Roofing calculator

How we work it out

A roofing square is 100 sq ft of roof surface. The surface is the footprint, measured from above with the eaves, times the pitch factor.

surface = footprint × √(1 + (pitch ÷ 12)²)

bundles = squares × (1 + waste) × bundles per square

Every coverage is the maker's own figure: 3 bundles a square for Owens Corning's Duration and Supreme shingles, 105 ft a bundle of their Starter Strip Plus, 20 ft a carton of DuraRidge hip and ridge cap, 929 sq ft a roll of ProArmor synthetic underlayment after its laps, and 408 sq ft a roll of TAMKO's #15 felt (#30 covers 2 squares). Owens Corning says to add 10% to 15% to every material for cuts; we use 10% for a plain gable and 15% for hips, valleys and dormers. Nails are 4 a shingle, or 6 where the code or a very steep roof asks for it.

For clay and concrete tile, the tiles per square and weight are the makers': 88.5 a square for Eagle's Capistrano and Bel Air concrete tile (903 and 970 lb), 114 for Ludowici's Spanish clay S-tile (800 lb, 4/12 and steeper) and 163 pieces for its two-piece Mission barrel (1,165 lb, 5/12 and steeper). The rest follows the tile industry's installation manual: tile below 3/12 is only decorative, under 4/12 it gets two layers of underlayment and from 4/12 up one, the minimum is #30 felt or mineral-surfaced roll roofing, battens are required above 7/12, and nails follow its Table 1A. For lift and relay, where the old tiles go back on new underlayment, nobody publishes how many break, so you type in the share.

Ice and water barrier runs from the eave to 24 in inside the wall, as Owens Corning's instructions put it. We add the overhang and a 6 in wall to that, measured flat, then along the slope.

The address search reads the roof from USGS lidar, as on our roof pitch page: its area came within 3% and 6% of the building outlines San Diego's regional planning agency traced from 2017 aerial photos. It doesn't measure edge lengths yet, so put those in from the roof.