Gambrel Roof Calculator
A gambrel roof calculator adds each slope's run ÷ cos(arctan(pitch/12)), × length × 2 sides. A 30 ft barn with 4 ft at 24/12 and 6 ft at 6/12 has 939 sq ft.
How do you calculate gambrel roof area?
A gambrel roof calculator finds the true surface area of a barn-style roof by working out the sloped length of each of its two sections. Sloped length = run ÷ cos(arctan(pitch ÷ 12)). Total area = (lower sloped length + upper sloped length) × building length × 2 sides. A 30 ft long building with a lower slope of 4 ft run at 24/12 and an upper slope of 6 ft run at 6/12 has about 939 square feet of roof.
Plan-view measurements underestimate any pitched roof. On a gambrel the steep lower slopes make the gap large, so working each slope separately is the only way to get an accurate shingle, metal, or underlayment order.
The formula in plain words
- Pitch to angle: a pitch of 6/12 means the roof climbs 6 in for every 12 in across. arctan(6 ÷ 12) = 26.6°.
- Sloped length: run ÷ cos(angle). For 6/12, cos(26.6°) = 0.894, so 6 ft of run is 6.71 ft along the roof.
- Repeat for the other slope.
- One side = (lower sloped length + upper sloped length) × building length.
- Total = one side × 2.
Worked examples
Barn: 30 ft long, lower 4 ft run at 24/12, upper 6 ft run at 6/12
- Lower sloped length: 4 ÷ cos(63.4°) = 4 × 2.236 = 8.94 ft
- Upper sloped length: 6 ÷ cos(26.6°) = 6 × 1.118 = 6.71 ft
- One side: (8.94 + 6.71) × 30 = 469.6 sq ft
- Total: 939 sq ft, or 9.39 squares
- Building width: 2 × (4 + 6) = 20 ft
Backyard shed: 24 ft long, lower 3 ft run at 18/12, upper 4 ft run at 6/12
- Lower: 3 × 1.803 = 5.41 ft
- Upper: 4 × 1.118 = 4.47 ft
- One side: (5.41 + 4.47) × 24 = 237.1 sq ft
- Total: 474 sq ft, about 4.74 squares, on a 14 ft wide shed
Pitch multipliers
Multiply horizontal run by these factors to get sloped length.
| Pitch | Angle | Multiplier |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 12/12 | 45.0° | 1.414 |
| 18/12 | 56.3° | 1.803 |
| 24/12 | 63.4° | 2.236 |
| 30/12 | 68.2° | 2.693 |
Typical gambrel pairs put a steep lower slope (often 18/12 to 30/12) under a gentle upper slope (often 4/12 to 8/12). Any pitch you choose is fine for the math, but check shingle and metal panel minimum-slope rules for the upper section.
How to measure an existing gambrel
- Building length: measure along the eave, including any gable overhang if you want it roofed.
- Runs: from the ground, measure the building width. Then measure inside the loft, or from drawings, to find where the break sits.
- Pitch: hold a level and tape against each slope, or use the roof pitch calculator.
- Check: lower run + upper run should equal half the width.
Common mistakes
- Entering sloped length as run. That double-counts the slope and overstates the area.
- Using one pitch for both slopes. The steep lower slope can be twice as long as its run.
- Forgetting both sides. The calculator already doubles; do not double again.
- No waste allowance. Add about 10 to 15% for cuts, the break line, and starter courses.
Turn the area into bundles with the roofing shingle calculator, and size felt or synthetic underlayment with the roofing underlayment calculator. For hip-style roofs, use the hip roof calculator.
Frequently asked questions
What makes a gambrel roof different from a gable roof?
A gable roof has one slope on each side. A gambrel has two: a steep lower slope near the eaves and a shallower upper slope up to the ridge, the classic barn shape. The steep lower section gives far more headroom in the loft than a gable over the same width.
Why does the calculator need a run and pitch for each slope?
The two sections are different widths and different angles, so each has its own sloped length. A 4 ft run at 24/12 is 8.94 ft along the roof, while a 6 ft run at 6/12 is 6.71 ft. The calculator finds both, adds them, and multiplies by building length and two sides.
How do I measure the run for each slope?
Run is horizontal distance, not the length along the roof. The lower run goes from the outside of the wall to the point directly below the break (where the slopes meet). The upper run goes from there to the ridge centerline. Lower run + upper run should equal half the building width.
How many squares of shingles does a gambrel roof need?
Divide the roof area by 100 for squares. The 939 sq ft example is 9.39 squares, or about 28.2 bundles at 3 bundles per square before waste, starter strip, and ridge cap. Very steep lower slopes may need extra nails per shingle; follow the shingle maker's steep-slope instructions.
Does this include overhangs, ridge caps, or flashing?
No. Enter runs that include the eave overhang if you want it counted. Ridge cap, the flashing or trim at the slope break, drip edge, and gable-end rake trim are ordered separately.
Estimates only. Verify quantities with your supplier before purchasing.