Roofing is bought in squares — one square is 100 sq ft of roof. The catch is that the roof is bigger than the footprint it sits on, because it slopes. Get the footprint, multiply by the pitch factor for the real area, and the squares and bundles follow. Here is the whole method.
Squares, and the pitch factor
The footprint is the flat ground the roof covers; the actual roof is larger by the pitch factor = √(rise² + 12²) ÷ 12. A 40 × 30 ft footprint (1,200 sq ft) at a 6:12 pitch has a factor of about 1.118, so the roof is about 1,342 sq ft — 13.4 squares, or about 15 with 10% waste.
| Pitch | Pitch factor |
|---|---|
| 3:12 | 1.031 |
| 4:12 | 1.054 |
| 6:12 | 1.118 |
| 8:12 | 1.202 |
| 12:12 | 1.414 |
Skipping the pitch factor is the most common way people come up short on shingles.
Bundles per square
Three bundles per square for most architectural (laminate) and 3-tab shingles — so a 15-square roof needs about 45 bundles. Heavy designer shingles can run four or five bundles per square. Order by the bundle, and buy a few spare from the same lot for repairs.
Roofing Calculator (Shingles & Squares)
Enter your footprint and pitch for the roof squares, shingle bundles and underlayment to buy.
How much waste to allow
About 10% for a simple gable roof; 15% or more for a hip roof, lots of valleys and hips, or a cut-up roof — the extra covers the starter course, ridge caps and the offcuts at valleys. Architectural shingles waste a little less than 3-tab. Always round up to whole bundles.
Underlayment
Underlayment covers the same roof area as the shingles. A roll of synthetic underlayment typically covers about 10 squares (1,000 sq ft); a roll of #15 felt about 4 squares (400 sq ft). Divide the roof area by the roll coverage. Add ice-and-water shield at the eaves and valleys separately where code requires it.
Ridge caps, starter and flashing
The waste allowance covers the starter course and ridge caps on a straightforward roof, but their exact count depends on the ridge, hip and eave lengths, which vary with the roof shape. For a cut-up roof, measure the ridge and hip runs and add cap shingles — plus flashing at walls and penetrations — on top of the field shingles the calculator sizes.
Not sure of the pitch?
Measure the rise over 12 inches of run — a 6 inch drop over 12 is a 6:12 — or work it out with the roof pitch calculator, then bring the x:12 figure here. The pitch is the one input that most changes the answer, so it is worth getting right.