Roof calculator: roof area, squares and shingles

How much roof do you have, and how much material?

A sloped roof is bigger than the footprint of the house. This roof area calculator takes the footprint, the overhang and the pitch, and returns the area of each roof plane, the total area, the rafter length, the ridge height, and the ridge and hip lengths. It then counts squares, shingle bundles or tiles, ridge pieces, underlayment rolls and battens, with waste included. A diagram shows the roof in plan view and in cross-section.

How to use it

1. Pick a shape: Shed (one slope), Gable (two slopes), Hip (four slopes) or Flat roof.
2. Enter the Length and Width of the house, measured outside the walls, then the Overhang.
3. Enter the Pitch as rise per 12 (type 6 or 6/12), in degrees or in percent. The conversion appears under the field.
4. Choose the Roof covering. With Asphalt shingles, the tool counts bundles at 3 per square; with tiles, enter how many you need per square or per m². Set the Waste (10% by default).
5. Check the underlayment roll size and lap, then read the results and the step-by-step working.

Worked example: 40 × 30 ft house, 6/12 gable roof

With a 1 ft overhang all round, the roof covers 42 × 32 = 1,344 ft² in plan. A 6/12 pitch is 26.57° or 50%, and its pitch factor is √(12² + 6²) ÷ 12 = 1.1180.

• Rafter length: 16 ft of run × 1.1180 = 17.89 ft.
• Total roof area: 1,344 × 1.1180 = 1,502.64 ft², which is 15.03 squares.
• Ridge height above the walls: 15 ft × 0.5 = 7.5 ft. Ridge length: 42 ft.
• Shingles: 15.03 squares × 3 = 45.08 bundles, plus 10% waste, gives 50 bundles.
• Underlayment in 4 × 250 ft rolls with a 4 in lap: each roll covers (4 − 0.333) × 250 = 916.67 ft², so 2 rolls.

The same tool works in metric. A 10 × 8 m house with a 0.5 m overhang and a 35° gable roof has 120.86 m² of roof (1,300.9 ft², or 13.01 squares). At 13 tiles per m² plus 10% waste, that is 1,729 tiles.

Hip roofs: trapezoids, triangles or one formula

A hip roof has two trapezoid planes on the long sides and two triangles at the ends. The calculator works out each one, and the length of every hip. But when all four planes share the same pitch, there is a shortcut: every square foot of plan becomes pitch factor × 1 ft² of roof, whatever the shape. So roof area = footprint × pitch factor, and the 40 × 30 ft house above has 1,502.64 ft² of roof as a hip roof too. Give the ends a steeper pitch in Hip end pitch and the shortcut no longer applies, so the tool adds the planes one by one.

Before you order

Three-tab and architectural shingles are usually sold at 3 bundles per square, but check the wrapper. Tile counts per square depend on the product and the pitch, and the manufacturer's data sheet has the final say. Starter strip, drip edge, flashing and hip and ridge caps are not counted: the tool gives the ridge and hip length so you can size the caps from the bundle label. Add more waste for valleys, dormers and cut-up roofs.

FAQ

How many squares is my roof?

Divide the roof area in ft² by 100. A 1,502.64 ft² roof is 15.03 squares before waste.

How do I convert a 6/12 pitch to degrees?

Divide the rise by 12 and take the arctangent: arctan(0.5) = 26.57°. In percent, 6/12 is 50%.

Should I include the overhang?

Yes. Shingles cover the eaves and rakes too. A 1 ft overhang turns a 1,200 ft² footprint into 1,344 ft².

How much waste should I add?

About 10% for a simple gable roof, and 15% or more for hip roofs and roofs with valleys or dormers.