How to Measure a Roof for Shingles (From the Ground, With Pitch Factors and a Worked Example)

Roofing

How to Measure a Roof Without Getting on It

A roofer measures your roof in about four minutes from the ground, with a tape, a glance at the gable end and a phone. You can do the same with a tape measure and a pitch table, and come within 5% of what their satellite report says. That number is what every quote should be built on, and knowing it is how you catch a quote that was built on something else.

Feet and inches, because that is what every roofer in Ontario works in. Squares are 100 square feet.

Step 1: the footprint

Walk the outside of the house with a 25 or 50 ft tape and measure the length of each wall, including the attached garage, the porch roof and anything else with a roof over it. Sketch it as you go. For a rectangle, length times width. For an L, two rectangles added. A 40 ft by 32 ft bungalow with a 24 by 24 garage is 1,280 + 576 = 1,856 sq ft of footprint. Do not use the floor area from the listing; a two-storey house has half the footprint of its floor area, and a bungalow with a finished basement has the same footprint as its main floor.

Step 2: the overhangs

The roof hangs past the walls at the eaves (the low edges) and the rakes (the sloped edges on a gable). A typical Ontario house has 16 to 24 in. of overhang. Measure it at one corner with the tape held out from the wall to the drip edge, then add it to each side of the footprint. Or, if you cannot be bothered, add 8% to the footprint, which is what a 20 in. overhang works out to on an average house. 1,856 x 1.08 = 2,004 sq ft of plan area. On a house with no overhangs (some 1960s boxes and most modern flat-roofed ones), skip this.

Step 3: the pitch

Pitch is rise over run: a 6/12 roof rises 6 in. for every 12 in. it runs. Stand at the gable end and look: a roof that rises half as fast as it runs is 6/12; one that rises as fast as it runs, a 45-degree roof, is 12/12. For a better number, hold a 12 in. level horizontally against the underside of the rake at the gable, level it, and measure from the far end of the level straight up to the roof edge: that is the rise in inches over 12. From inside the attic, same thing against a rafter. Then the factor:

Pitch	Angle	Factor	Pitch	Angle	Factor
2/12	9.5 deg	1.014	8/12	33.7 deg	1.202
3/12	14.0 deg	1.031	9/12	36.9 deg	1.250
4/12	18.4 deg	1.054	10/12	39.8 deg	1.302
5/12	22.6 deg	1.083	11/12	42.5 deg	1.357
6/12	26.6 deg	1.118	12/12	45.0 deg	1.414
7/12	30.3 deg	1.158	14/12	49.4 deg	1.537

The factor is the square root of (1 + (rise/12) squared), for anyone who wants to check. Plan area times the factor is the sloped area. 2,004 x 1.118 = 2,240 sq ft, or 22.4 squares, for that bungalow at 6/12.

Step 4: the complications

Hip roofs. Same arithmetic. A hip roof over a given footprint has the same area as a gable over the same footprint at the same pitch, which surprises people. What changes is the waste (more cuts) and the flashing (hips need cap).
Different pitches on different parts. Garage at 4/12, house at 8/12: measure the two footprints separately and apply the two factors.
Dormers. Small ones add almost nothing to the area and a lot to the labour; a shed dormer across half the roof adds its own footprint times its own factor, plus the wall it creates to flash. Add the dormer roof area and let the roofer price the flashing.
Porches, bay windows, bump-outs. Each one is a little roof: footprint times factor. Easy to forget and together they can be two squares.
Flat roofs. Factor is 1. Measure the footprint and add any parapet wall area that will be membraned.

The satellite shortcut

Most roofers now order a satellite or aerial measurement report (EagleView, Roofr, Hover and a few others) for $20 to $60, which measures every plane, ridge, hip, valley and eave from aerial photographs to within a couple of percent, and prints it on the quote. Ask to see it. If one roofer’s report says 22.4 squares and another’s quote says 26, the second one is either counting waste as roof or has not measured at all. You can get close for free with the measuring tool in Google Earth on a computer: trace each roof plane, read the area, multiply by the pitch factor. It is the plan area you are tracing, so the factor still applies.

Step 5: waste, and what you pay for

The material order is the roof area plus waste: 10% for a plain gable, 12 to 13% for a hip, 15% for a cut-up roof with valleys and dormers. That is shingles cut at rakes, valleys and hips and the half-bundle at the end of every course. So the 22.4-square bungalow needs about 25 squares of shingles, 75 bundles. The installed price, though, is on the 22.4. A quote that applies $675 a square to 25 squares instead of 22.4 is charging you $1,750 for the installed rate on offcuts. It happens; it is the first thing the per-square page tells you to check.

The two mistakes that put people 20% out

Using the floor area. A 2,400 sq ft two-storey has a 1,200 sq ft footprint. Its roof is about 14 squares, not 27. People budget double what they need, which is the pleasant mistake, until a roofer quotes the 27 and they pay it.
Forgetting the pitch. A 10/12 roof has 30% more surface than its footprint. A homeowner who measured the footprint and called it the roof is 30% short, and the quote looks like robbery when it is just geometry.

Questions people ask

How do I calculate roof square footage from the house square footage?

You cannot directly. Use the footprint (ground-floor outside dimensions), add 8% for overhangs, and multiply by the pitch factor: 1.118 for 6/12, 1.202 for 8/12, 1.414 for 12/12. A 1,500 sq ft bungalow at 6/12 is about 1,810 sq ft of roof.

How many squares is a 2,000 square foot house?

If it is a bungalow at 6/12, about 24 squares. If it is a two-storey with a 1,000 sq ft footprint, about 12. The floor area tells you nothing without the footprint and the pitch.

How do I find my roof pitch?

A 12 in. level held horizontal under the rake or against a rafter in the attic, then measure straight up from the end of the level to the roof: that is the rise over 12. Or compare the gable to the table: half as tall as wide is 6/12, as tall as wide is 12/12.

How much waste should I add for shingles?

10% for a simple gable, 12 to 13% for a hip, 15% for a complex roof. Add it to the material order only; the installed price is on the roof area.

Are satellite roof measurements accurate?

Within 2 to 3% on most houses, which is better than a tape from the ground. Roofers pay $20 to $60 a report. Ask for the report with the quote.

Then price it

Footprint, 8%, pitch factor, divide by 100. The calculator does the arithmetic and prices the result in whatever you are thinking of putting on it.

Price the roof What the per-square rate includes

Sources and further reading

Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
Canadian Roofing Contractors Association: the national association; its roofing specifications are what good quotes are written to.

How to Measure a Roof From the Ground: Footprint, Pitch Factor, Squares, and the Satellite Shortcut