Roofing Calculator

Add a 1 ft overhang on each side: 42 x 32 = 1,344 sq ft of footprint. Multiply by the 6/12 factor of 1.118 for 1,503 sq ft of roof. Add 10% waste and round up to the next third of a square: 16.7 squares, which is 50 bundles of 3-bundle shingles.

Roof area = (length + 2 x overhang) x (width + 2 x overhang) x sqrt(1 + (pitch / 12)^2). Squares = area / 100, then add waste.

1. Measure the footprint. Measure the house length along the ridge and the width eave to eave, from the ground or a plan, then add the overhang on every side (usually 12 in).
2. Find the pitch. Hold a level 12 in out from the roof and measure straight up to the roof surface. That rise in inches is the pitch, written x/12.
3. Apply the pitch multiplier. Multiply the footprint by the square root of 1 + (pitch / 12) squared: 1.118 for 6/12, 1.414 for 12/12.
4. Add waste. Add 10% for a simple gable, 15% for a hip or cut-up roof and 20% for many valleys and dormers.
5. Convert to squares and bundles. Divide by 100 for squares, then multiply by 3 bundles per square for most shingles (4 for heavy architectural).

Starter strip covers the eaves and rakes, ridge cap covers the ridge and hips (about 30 linear ft per bundle), drip edge comes in 10 ft pieces, ice and water shield runs 3 ft up every eave and along valleys at 200 sq ft per roll, and nails run about 320 per square at 4 per shingle (2.5 lb).

Up to 6/12 is walkable with care; above 8/12 you need roof jacks and planks, and you should be in a harness at any pitch. Most codes allow two layers of asphalt at most, so a second overlay means a full tear-off. In cold climates the code requires ice barrier from the eave to 24 in inside the warm wall. Check the asphalt shingle roofing cheat sheet for nailing and valleys, the attic ventilation guide for intake and ridge vent, and the permit and inspection cheat sheet before you start. New gutters go with a new roof: size them with the gutter calculator.