Measurement tool
Gambrel Roof Calculator
A gambrel roof has two pitches per side: a shallow upper slope and a steep lower one. Enter both to get true surface area and material quantities.
Gambrel roof area is the sum of the upper and lower slopes on both sides. Each slope is its horizontal run multiplied by its own pitch multiplier, so a gambrel must be calculated in two parts and added.
Calculate a gambrel roof
Enter the horizontal run and pitch for each of the two slopes.
Roof area
1,891 sq ft
both slopes, both sides
Why gambrel roofs are calculated in two parts
A gambrel is really two roofs stacked. The lower slope is steep, often 18/12 or 24/12, and the upper slope is shallow, typically 3/12 to 6/12. Because the pitch multiplier differs between them, you cannot apply one multiplier to the whole footprint. Each slope gets its own calculation, and the results are added.
The payoff is headroom. That steep lower slope pushes the walls nearly vertical for the first several feet, which is what makes barn lofts and gambrel attics usable as living space where a standard gable would give you a crawl space.
Material implications
The steep lower slope is above the walkable threshold, so plan on staging and fall protection. Labor per square is meaningfully higher than on a standard roof. Use a 15% waste factor to cover cuts at the break line where the two slopes meet.
The break itself needs proper flashing detail. It is a horizontal transition on a roof, which is exactly the geometry that traps water, and it is the first place a poorly built gambrel leaks.