How to Measure a Roof: How Many Squares Is Your Roof?

Roofers measure in squares. One square is 100 square feet of roof surface. A roof with 2,800 square feet of surface is a 28-square roof. Shingles, underlayment and labor are all priced per square, which is why the square count is the first number on any honest quote. Atrium's published rates on the roof cost guide are per square, installed.

The catch is the word "surface." Square footage of the house is not square footage of the roof. Two corrections turn one into the other: the roof's plan area, and its pitch.

Plan area is the outline of the roof seen from directly above. Three things make it differ from the number on your appraisal:

• Overhangs. The roof extends past the walls at every eave and rake. On a 60 by 40 foot house, a 1-foot overhang all around adds 204 square feet of plan.
• Garages and porches. They are not living area, but they are roof.
• Stories. A two-story house has a roof over its footprint, not over its total living area. A 3,000 square foot two-story home might sit on a 1,500 square foot footprint.

Here is a fact that surprises people: if every plane of the roof has the same pitch, the shape of the roof doesn't change its area. A hip roof and a gable roof over the same plan, at the same pitch, have the same surface area. Each plane's true area is its plan area times the same multiplier, so the total is too. What hips and valleys do change is waste and the length of ridge and hip cap.

Pitch is written as rise over a 12-inch run. A 6/12 roof climbs 6 inches for every 12 inches it runs horizontally. The sloped length is the hypotenuse of that triangle, so the multiplier is the square root of 1 + (rise/12)². Multiply plan area by it to get surface area.

Two thresholds matter. The code allows asphalt shingles down to a 2/12 slope, and from 2/12 up to 4/12 it calls for two layers of underlayment.¹² OSHA treats any roof steeper than 4 in 12 as a "steep roof" for fall protection.³ Steep roofs take more labor and safety equipment, which is why pitch shows up in price twice: once in the area, once in the labor.

Measuring pitch without climbing. From the attic, hold a level horizontally against the underside of a rafter, mark 12 inches along the level, and measure straight up from that mark to the rafter. That vertical distance is the rise. Do not walk the roof to measure it. If you must use a ladder at a gable end, read our ladder safety guide first.

You can't buy exactly the surface area. Shingles get cut at every rake, hip and valley, and the off-cuts are rarely reusable. Starter strips run along the eaves and rakes, and ridge cap covers the hips and ridges. The calculator on our cost guide applies a 10% allowance for cuts, starters, hips, valleys and ridge.

A simple gable roof wastes less than a cut-up roof with many hips, valleys and dormers. That is the main reason two roofs with the same surface area can need different amounts of material. When you compare quotes, compare the measured squares and the waste percentage separately. A low square count with a high waste factor and a high square count with a low one can land on the same total, and you should know which you are looking at.

Area is only half of a roof measurement. Several components are bought by the linear foot, so a good measurement also records the length of every edge and intersection:

• Eaves and rakes get starter strip and drip edge. The code requires drip edge at the eaves and rake edges of shingle roofs.¹ See what drip edge is.
• Ridges and hips get ridge cap shingles, and ridges often get ridge vent.
• Valleys get extra underlayment treatment and, on some roofs, metal.
• Walls and penetrations get step flashing, pipe boots and other flashing, counted piece by piece.

For the example house below, built as a simple gable, that is 124 feet of eave, about 94 feet of rake (four sloped edges), 62 feet of ridge, and about 218 feet of drip edge in total. The same house built as a hip roof has the same area but swaps rakes for hips, so its line items change even though its squares don't.

How the measuring is actually done. Roofers use three methods: a tape on the roof, the home's construction plans, or an aerial measurement taken from imagery. Each can be accurate, and each can be wrong. A measurement report from any of them should list the area of each plane, the pitch of each plane, and the edge and intersection lengths. If a quote gives you only a single square count, ask for the breakdown behind it.

Take a single-story house, 60 by 40 feet, with a 1-foot overhang on every side and one uniform 6/12 pitch.

1. Footprint: 60 × 40 = 2,400 sq ft.
2. Plan area with overhangs: 62 × 42 = 2,604 sq ft.
3. Pitch multiplier for 6/12: ×1.118.
4. Roof surface: 2,604 × 1.118 = 2,911 sq ft, or 29.11 squares.
5. With 10% waste: 29.11 × 1.1 = 32.02 squares.

A quick sanity check: on a simple gable, each rafter runs 21 feet horizontally from the ridge to the eave edge and is 23.5 feet long on the slope. Two planes, each 62 feet long, give the same 2,911 square feet.

At Atrium's published laminated shingle rates, 32.02 squares is $16,000–$19,200 installed, and Class 4 impact-resistant shingles are $19,200–$22,400. Your roof will have its own number. The cost guide runs the same math from your address.

The address estimator on our roof cost page measures the roof from aerial imagery, adds waste and applies our per-square rates. The quick slider below it uses three pitch categories instead of an exact pitch:

• Low: ×1. A flat-plane approximation; a real 3/12 roof is ×1.031 and a 4/12 roof is ×1.054.
• Medium: ×1.1. Between a 5/12 roof (×1.083) and a 6/12 roof (×1.118).
• Steep: ×1.25. Exactly a 9/12 roof (×1.250); a 12/12 roof is ×1.414.

On the example house, the slider at medium pitch with the 2,400 sq ft footprint gives 29.04 squares, or $14,500–$17,400 for laminated shingles. Entering the 2,604 sq ft plan area instead gives 31.51 squares, close to the measured 32.02. That gap is the overhangs and the rounded pitch. It is why the slider is a ballpark and a written proposal starts from a measured roof.

After a hail or wind storm, your insurer's estimate will list the roof in squares too. Compare that figure with your contractor's measurement before you compare dollars. A difference in square count, pitch or waste can explain a large part of the gap between two estimates, and it is the easiest kind of gap to settle, because both sides can show their math. Our guide to how roof insurance claims work covers the rest of that process.

Central Texas summers also make the attic method the right way to measure pitch. A sun-baked roof surface is not a place to stand with a tape measure, and on a steep roof it isn't a place to stand at all.

Because roofing is priced per square, every square of measurement error is money. On the example house, the difference between the footprint-only slider figure (29.04 squares) and the measured roof (32.02 squares) is 2.98 squares. Use the address estimator on the cost guide for a ballpark, then expect a replacement proposal to state the measured squares it is priced on. A free inspection includes a written report you keep.