Roof squares = roof area in square feet ÷ 100. A roofing square is 100 sq ft of roof surface, so a 40 × 30 ft house with 1 ft overhangs and a 6/12 pitch has 1,503 sq ft of roof, or 15.03 squares. With 10% waste you buy 16.53 squares, which is 50 bundles of architectural shingles at 3 bundles per square.
How roof squares are calculated
Start with the floor plan, not the roof. Measure the outside of the walls, add the overhangs, and you have the level footprint the roof covers. Multiply it by the pitch multiplier to get the sloped surface, then divide by 100.
Roof area = footprint × √(1 + (pitch ÷ 12)²) · Squares = roof area ÷ 100 · Squares to buy = squares × (1 + waste)
Worked example with the calculator's defaults: the footprint is (40 + 2) × (30 + 2) = 1,344 sq ft. A 6/12 roof has a multiplier of 1.1180, so the roof is 1,344 × 1.1180 = 1,502.6 sq ft, or 15.03 squares. Ten percent waste makes 16.53 squares. Architectural shingles come at 3 bundles per square (GAF Timberline HDZ and Owens Corning Duration both list it), so 16.53 × 3 = 49.6, rounded up to 50 bundles.
Roof squares by house size
Most people know their floor area, not their roof. The Floor area tab assumes a square footprint of floor area ÷ stories, a 1 ft overhang and a 6/12 pitch. These are the results, before waste:
| House floor area | Stories | Roof area | Roof squares |
|---|---|---|---|
| 1,500 sq ft | 1 | 1,855 sq ft | 18.55 |
| 2,000 sq ft | 1 | 2,441 sq ft | 24.41 |
| 2,500 sq ft | 1 | 3,023 sq ft | 30.23 |
| 3,000 sq ft | 1 | 3,604 sq ft | 36.04 |
A two-story house has half the footprint, so a 2,000 sq ft two-story house has about half the roof of the single-story figure above: the walls go up, the roof does not grow. A long thin house has more roof than the square-footprint assumption, and a house with a garage, porch or dormers adds planes of its own. For those, use the House footprint tab and the roofing calculator, or measure each plane and use the Roof area tab.
Pitch multiplier for roofing squares
The steeper the roof, the more surface sits over the same footprint. The multiplier is the sloped length of a triangle whose level side is 1. If you are not sure of your pitch, find it with the roof pitch calculator.
| Pitch | Multiplier | Squares per 1,000 sq ft of footprint |
|---|---|---|
| 4/12 | 1.0541 | 10.54 |
| 6/12 | 1.1180 | 11.18 |
| 8/12 | 1.2019 | 12.02 |
| 12/12 | 1.4142 | 14.14 |
How many bundles are in a square?
It depends on the shingle. Owens Corning lists 80 three-tab shingles and 3 bundles per square for Supreme, 64 shingles and 3 bundles for Duration, and 90 shingles in 6 bundles for its heavier Woodcrest designer shingle. The calculator's Shingles menu switches between them, so 16.53 squares is 50 bundles of 3-tab or architectural shingles but 100 bundles of designer shingles.
How much waste to add
Waste is the cut-off at rakes, hips, valleys and the ridge, plus the odd damaged bundle. Ten percent suits a plain gable roof. A hip roof has more cuts and most roofers start near 15%. A roof with many valleys, dormers or a steep pitch can go higher. Rounding up to whole bundles adds a little more on its own.
What to work out next
Roof squares are the input to almost every roofing quantity. The shingle calculator goes from squares to bundles, nails and starter. Underlayment, ice barrier and ridge cap are in the roofing calculator. Under the shingles you need roof sheathing if the deck is new, and a hip roof has its own rafters and hip lengths in the hip roof calculator. To put a price on the job, use the roof replacement cost calculator. Every roofing tool is listed in the roofing and siding calculators.