How Many Roofing Squares Do I Need?

Roofers quote in squares, suppliers sell in bundles, and the number on your house plans is the ground footprint rather than the roof itself. Those three units account for most of the gap between the figure in someone's head and the quote that arrives.

A square is 100 square feet, and the footprint understates it

Take a plain gable roof over a 40 by 30 foot house. The footprint is 1,200 square feet. The roof is larger than that because it slopes, so multiply by the pitch factor. At 6/12, meaning six inches of rise for every twelve inches of run, the factor is 1.118. That gives 1,342 square feet of roof, or 13.4 squares.

Now steepen it. The same house with a 10/12 pitch has a factor of 1.302, so the surface is 1,562 square feet, or 15.6 squares. Two extra squares of material, and a much slower day for whoever is walking on it. Pitch is the reason two houses with identical floor plans get different roofing quotes.

Pitch factors worth writing down

4/12 is 1.054. 6/12 is 1.118. 8/12 is 1.202. 10/12 is 1.302. A 12/12 roof, which sits at 45 degrees, is 1.414. Anything above about 9/12 usually carries a steep-slope labour surcharge on top of the material, because the crew needs staging to stand on.

Waste is not padding

Shingles get cut at hips, valleys, rakes and around every pipe and vent, and the offcuts do not go back on the roof. A simple two-plane gable runs around 10 percent waste. A hip roof with dormers and valleys runs 15 to 18 percent. On that 13.4 square roof, 10 percent waste adds 1.34 squares and 18 percent adds 2.41, so the gap between the two is just over one square. At $130 a square for shingles that is about $140, which is cheap next to a second delivery on a Saturday.

Ridge cap and starter strip are sold by the linear foot rather than the square, and they are the two items people forget. A 40 foot ridge plus 60 feet of eave gives you two more line items before the job is properly priced.

Count the layers before you count the squares

Tear-off is priced per square as well, and a roof carrying two existing layers costs about twice as much to strip and dispose of. Lift a shingle at the eave and look at the edge before you accept anyone's estimate.

Using the roofing calculator

The calculator asks for the footprint dimensions of each roof plane, the pitch, and a waste percentage. Take the dimensions with a tape measure at ground level along the exterior walls, and include the overhang, usually 12 to 24 inches per side, because shingles run out to the drip edge. An L-shaped house is two rectangles; enter them separately and add the results. The same habit of splitting an awkward shape into rectangles that works for measuring interior square footage works here.

Pitch comes from the attic if you can get into it. Hold a level out 12 inches from a rafter and measure the drop at the far end: six inches of drop is a 6/12. It is the same rise over run arithmetic used in stair rise and run, just at a larger scale. Set waste to 10 percent for a simple roof and 15 for anything with valleys, then round the final square count up to a whole number, because that is how material is sold.

Common questions

How many bundles are in a roofing square? Three, for standard three-tab and most architectural shingles. Heavier designer profiles and slate-look products often run four or five bundles per square, so read the wrapper rather than assuming.

Can I measure a roof without getting on it? Yes. Footprint from the ground plus pitch from the attic gives the calculator everything it needs. Satellite measuring services are accurate on simple roofs and less reliable once tree cover or a run of dormers gets in the way.

Do I include the overhang? Yes, measure to the outside of the fascia rather than the wall. A 24 inch overhang on that 40 by 30 house makes it 44 by 34, so the footprint goes from 1,200 to 1,496 square feet. That is nearly three extra squares before pitch is even applied.

Why is the quote so much higher than my material total? Underlayment, ice and water shield, drip edge, flashing, vents, nails, tear-off, skip hire and labour are all separate lines. Shingles are frequently less than half the invoice.