What is a roofing square?
A roofing square is a unit of roof area equal to 100 square feet. The National Roofing Contractors Association glossary defines it two ways: a unit for measuring roof area, and a quantity of material that covers 100 square feet of roof deck.
Shingles, underlayment and labor are all priced by the square, so every estimate starts with this number. A roof of 2,000 square feet is a 20 square roof.
Why can you not just use the floor area of the house?
A pitched roof has more surface than the footprint under it. The steeper the roof, the bigger the gap. A flat footprint of 1,440 square feet becomes about 1,610 square feet of actual roof at 6/12 pitch.
Footprint means the outside dimensions of the roof planes, measured on the flat, including eave and rake overhangs. Do not use the living area from a listing, because it leaves out the garage, porches and overhangs that the roof still covers.
How much waste should you add?
Waste covers cut-offs at hips, valleys, rakes and penetrations, plus damaged bundles. One published guide lists about 10 percent for a simple gable, 12 to 15 percent for a hip roof and 15 to 20 percent for hips and valleys with dormers. Heavily cut roofs with turrets, skylights and many penetrations can run higher.
Treat these as starting points. Your crew's cutting habits, the shingle pattern and the supplier's return policy all move the number, so keep your own history and adjust it.
What does a worked example look like?
Take 1418 Ridge Rd, a 6/12 gable and hip roof. The footprint with overhangs is 48 ft by 30 ft.
Footprint: 48 x 30 = 1,440 sq ft. Roof surface: 1,440 x 1.118 = 1,609.9 sq ft, or 16.1 squares. Add 12 percent waste: 16.1 x 1.12 = 18.03 squares. At three bundles per square that is 18.03 x 3 = 54.1 bundles, so order 55 bundles.
Tear-off and labor are normally priced on the 16.1 measured squares. The shingle order uses the 18 squares that include waste. Starter, ridge cap, underlayment and drip edge are ordered by length or roll coverage, not by the square.
When should you confirm with a measurement report?
A footprint calculation is good for a quick budget number at the kitchen table. Before you order material or sign a fixed price, confirm with a measurement report or a physical measure of each roof plane. Reports list every facet, the pitch of each and the length of ridges, hips, valleys and eaves, which a single footprint number hides.
If the roof has more than one pitch, calculate each section on its own and add the results.
Step by step
- Measure the footprint. Measure the outside dimensions of each roof section on the flat, including eave and rake overhangs, and multiply length by width.
- Find the pitch. Read the rise per 12 inches of run for each section, for example 6/12.
- Apply the pitch multiplier. Multiply each footprint by its multiplier, for example 1.118 for 6/12, to get roof surface in square feet.
- Convert to squares. Divide the total roof surface by 100.
- Add waste. Multiply by 1.10 to 1.20 depending on how many hips, valleys and penetrations the roof has.
- Convert to bundles. Multiply squares by the bundles per square printed on the product, usually three, and round up to whole bundles.
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Sources: NRCA Roofing Guidelines Glossary: Square; Vertigraph: How to Calculate Roofing Squares