Roofing Calculator

1. Measure the roof footprint. Measure the length and width of your house from the ground or from architectural plans. These are the horizontal dimensions, not the actual roof surface. For a simple gable roof, the footprint is the same as the house floor plan.
2. Select the roof pitch. Pitch is the slope expressed as rise over 12 inches of run. A 4:12 pitch rises 4 inches for every 12 horizontal inches. Most residential roofs fall between 4:12 and 8:12. Check your existing roof with a speed square, or ask your roofer. The pitch multiplier converts horizontal area to actual sloped surface area.
3. Set the waste factor. Use 15% for simple gable roofs with no penetrations. Use 20% for hip roofs. Use 20 to 25% for roofs with multiple valleys, skylights, dormers, or complex geometry. Waste accounts for cuts and offcuts at edges, hips, and valleys.
4. Read the materials list. The calculator outputs roofing squares (1 square = 100 sq ft), shingle bundles (3 per square for standard three-tab or architectural shingles), underlayment rolls, and ridge cap bundles. Always buy an extra bundle of shingles to keep for future repairs.

Example: a 40 ft x 30 ft house with a 6:12 pitch has 1,200 ft² horizontal area. Multiply by 1.118 to get 1,341.6 ft² actual area. Add 15% waste for 1,543 ft², which is 15.43 squares, 47 bundles of shingles, and 4 rolls of underlayment.

The core roofing calculation converts a flat footprint area to actual sloped surface area using a pitch multiplier derived from the Pythagorean theorem. The multiplier equals the hypotenuse of a right triangle with legs of 12 and the rise number, divided by 12.

Standard three-tab and architectural (dimensional) shingles both use 3 bundles per square. Premium designer shingles may use 4 bundles per square. Ridge cap shingles cover the peak of the roof and are sold separately; one bundle covers roughly 35 linear feet. Drip edge runs along the eaves and rakes and is measured in linear feet, not squares.