What is a Gambrel Roof?
A gambrel roof, sometimes called a barn roof, features two different slopes on each side of the ridge. The lower slope is steep (typically 60 to 70 degrees from horizontal), while the upper slope is shallow (typically 20 to 30 degrees). This dual-pitch design creates significantly more usable space in the attic or upper floor compared to a standard gable roof.
Gambrel roofs are most commonly associated with barns, Dutch Colonial houses, and storage sheds. The design has been used for centuries because it provides the most interior volume with the least amount of structural material. The steep lower section also sheds rain and snow effectively.
Gambrel Roof Formulas
Each side of a gambrel roof has two rafter sections. The lower rafter runs from the eave to the break point, and the upper rafter runs from the break point to the ridge.
Advantages and Disadvantages
- Maximum interior space: The steep lower walls provide nearly vertical interior walls, maximizing usable floor area on the upper story.
- Cost-effective: Uses less material than a full second story while providing comparable interior volume.
- Classic aesthetic: Provides a distinctive, traditional appearance popular in Colonial and farmhouse architecture.
- Wind vulnerability: The flat upper section can be vulnerable to wind uplift in hurricane-prone areas.
- Complex framing: Requires more precise cutting and fitting than a simple gable roof.
Typical Gambrel Pitches
| Style | Upper Pitch | Lower Pitch | Best For |
|---|---|---|---|
| Traditional Barn | 4:12 to 6:12 | 18:12 to 24:12 | Barns, storage buildings |
| Dutch Colonial | 6:12 to 8:12 | 12:12 to 18:12 | Residential homes |
| Shed/Garage | 4:12 to 6:12 | 16:12 to 20:12 | Garages, workshops |
Worked Example
A 24-foot-wide, 40-foot-long barn with upper pitch 6:12 and lower pitch 18:12, break point at 50%:
Frequently Asked Questions
What is the ideal angle for a gambrel roof?
The most common configuration uses a 60-degree lower slope and a 30-degree upper slope. This creates an equilateral triangle cross-section and provides a good balance between interior space and structural stability.
Are gambrel roofs good for snow?
The steep lower section sheds snow well, but the shallow upper section can accumulate snow. In heavy snow areas, the upper pitch should be increased to at least 6:12 or steeper, and the structure must be engineered for the expected snow load.
How many roofing squares do I need?
One roofing square equals 100 square feet. Divide the total roof area by 100, then add 10-15% for waste, ridges, and overlaps. A 1,400 sq ft gambrel roof needs approximately 14 squares plus waste, so order 16 squares.
Can I convert a gable roof to a gambrel?
It is possible but requires significant structural modification, including new rafters, knee walls, and potentially reinforced walls and foundation to handle the different load distribution. Consult a structural engineer before attempting this conversion.