Shingle Calculator

Squares — the unit the whole trade uses

One square is 100 square feet of roof. Not one square foot, not a square of any other size. Every shingle carton, every underlayment roll, every roofing quote you will ever receive is denominated in squares, and a price of "$450 a square installed" is completely normal while "$450 a square foot" would be absurd.

Architectural (laminated) shingles come three bundles to the square. Some heavier designer profiles run four or five bundles, and old-style three-tab is also three. Always read the coverage printed on the wrapper before you order, because it is the only number that is definitely right for the product in front of you.

The pitch multiplier — the mistake that costs a delivery

Here is where most people go wrong. You measure your house on the ground, or off a plan, or from a satellite image. That gives you the footprint — the flat outline. But a roof is sloped, and the sloped surface is always longer than the flat distance underneath it. Ordering shingles for the footprint leaves you short by ten to thirty percent.

The conversion is the same triangle a framer uses:

Pitch factor = √(rise² + 12²) ÷ 12

Roof area = footprint area × pitch factor

For common pitches: 4/12 is 1.054, 6/12 is 1.118, 8/12 is 1.202, 10/12 is 1.302, 12/12 is 1.414. A 12/12 roof — a 45-degree roof — has forty-one percent more surface than its footprint. That is not a rounding error, it is four extra squares on a modest house.

A worked example

A 1,800 sq ft footprint at 6/12, moderate complexity, 60 ft of ridge and hip, 24 ft of valley, 160 ft of eave and rake, one existing layer to tear off.

• Pitch factor: √(36 + 144) ÷ 12 = √180 ÷ 12 = 1.118
• Roof area: 1,800 × 1.118 = 2,012 sq ft
• Squares: 2,012 ÷ 100 = 20.1
• Add 13% waste for the hips and valleys: 22.7 squares
• Bundles: 22.7 × 3 = 69 bundles
• Ridge cap: 60 ft ÷ 20 ft per bundle = 3 bundles
• Starter: 160 ft, about 2 bundles
• Underlayment: 2,012 sq ft with lap ÷ 1,000 = 3 rolls
• Drip edge: 160 ÷ 10 = 16 lengths
• Nails: about 57 lb, so two 50 lb boxes

Waste, and why complexity drives it

Ten percent is the floor, and it only applies to a plain gable roof with two rectangular planes. Every hip and every valley forces angled cuts, and the triangle you cut off is scrap. A cut-up roof with dormers, multiple ridges and several valleys justifies eighteen percent and sometimes twenty. If you are hand-cutting your own valley shingles rather than using a closed-cut method, budget more again.

Waste also covers the starter course if you are cutting it from field shingles rather than buying dedicated starter, damaged bundles, and the two or three bundles you should leave in the garage for future repairs.

The pieces that are not field shingles

A roof is more than the bundles, and the accessories are where an order comes up short:

• Starter strip at every eave and rake. The self-sealing strip along the leading edge stops wind lifting the first course, which is where blow-offs begin.
• Ridge cap, sold separately and covering about 20 to 25 linear feet a bundle. Cutting three-tab into caps is legal on some products and voids the warranty on others.
• Underlayment, synthetic on a 10-square roll. Add ice-and-water shield in the valleys, at the eaves in cold climates, and around every penetration.
• Drip edge in 10 ft lengths, at eaves under the underlayment and at rakes over it.
• Nails, about 2.5 lb a square at four nails per shingle, and six per shingle in high-wind zones.

Before you order

Measure or verify the roof rather than trusting a plan, count the penetrations, and photograph every flashing detail before the tear-off starts. Order the accessories with the shingles from the same supplier so the dye lot on the ridge cap matches the field. And confirm the disposal plan — a 20-square tear-off of one layer is roughly three tons of debris and needs a real dumpster, not a trailer.