Enter Gambrel Roof Dimensions
Example Data Table
| Span | Lower Run | Lower Rise | Upper Rise | Lower Angle | Upper Angle | Total Height |
|---|---|---|---|---|---|---|
| 24 ft | 5 ft | 7 ft | 5 ft | 54.46° | 35.54° | 12 ft |
| 30 ft | 6 ft | 8 ft | 6 ft | 53.13° | 33.69° | 14 ft |
| 36 ft | 7 ft | 9 ft | 7 ft | 52.13° | 32.47° | 16 ft |
Formula Used
A gambrel roof has two slopes on each side. This calculator treats one side as two right triangles. The lower triangle uses lower run and lower rise. The upper triangle uses upper run and upper rise.
Half Span = Total Span / 2Upper Run = Half Span - Lower RunLower Angle = atan(Lower Rise / Lower Run)Upper Angle = atan(Upper Rise / Upper Run)Pitch = Rise / Run × 12Rafter Length = √(Run² + Rise²)Overhang Length = Horizontal Overhang / cos(Lower Angle)Roof Area = 2 × Side Rafter Length × Roof LengthArea With Waste = Roof Area × (1 + Waste Percent / 100)
Angles are shown in degrees. Pitch is shown as inches of rise per 12 inches of horizontal run.
How To Use This Calculator
- Choose feet or meters as the working unit.
- Enter the total span of the building.
- Enter the lower run from the wall line to the gambrel bend.
- Enter lower rise and upper rise values.
- Add overhang, roof length, waste allowance, and rafter spacing.
- Press the calculate button to view angles and lengths.
- Use CSV or PDF export for estimates and records.
Always confirm final cuts with local code, lumber size, ridge details, and structural design requirements.
Gambrel Roof Planning Guide
What A Gambrel Roof Needs
A gambrel roof uses two different slopes on each side. The lower slope is usually steeper. The upper slope is usually flatter. This shape gives more room under the roof. It is common on barns, sheds, garages, and storage lofts. Good layout starts with span, rise, run, and roof length.
Why Angles Matter
Roof angle controls the rafter cut. It also affects headroom and roof appearance. A steep lower angle creates tall side space. A flatter upper angle creates a wider roof crown. Both angles must work together. Small changes can shift the break point. They can also change the required rafter length.
Run And Rise Balance
The lower run should not exceed half the span. The upper run uses the remaining distance. A short lower run makes the lower roof steep. A long lower run makes it gentler. Rise values set the height of each roof section. The total roof height equals both rises added together.
Rafter Length And Area
Rafter length is found with the right triangle method. Each slope has its own run and rise. The calculator also adds lower overhang length. Roof area uses both sides of the roof. Waste allowance helps with cuts, laps, and mistakes. This gives a stronger material estimate.
Practical Framing Notes
Use the results as a layout guide. Mark each cut carefully. Check the bend angle before cutting several rafters. Build one test pair first. Match framing to local loads and codes. Snow, wind, lumber grade, and fasteners matter. For major buildings, ask a qualified professional.
FAQs
1. What is a gambrel roof angle?
It is the slope angle of each roof section. A gambrel roof usually has a steep lower angle and a flatter upper angle.
2. What does lower run mean?
Lower run is the horizontal distance from the wall line to the bend point on one side of the roof.
3. How is upper run calculated?
Upper run equals half the total span minus the lower run. It reaches from the bend point to the ridge line.
4. Can I use meters instead of feet?
Yes. Select meters from the unit field. Use the same unit for span, rise, run, overhang, and roof length.
5. What is pitch in this calculator?
Pitch is rise per 12 inches of horizontal run. It helps compare roof steepness with common framing measurements.
6. Does the calculator include overhang?
Yes. It converts horizontal overhang into sloped rafter length using the lower roof angle.
7. Is the bend bevel final for cutting?
It is an estimate based on equal splitting of the included bend angle. Verify cuts with your saw setup and framing method.
8. Should I use this for structural approval?
No. It is a planning tool. Final framing should follow local building codes and professional structural guidance.