste1,328 sq ft (13.3 squares)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Model: each half-span is split equally between the steep lower slope and the shallow upper slope — the classic gambrel proportion. Rafter length = run × √(1 + (pitch/12)²).
How a gambrel roof is measured
A gambrel has two slopes on each side: a steep lower slope (often 20/12 to 30/12) and a shallow upper slope (4/12 to 8/12). This calculator uses the standard symmetric layout where the horizontal run of each slope is one quarter of the building width — the lower rafter and upper rafter each cover the same run, just at different angles. Each rafter length is its run times the slope factor √(1 + (pitch/12)²).
Total roof area is the sum of all four planes times the building length. Because the lower slopes are nearly vertical, a gambrel packs far more shingle area over the same footprint than a plain gable — that is the price of the loft space it creates.
Ordering materials from the numbers
Shingles are sold in squares (100 sq ft) and bundles (3 bundles per square, so 33.3 sq ft per bundle). Add waste before you convert: 10% is the usual allowance for straight gambrels, and 15% is safer if the roof has dormers, which are common on gambrel barns and garages. Rafter lengths here are cut lengths along the slope with no overhang — add your eave and any purlin allowances before buying stock.
How it’s calculated
Half-span = width/4 per slope. Lower rafter = (width/4) × √(1 + (lower pitch/12)²); upper rafter = (width/4) × √(1 + (upper pitch/12)²). Roof area = 2 × (lower + upper rafter) × length. Area with waste = area × (1 + waste%/100). Bundles = area with waste ÷ 33.3 sq ft, rounded up (3 bundles per 100 sq ft square).
Assumes a symmetric gambrel with the half-span split equally between the two slopes and no overhangs, dormers, or gable-end trim — real cut lists need a framing square or truss drawings.
Roof area for a 24 x 30 ft footprint by pitch pair
| Lower pitch | Upper pitch | Roof area |
|---|---|---|
| 18/12 | 4/12 | ≈ 1,029 sq ft |
| 20/12 | 8/12 | ≈ 1,132 sq ft |
| 24/12 | 6/12 | ≈ 1,207 sq ft |
| 30/12 | 5/12 | ≈ 1,359 sq ft |
Computed with this calculator's formula (half-span split equally between slopes); rounded to the nearest sq ft.
Common mistakes
- Using the footprint area (width × length) for shingles — the steep lower slopes make the real roof surface 40-90% larger.
- Entering pitch in degrees. This tool wants rise per 12 in of run: a 63° lower slope is about 24/12, not 63.
- Forgetting that rafter lengths shown have no overhang — add 12 in or more per end before cutting.
- Skipping the waste factor on a roof with dormers; gambrel barns usually have them and they eat shingles at every valley.
Frequently asked questions
What is the gambrel roof formula?
Each rafter length is run × √(1 + (pitch/12)²), where the run is one quarter of the building width. Roof area is 2 × (lower rafter + upper rafter) × building length, since both sides mirror each other.
What pitches make a classic gambrel?
A steep lower slope around 24/12 (about 63°) paired with a shallow upper slope around 6/12 (about 27°) is the traditional barn profile. Splitting the half-span equally between the two slopes keeps the proportions right.
Why is the roof area so much bigger than the building footprint?
The lower slopes stand nearly upright, so they add wall-like surface that a plain gable would not have. A 24 x 30 building (720 sq ft footprint) carries about 1,200 sq ft of gambrel roof at the default pitches.
How many shingle bundles does a gambrel roof need?
Divide the waste-included area by 33.3 sq ft per bundle and round up. Three bundles cover one square (100 sq ft) for standard 3-tab and most architectural shingles — check the wrapper, some heavyweight lines run 4 bundles per square.
Does this work for a gambrel shed or tiny barn?
Yes — the geometry scales. For sheds many kits use a semi-octagon layout where all four rafters are the same length; enter matching pitches of about 22.5° and 67.5° equivalents (5.0/12 and 29/12) to approximate it.