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How to calculate roofing squares
- Measure the footprint. Tape the outside walls, then add the overhang on every side. A 40 × 30 ft house with 1 ft eaves is 42 × 32 ft, or 1,344 sq ft, in plan.
- Apply the pitch. Multiply by the pitch multiplier. At 6/12 that is 1.118, so the roof surface is 1502.6 sq ft. Not sure of the pitch? Use the roof pitch calculator.
- Convert to squares. Divide by 100: 15.03 squares.
- Add waste. With 10% for a gable roof you order 16.53 squares.
- Count bundles. At 3 bundles per square that is 50 bundles.
The rest of the list for that roof: 2 rolls of synthetic underlayment, 17 pieces of drip edge for 84 ft of eaves and 72 ft of rakes, 2 bundles of starter, 2 bundles of ridge cap for the 42 ft ridge, and 2 rolls of ice & water shield along the eaves.
Footprint or sections?
House footprint works for a rectangular house with one gable or hip roof. Because the shape is known, the calculator also works out every edge: eaves, rakes, ridge and hips, so drip edge and ridge cap come out without extra measuring.
Roof sections is for everything else: L-shaped houses, porches, dormers and garages at a different pitch. Enter each plane's plan size, or tick measured on roof if you measured along the slope, and type in your edge lengths. Two planes of the same size, like both sides of a gable, can be one row with How many set to 2.
What goes into the materials list
| Item | Worked out from | Default coverage |
|---|---|---|
| Shingles | Order squares × bundles per square | 3 bundles per square |
| Synthetic underlayment | Roof area plus waste | 1,000 sq ft per roll |
| Ice & water shield | 3 ft up the eaves, 3 ft in valleys | 200 sq ft per roll |
| Starter strip | Eaves and rakes | 100 ft per bundle |
| Drip edge | Eaves and rakes, +5% laps | 10 ft pieces |
| Hip & ridge cap | Ridges and hips | 25 ft per bundle |
Coverage varies by product, so every default is editable under Product coverage, and the link you share keeps your settings. The IRC requires an ice barrier at the eaves where ice dams are common (R905.1.2); untick it if your area does not need one. Once you know your squares, see what the job costs in asphalt, metal or tile with the roof cost calculator.
Questions people ask
What is a roofing square?
A square is 100 square feet of roof surface. Shingles, underlayment and most roofing labor are priced by the square, so a 2,000 sq ft roof is 20 squares.
How many bundles of shingles are in a square?
Three for most standard and architectural (laminated) shingles. Some heavier designer shingles take four or five bundles per square; the wrapper says. You can change bundles per square under Product coverage.
How do I work out roof squares from my house footprint?
Add the overhang to each side of the footprint, multiply length by width, then multiply by the pitch multiplier for your roof, which is √(1 + (rise ÷ 12)²). Divide by 100. A 42 × 32 ft plan at 6/12 is 1,344 × 1.118 = 1,502.6 sq ft, or 15.03 squares.
How much waste should I add?
Roofers commonly add about 10% for a simple gable roof, about 15% for a hip roof, and more for roofs cut up by valleys and dormers, because shingles are trimmed at every hip, valley and rake. Use what your roofer or supplier recommends.
Does a hip roof have more area than a gable roof?
Not at the same pitch over the same footprint: both equal footprint × pitch multiplier. A hip roof does have more waste and more hip-and-ridge cap, and its eaves run all the way around, so it needs more drip edge and starter at the eaves.
How do I measure a roof I cannot get on?
Measure the house walls from the ground, add the overhang, and use the footprint mode with your pitch. Satellite views and house plans also give plan sizes. Use the roof pitch calculator to get the pitch from a photo.
How many rolls of underlayment do I need?
Divide the roof area, plus waste, by the coverage on the roll. Many synthetic underlayments cover 1,000 sq ft (10 squares) per roll, while #15 felt covers about 400 sq ft. Set the coverage of the product you buy and the count updates.
How much drip edge do I need?
Drip edge runs along the eaves and the rakes. Add their lengths, allow about 5% for laps, and divide by the piece length, usually 10 ft.