Roof Area Calculator
Plan area × sec(slope) → actual roof area. Add eave perimeter and overhang to include eave area.
Result
Enter plan area and slope angle.
What this tool does
The roof area calculator computes actual roof surface area from the floor footprint and roof pitch. Because a roof slopes, its real area is always larger than the top-down projection — this is the basis for estimating tiles, shingles, metal panels, and waterproofing. The tool accounts for eave overhang to give the true installation area.
Who uses this
- Roofing quotes: tile, asphalt shingle, metal panel quantity estimation
- Waterproofing: sloped or flat roof area plus waste factor
- Solar panels: pre-check installable roof area
- Renovation quotes: existing roof replacement area
- Building design: roof load and insulation area baseline
How to use (4 steps)
- 1Enter the building's floor width and depth in meters — the basis for the projected area the roof covers.
- 2Enter the roof pitch. Korea uses 'x/10' notation (e.g., 4-chi = 4/10 = 21.8°); US uses 'x/12'. You can also enter degrees directly.
- 3Enter the eave overhang length, typically 30-60cm. Longer eaves mean a larger actual roof area.
- 4Click Calculate. You'll get the pitch-corrected actual roof area plus the area including material waste (typically 10%).
Roof area formula
Pitch correction factor = √(1 + (rise/run)²) Example: 4/10 pitch → √(1 + 0.16) = 1.077 Actual roof area = projected area × correction factor (Projection with eaves = (width + 2×eave) × (depth + 2×eave)) Pitch notation: Korea 'x-chi' = x/10 → angle = atan(x/10) US 'x/12' = x/12 → angle = atan(x/12) e.g. 4-chi = 21.8°, 5/12 = 22.6°, 6/12 = 26.6° Material quantity = actual area × (1 + waste factor) Waste: tile 10%, shingle 10-15%, metal 5-10%
Real examples
Example 1: 10m × 8m building, 4/10 pitch, 0.5m eaves
Projection with eaves = (10+1) × (8+1) = 99㎡. Correction = 1.077. Actual roof = 106.6㎡. With 15% shingle waste → order 122.6㎡.
Example 2: Steep 6/12 pitch
6/12 = 26.6°. Factor = √(1+0.25) = 1.118. Same 99㎡ projection becomes 110.7㎡ actual. Steeper pitch increases area, material, and cost.
Example 3: Flat roof waterproofing
Near-zero pitch → factor ≈ 1.0. Projected = actual. But parapet vertical waterproofing area is added separately. Waterproofing adds 10% waste plus overlap zones.
Frequently asked questions
Why is roof area larger than floor area?+
A sloped roof's hypotenuse is longer than its base. A 4/10 (21.8°) pitch adds ~7.7%; a 6/12 (26.6°) adds ~11.8%. It looks the same from above, but the actual installation area is always larger.
How do Korean 'chi' and US 'x/12' notations differ?+
Both express slope but with different denominators. Korea: rise per 10 horizontal (x/10). US: rise per 12 (x/12). E.g., Korea 4-chi = 21.8°, US 5/12 = 22.6°. The calculator supports both.
Why include eaves in the area?+
Eaves (roof overhang beyond the wall) must also be covered with material, so they count as installation area. A 0.5m eave adds 1m to each dimension, growing a 10×8 building to an 11×9 projection.
Why is a waste factor needed?+
Cutting loss, overlap (shingles overlap top-to-bottom), trimming waste, and breakage allowance. Tile 10%, asphalt shingle 10-15%, metal 5-10%; complex roofs (polygonal, dormers) need more.
How do I calculate hip or complex roofs?+
This calculator handles simple sloped roofs (gable, shed). For hip or complex roofs, calculate each plane separately and sum. Valleys and ridges need extra waste allowance.
Can I use this for solar panel area?+
Get the actual roof area here, but installable panel area must subtract shading, vents, and setback distances. Typically 60-70% of actual roof area is usable for panels.
Cautions
- •Handles simple sloped roofs (gable, shed). Hip/complex roofs need plane-by-plane summation.
- •Ridges, hips, and valleys need extra material and waste.
- •Waterproofing adds parapet vertical area separately.
- •Waste factor adjusts by roof shape/complexity. A flat 10% is the floor.
- •Structural and snow load review requires separate structural calculation.
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Last reviewed: 2026-05-30