Gable Roof
Two equal rectangular slopes meeting at a ridge. The most common residential roof shape.
Gable Area = 2 × Length × Slope Length
Slope Length = (Width ÷ 2) ÷ cos(pitch angle)
Example: 40'×30' with 6/12 pitch → Slope = 15 ÷ cos(26.57°) = 16.77'
Area = 2 × 40 × 16.77 = 1,342 sq ft
Hip Roof
Four sloping sides — two trapezoids (front/back) and two triangles (ends). All sides slope from ridge to eaves.
Hip Area = 2 × Trapezoidal Sides + 2 × Triangular Ends
Trapezoid area = ((Length + Ridge) ÷ 2) × Slope Length
Triangle area = (Width ÷ 2) × Slope Length
Ridge Length = Building Length − Building Width
Gambrel Roof
Two slopes on each side — steep lower slope (~60°) and shallow upper slope. Maximizes attic space.
Gambrel Area = 2 × Length × (Lower Slope Length + Upper Slope Length)
Lower slope covers ~40% of half-width at ~60°
Upper slope covers remaining ~60% at configurable pitch
Mansard Roof
Four-sided gambrel with steep walls on all sides (~70°) and a flat or low-slope top.
Mansard Area = Steep Walls (all 4 sides) + Flat Top Area
Steep wall height = min(L,W) × 0.35
Flat top = (L − 2×inset) × (W − 2×inset)
Flat Roof
Level or nearly-level surface. Area equals the footprint. Requires membrane materials.
Flat Roof Area = Length × Width
Add perimeter for parapet/flashing calculations
Minimum slope: ¼" per foot for drainage
Shed Roof (Skillion)
Single sloping surface from high wall to low wall.
Shed Area = Length × Slope Length
Slope Length = Width ÷ cos(pitch angle)
High Side Height = Width × tan(pitch angle)
Saltbox Roof
Asymmetrical gable — short steep front slope and long shallow back slope. Ridge offset toward front.
Saltbox Area = Length × (Front Slope + Back Slope)
Front run = Width × 0.35, Back run = Width × 0.65
Front pitch is steeper; back pitch is calculated from same ridge height
Key Principles
- Always add overhangs to each dimension before calculating
- Apply a waste factor (5-20%) based on roof complexity
- For complex roofs, break into simpler shapes, calculate each, then sum
- Convert to roofing squares by dividing by 100
Use Our Shape Calculators
Select your roof shape and enter dimensions for instant area calculations.