Shingles are sold by the square — 100 square feet of finished roof — and packaged three bundles to a square for standard weights. Enter your actual sloped roof area, not the footprint. If you have not worked that out yet, do the pitch calculation first.
Squares, bundles and weight
One square is 100 square feet. Architectural and three-tab shingles run three bundles to the square at about 33.3 square feet each. Heavier designer shingles go four bundles to the square.
A bundle weighs 65 to 85 pounds. Most shingles are three bundles to the square, so a square is 195 to 255 pounds; the heavy laminated ones run four bundles and reach about 340. A 20-square roof is two to three and a half tons of material either way.
Waste is about roof shape
A simple gable roof with two planes wastes very little — 10 to 12 percent covers cuts and the starter course.
Every valley and every hip means cutting shingles at an angle and throwing away the offcut. A roof with several dormers and valleys can waste 20 percent.
The extras nobody counts
Starter strip runs along the eaves and rakes and is a separate product. Hip and ridge cap is another. Neither is included in the field shingle count.
Add roughly one bundle of cap for every 20 to 25 linear feet of ridge and hip, and starter for the full eave plus rake length.
Check what is underneath before ordering
Most jurisdictions allow two layers of shingles maximum. If there are already two up there, the job includes a tear-off and a dumpster, which is often the largest single line on the bill.
Tear-off also exposes the deck. Budget for replacing some sheathing — there is almost always soft plywood somewhere, usually at the eaves.
Nailing is what the warranty rests on
Almost every shingle warranty claim that gets denied comes down to nailing — too few nails, nails too high, or nails driven through the shingle face.
Standard is four nails per shingle, six in high wind zones, driven in the nail strip which is a narrow band the manufacturer marks. Above the strip and the nail holds nothing; below it and the head is exposed.
Overdriven nails cut through the mat. Underdriven nails hold the course above off the deck. Both are visible on inspection and both are common on a fast job.
Tear-off, and what is underneath
A second layer of shingles over an existing roof is permitted in some places and it is almost always a false economy. It hides the deck, so any rot stays, and it adds weight the framing was not designed for.
Tearing off also means disposal, which is a real cost — a roof is heavy and most landfills charge by weight.
Plan for some deck replacement. On any roof old enough to need replacing, there is usually some soft sheathing around the penetrations and along the eaves, and finding it after the delivery has arrived is better than discovering it at the last minute.