Gambrel Roof Calculator
Rafter lengths, angles and area for a barn-style roof.
Enter your values
Enter the building span in ft.
Enter the lower slope angle in °.
Enter the upper slope angle in °.
Enter the break point from wall in % of half span.
Enter the building length in ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Lower rafter
- 8.4 ft
- Upper rafter
- 9.007 ft
- Total ridge height
- 11.778 ft
- Break point height
- 7.275 ft
- Roof area
- 1,253 sq ft
- Shingle squares
- 12.5
Quick answer
A gambrel roof has two slopes each side — typically a steep 60° lower rafter and a shallow 30° upper rafter meeting at the ridge.
rafter = √(rise² + run²) per slope
At a glance
| What it does | Rafter lengths, angles and area for a barn-style roof. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Building span, Lower slope angle, Upper slope angle, Break point from wall, Building length |
| Main output | Lower rafter |
| Formula | rafter = √(rise² + run²) per slope |
| Cost | Free — no sign-up, no download |
How to use the Gambrel Roof Calculator
- 1Enter the building span wall to wall.
- 2Set the lower and upper slope angles — 60° and 30° is traditional.
- 3Move the break point to change the loft headroom.
Inputs explained
- Building span(ft)
- Enter the building span in ft.
- Lower slope angle(°)
- Enter the lower slope angle in °.
- Upper slope angle(°)
- Enter the upper slope angle in °.
- Break point from wall(% of half span)
- Enter the break point from wall in % of half span.
- Building length(ft)
- Enter the building length in ft.
Worked example
Using the values the calculator loads with:
Inputs
- Building span24 ft
- Lower slope angle60 °
- Upper slope angle30 °
- Break point from wall35 % of half span
- Building length36 ft
Results
- Lower rafter8.4 ft
- Upper rafter9.007 ft
- Total ridge height11.778 ft
- Break point height7.275 ft
- Roof area1,253 sq ft
- Shingle squares12.5
Frequently asked questions
What angles are used on a gambrel roof?
Most barns use 60° for the lower slope and 30° for the upper, which makes an even semicircle of the truss.
Why choose a gambrel roof?
It maximises usable loft space under the roof compared with a plain gable of the same span.
What do I need to enter into the Gambrel Roof Calculator?
Just 5 values: building span, lower slope angle, upper slope angle, break point from wall, building length. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Gambrel Roof Calculator work out the answer?
A gambrel roof has two slopes each side — typically a steep 60° lower rafter and a shallow 30° upper rafter meeting at the ridge. It applies the formula rafter = √(rise² + run²) per slope and shows the working so you can check each step by hand.
Is the Gambrel Roof Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.