Roofing Calculator

How Much Roofing Do I Need?

1. Plan area: your house’s footprint plus the overhang on every side.
2. Roof area: plan area × the slope factor for your pitch.
3. Squares: roof area ÷ 100, plus 10–15% for waste.
4. Bundles: squares × 3 for most asphalt shingles.

Worked example. A 50 × 40 ft house with 1 ft overhangs is 52 × 42 = 2,184 sq ft from above. At a 6/12 pitch (slope factor 1.118) that’s 2,442 sq ft of roof, or 24.4 squares. With 10% waste for a simple gable, order 26.9 squares: 81 bundles. With the ridge along the 50 ft length, the two eaves total 104 ft and the four sloped rakes total about 94 ft: 198 ft of edge, or 20 ten-foot pieces of drip edge.

Slope Factors

Steeper roofs have more surface over the same footprint:

If you’re not sure of your pitch, the roof pitch calculator works it out from rise and run, or from an angle.

Shingles, Squares and Bundles

A square is 100 sq ft of roof. GAF says most asphalt shingles come 3 bundles to a square, about 33.3 sq ft per bundle, but some heavier designer shingles need 4 or 5. GAF also says contractors typically add 10–15% waste, more for steep roofs and roofs with rakes, valleys, and dormers.

Low-Slope Roofs

The Asphalt Roofing Manufacturers Association says never to install asphalt shingles below 2/12. From 2/12 up to 4/12, shingles need either two layers of underlayment or a self-adhering membrane. Flatter roofs need membrane roofing instead.

What This Estimate Leaves Out

Drip edge for rectangular gables includes the sloped rakes and assumes the ridge runs along the entered length. Other roof types and triangular sections use the footprint perimeter with overhang as a rough estimate. Complex roof edges need a measured report; the piece count does not include overlap or cutting waste.

Starter strip, ridge cap, hip shingles, flashing, vents, and ice-and-water shield depend on your roof’s details. Roofers order from a measured report, from aerial imagery or a site visit. Use this to check their numbers or budget. The roof cost calculator turns the same inputs into a replacement estimate.