Roof area calculator
This roof area calculator works out the area of a gable, shed or hip roof from the building size, the pitch and the overhang. A house of 40 × 30 ft with a 6-in-12 pitch (26.6°) and a 1.5 ft overhang has about 1,587 sq ft of roof, or 15.9 roofing squares.
Example: Roof type: Gable roof · Building length (along the ridge) 40 · Building width (gable end) 30 · Roof pitch 26.57 ° · Overhang all round 1.5 · Unit: Feet → Roof area: Roof area: 1,586.6 sq ft. Source: Wikipedia – Roof pitch (angle, percent and rise-over-12 notation). Updated: .
How it is calculated
How to calculate roof area
A roof is larger than the footprint because it slopes. If all roof planes share the same pitch α, then for gable, shed and hip roofs: roof area = footprint ÷ cos α. The footprint is length × width including the overhang.
Example
House 40 × 30 ft, overhang 1.5 ft, pitch 26.6° (6 in 12): footprint with overhang 43 × 33 = 1,419 sq ft; roof area = 1,419 ÷ cos 26.6° = 1,587 sq ft. The rafter length is half the width divided by cos α, and the ridge height half the width times tan α.
Roof types
- Gable roof: two planes, ridge over the full length.
- Shed roof: one plane across the whole width, so the rafter is twice as long as on a gable roof.
- Hip roof: four planes with equal pitch; the ridge is shorter by the difference between length and width, and a point on a square plan.
Pitch as angle, percent and rise in 12
Pitch in percent is tan α × 100, and rise per 12 in of run is tan α × 12: 26.57° is 50%, or 6 in 12; 45° is 100%, or 12 in 12. Roofing manufacturers give a minimum pitch for each covering – check it before you plan.
Honest limits
Dormers, skylights, chimneys and gutters are neither subtracted nor added. Waste, starter courses, ridge caps and valleys come on top of the net area when you order material. For roofs with different pitches, calculate each plane separately.
Frequently asked questions
How do I calculate the roof area of a gable roof?
Divide the footprint (length × width including overhang) by the cosine of the pitch angle. At 35° the factor is 1.22.
How do I convert pitch like 6/12 to degrees?
Degrees = arctan(6 ÷ 12) = 26.6°. In percent that is 50%.
How many roofing squares do I need?
One square covers 100 sq ft of roof. Divide the roof area by 100 and add waste, typically 10–15%.
How do I find the roof area from a satellite map?
Measure the footprint from the map and divide by the cosine of the pitch; the pitch itself must come from a site measurement or plans.
Does the overhang count as roof area?
Yes, it is covered too. Enter it here; use 0 only for the area without overhang.
Sources and legal basis
As of: · Editorial policy
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