Roofing Shingle Calculator

What this calculation does not cover

• The slope multiplier is applied to exactly the rectangle you enter, and nothing is added for the eaves and rakes, so the two dimensions have to be the roof's plan to the drip edge rather than the building's footprint at the walls. Taken at the wall faces they leave the overhangs out: on a 10 by 10 m building with 500 mm of overhang all round the roof plan is really 11 by 11 m, and the 6/12 job that reads 41 bundles at the wall dimensions needs 49.
• One length times one width times one multiplier is the entire geometry model, so anything that is not a single rectangle under a single pitch has to be split up by hand: an L-plan, a cross gable, a lower porch or garage roof at a shallower pitch, and a dormer cheek are all outside what two footprint figures can describe. Run each rectangle at its own pitch and add the squares. No perimeter is ever derived either, so the code has no idea how long your ridge, hips, valleys, rakes or eaves are.
• Pitch is a six-item list — flat, 4, 6, 8, 10 and 12 in 12 — not a number you can type, so a 7/12 or 9/12 roof has to be forced onto the nearest rung; choosing 6/12 rather than 8/12 on the default job moves the order from 41 bundles to 44, with the true 7/12 answer sitting between the two. The flat entry sets the multiplier to exactly 1 and still returns a bundle count, because this is area arithmetic and makes no judgement about whether shingles belong on a slope that shallow.
• Three bundles to the square is a fixed constant in the code and nothing on the form asks which shingle you are buying. If the coverage printed on the wrapper is not one third of a square, the bundle line is wrong in direct proportion while the squares figure above it stays right, so divide the squares yourself by the coverage stated on the pallet. Rounding up happens only at the bundle, so the answer never rounds to a whole square, a full pallet or a supplier's minimum drop.
• The waste percentage is a flat multiplier on the area, which turns the allowance into more of the same field shingle and nothing else — at the 12 per cent default that is about 1.4 extra squares, roughly four more bundles of the identical product. That works if you are cutting starters and caps from field shingles; dedicated starter strip and hip-and-ridge cartons are sold by the linear metre and would sit outside this figure entirely. Because no edge length is known, the percentage also cannot respond to how much cut edge you have, so a plain gable and a hip roof with four valleys are charged the same 12 per cent unless you raise it.
• Nothing is deducted for what sits inside the roof plane: chimneys, skylights, roof windows and open metal valleys are all shingled straight over, since there is no field in which to enter them. Order from the squares figure at the top rather than from the surface-area rows in the breakdown, because those rows are the bare roof before waste — the 1,203 sq ft (112 m²) line on the default job is about 145 sq ft (13 m²) short of the 13.5 squares the headline is telling you to buy.