Gambrel Roof Calculator
Design the classic barn-style gambrel roof with precision. Enter your building width, length, lower and upper roof pitches, and eave overhang. Get exact rafter lengths for both upper and lower slopes, roof rise at each section, total roof area, materials needed in squares, and estimated cost. Perfect for sheds, barns, garages, and Dutch Colonial homes. The gambrel design maximizes attic headroom while maintaining beautiful proportions.
Total width of the building from outside wall to outside wall
Total length of the building for roof area calculation
Lower slope is typically steeper (6-10/12) for barn aesthetics
Upper slope is typically shallower (3-6/12) than lower slope
Roof overhang beyond the exterior wall
What is a gambrel roof?
A gambrel roof (also called a barn roof) has two slopes on each side — a steeper lower slope and a shallower upper slope. This creates the classic Dutch Colonial or barn silhouette. The gambrel design maximizes interior headroom under the roof, making it ideal for barns, sheds, garages, and homes with attic living space. The lower slope typically has a pitch of 6-10/12 and the upper slope 3-6/12. The ridge is centered and the upper slope connects to the ridge while the lower slope meets the eave.
What are the standard gambrel roof pitches?
Classic gambrel proportions: Lower slope 8/12, upper slope 4/12. This gives an approximately 60/40 height split — the steeper lower section is about 60% of total rise. Common variations: 10/12 lower with 5/12 upper (steep barn look), 6/12 lower with 3/12 upper (gentle slope). The upper pitch should always be shallower than the lower pitch. A good rule: upper pitch ≈ lower pitch ÷ 2. The transition point (where pitches change) is typically at about 60% of the half-span.
How do I calculate gambrel roof rafter lengths?
Each side has two rafter types: upper rafters (from ridge to pitch break) and lower rafters (from pitch break to eave). For the lower rafter: Horizontal run = half the building width × 0.6 (approx), Rafter length = run / cos(atan(pitch/12)). Upper rafter: Horizontal run = half width × 0.4 (approx), Rafter length = run / cos(atan(pitch/12)). Total rafter length = lower + upper. Add overhang to the lower rafter. Always use the actual rafter length (not horizontal distance) when cutting lumber.
What is the advantage of a gambrel roof over a gable roof?
Gambrel roofs offer 30-50% more usable interior space than a gable roof of the same building width and overall height. This extra space is ideal for bonus rooms, loft storage, or workshop lofts. The gambrel also sheds snow well — the steeper lower slope prevents snow buildup at the eaves. However, gambrel roofs require more complex framing, use more materials, and have higher wind uplift risk in hurricane zones. They are excellent for sheds, barns, and Dutch Colonial homes.