Roofing squares and shingle calculator

A square is 100 square feet of roof surface. A roof that measures 2,400 square feet of surface is 24 squares, and shingles, underlayment and labor are all usually priced by the square.

Multiply the flat footprint, including the overhangs, by the pitch factor. The factor is the square root of 1 plus the rise over 12, squared, so a 6/12 roof is about 1.118. A 2,000 square foot footprint at 6/12 is about 2,236 square feet of roof, or 22.4 squares before waste.

Most standard 3-tab and architectural shingles come 3 bundles to the square. Some heavier designer and laminated lines are 4 or 5 bundles to the square, so check the wrapper before you order.

A common rule of thumb is about 10 percent for a simple gable roof and about 15 percent for a hip roof or one cut up with valleys, dormers and penetrations. Starter strip and ridge cap are ordered separately.

Hold a level 12 inches long against the roof or rafter, level it, and measure straight up from the end of the level to the roof. That number is the rise, so 6 inches up over 12 inches across is a 6/12 pitch.

Add up the eaves and the rakes, the edges that run along the gutters and up the gable ends. Drip edge usually comes in 10 foot pieces, so divide by 10 and round up, and allow a little extra for overlap at the joints.