Quick answer: A gambrel roof has two slopes per side β a steep lower slope (~60Β°) and a shallow upper slope (~30Β°). Enter the width and wall height for rafter lengths and ridge height.
A gambrel roof (classic "barn roof") has two slopes per side β a steep lower slope and a shallower upper slope. Enter your building width and pitches to get rafter lengths, ridge height, and loft width.
A gambrel roof β often called a "barn roof" or "Dutch colonial roof" β has two pitch angles on each side instead of one continuous slope. The lower slope is steep (typically 9/12 to 12/12 or steeper) and the upper slope is shallow (typically 2/12 to 4/12), creating extra headroom and usable loft or attic space compared to a standard gable roof.
This calculator splits the building's half-width evenly between the lower and upper sections, then uses standard rise/run trigonometry (rafter length = β(runΒ² + riseΒ²)) to find each rafter's true length and the overall ridge height.
| Style | Lower Pitch | Upper Pitch | Best For |
|---|---|---|---|
| Standard Barn | 12/12 | 3/12 | Storage barns, equipment sheds |
| Loft-Maximizing | 14/12 β 16/12 | 2/12 β 3/12 | Hay lofts, livable second floors |
| Dutch Colonial | 9/12 β 10/12 | 4/12 β 5/12 | Residential homes, garages |
The two-slope design maximizes usable hay loft or storage space under the roof compared to a triangular gable roof of the same wall height, while still shedding rain and snow effectively on the steep lower slope.
Yes, typically 10β20% more due to the extra ridge/break-point framing, additional flashing at the slope transition, and more cut lumber waste. The added usable floor space often offsets this on barns and outbuildings.
Most gambrel roofs use a steep 9/12 to 12/12 lower slope and a shallow 2/12 to 4/12 upper slope. Steeper combinations (up to 16/12 lower) are common on barns built specifically to maximize loft storage.
Yes β standing seam and corrugated metal are the most common gambrel roofing materials because of their light weight and long spans. Use our metal roof calculator on each slope section separately since the pitch multiplier differs above and below the break line.
With the break at the quarter point of the width, a 12:12 lower slope and a 3:12 upper slope β the classic barn proportion.
| Building width | Lower run | Lower rise | Lower rafter | Upper rafter | Total roof height | Loft width |
|---|---|---|---|---|---|---|
| 16 ft | 4.0 ft | 4.0 ft | 5.7 ft | 4.1 ft | 5.0 ft | 8.0 ft |
| 20 ft | 5.0 ft | 5.0 ft | 7.1 ft | 5.2 ft | 6.3 ft | 10.0 ft |
| 24 ft | 6.0 ft | 6.0 ft | 8.5 ft | 6.2 ft | 7.5 ft | 12.0 ft |
| 28 ft | 7.0 ft | 7.0 ft | 9.9 ft | 7.2 ft | 8.8 ft | 14.0 ft |
| 32 ft | 8.0 ft | 8.0 ft | 11.3 ft | 8.2 ft | 10.0 ft | 16.0 ft |
| 36 ft | 9.0 ft | 9.0 ft | 12.7 ft | 9.3 ft | 11.3 ft | 18.0 ft |
| 40 ft | 10.0 ft | 10.0 ft | 14.1 ft | 10.3 ft | 12.5 ft | 20.0 ft |
Total roof height is measured from the eave, so add your wall height for ridge height above the floor. The gambrel shape buys headroom: a 24 ft barn gains about 12 ft of usable loft width.
Every constant this calculator uses and where it comes from, so you can check it rather than take our word for it.
Local codes amend the model IRC and adopt different cycles — your building department governs. Manufacturers revise yields; where your product's data sheet disagrees with a default here, use the product's number. See our full methodology.