You do not have to climb onto a roof to measure it. When every slope has the same pitch, the roof’s area is the ground it covers — the house’s footprint with the overhangs — times a slope factor, √(1 + (rise ÷ 12)²). At 6 in 12 that factor is 1.118, so a 1,200 sq ft footprint carries about 1,342 sq ft of roof. It is the same for a gable as for a hip: the shape changes the edges, not the area, and the area works for a footprint of any shape.
What is a square in roofing?
A roofing square is 100 square feet of roof, a patch 10 ft by 10 ft. Divide the roof’s area by 100 for the squares, so the 1,342 sq ft roof above is 13.4 squares.
Shingles come in bundles, and how many make a square depends on the shingle. Owens Corning’s TruDefinition Duration is three bundles to a square and Duration Premium four — and the square their bundles cover is 98.4 sq ft rather than a round hundred, so the page counts bundles from that coverage instead of from “three to a hundred”.
The edges
The edges come from the same measurements. The ridge and the hips take the cap shingles; the starter course goes along the eaves and up the rakes, and Owens Corning’s Starter Shingle Roll is 33 2/5 feet long; drip edge goes on the same edges, each piece lapping the next by 2 inches.
What the code says about slope
Two rules in the International Residential Code are worth knowing before choosing shingles: asphalt shingles go only on roofs of 2 in 12 or steeper, and from 2 up to 4 in 12 they need underlayment twice over. Where ice dams are a risk the code wants an ice barrier from the eaves to 24 inches inside the walls — the page works out its area from the overhang and the pitch.
How much extra to buy
How much extra to buy for cutting is the one number no shingle maker publishes. The page starts at 10 per cent; a roof with hips, valleys and dormers needs more.