A 40 × 28 ft house with 12 in overhangs and a 6/12 gable roof has 1,409 square feet of roof — 14.1 roofing squares, 15.5 once you add 10 % for waste. You don't need to climb up and measure the slopes: the plan size, the overhangs and the pitch are enough. The calculator above works out the roof area of gable, hip, shed and gambrel roofs and the materials that go on it.
How the calculator works
Choose the roof shape, enter the building length and width, the pitch and the overhangs. The result is the roof area in square feet and squares, the slope length from eave to ridge, the rise and angle, and the material list for the covering you pick: shingle bundles and nails or rolls, underlayment, ice barrier, starter, ridge and hip cap, drip edge.
The slope factor
Rain falls on the plan, but shingles cover the slope. The slope is longer than its run by √(1 + (pitch/12)²), so the roof area is the plan area times that factor. For a roof whose slopes share one pitch it does not matter how they are arranged: a hip roof on the same plan has exactly the same area as a gable.
| Pitch | Angle | Slope factor | Roof on a 1,260 ft² plan |
|---|---|---|---|
| 4/12 | 18.4° | 1.054 | 1,328 ft² |
| 6/12 | 26.6° | 1.118 | 1,409 ft² |
| 8/12 | 33.7° | 1.202 | 1,514 ft² |
| 12/12 | 45° | 1.414 | 1,782 ft² |
Overhangs
The plan includes the overhangs: eaves on the low edges, rakes on the gable ends. A 12 in overhang on every side turns a 40 × 28 ft house into a 42 × 30 ft roof plan — 140 ft² more before the slope factor, 12 % of the house.
Gambrel and shed roofs
A gambrel has a steep lower slope and a flatter upper one on each side, so each gets its own factor: on the same house with 5 ft lower runs at 24/12 and 6/12 above, the roof grows to 1,972 ft². A shed roof spans the whole width with one slope; its area is the plan times one factor, and its high edge needs drip edge or flashing instead of a ridge cap.
Squares and waste
Roofers price and order in squares of 100 ft². Add waste for starter cuts, rakes, hips and valleys: about 10 % on a simple gable, 15 % on a hip roof, more on a cut-up roof with dormers. Shingles come three bundles to the square, so 15.5 squares is 47 bundles.
Common mistakes
- Using the floor area. The roof covers the overhangs and the slope — 12 % to 40 % more than the house.
- Adding hip ends as extra triangles. Hip ends replace part of the main slopes; the total is still plan × slope factor.
- Ordering without waste. The last bundle you need is the one you didn't buy.
For the shingle order in detail, use the Shingle Calculator; for the pitch itself, the Roof Pitch Calculator; for the framing, the Rafter Length Calculator or the Hip Roof Calculator.