Taking measurements without going on the roof
Every measurement is taken from the ground. Do not go on the roof to measure.
- Length and width. Measure the exterior walls of the building, or take them from the certificate of location.
- Overhang. Measure how far the roof edge extends past the wall. On a flat roof without a cornice, leave it at zero.
- Pitch. Pitch is expressed in inches of rise per 12 inches of run. A 4/12 roof rises 4 inches every 12 inches. If you do not know it, pick the closest one by eye and ask us to confirm it.
The pitch factor
The pitch factor comes from the Pythagorean theorem. It is the square root of 1 plus the square of the slope. It equals 1.000 for a flat roof, 1.054 for a 4/12, 1.118 for a 6/12 and 1.414 for a 12/12. A 6/12 roof has an area about 12% larger than the house footprint.
The calculation gives the same area for a gable roof and a hip roof, as long as all slopes have the same pitch. For a roof that combines several pitches, calculate each part separately and add them up.
Flat roof or pitched roof
The City of Montreal considers any roof with a slope under 2/12 to be flat or low-slope. On a plex with a flat roof, the roof area is almost equal to the footprint, plus the parapets.
From square feet to squares
Roofers often count area in squares. One square equals 100 square feet of roof, or about 9.3 square metres. A 1,500-square-foot roof is 15 squares.
The calculator gives the roof area, not the quantity of materials to order. Cuts, valleys, dormers and starter courses add waste that only a visit to the roof can count.
With the area in hand
The area is the first figure to compare from one quote to another. If two quotes give very different areas for the same roof, ask how each was measured. To turn the area into a cost range, see the roof replacement cost page. Our quote states the measured area.
Frequently asked questions
How do you calculate the area of a roof?
Multiply the length by the width of the building, overhangs included, then multiply the result by the pitch factor. For a 4/12 roof, the factor is 1.054. The calculator above does it in feet or metres.
How many squares is my roof?
Divide the roof area in square feet by 100 to get the number of squares. A roof of 1,500 square feet is 15 squares.
How many square feet are in a roofing square?
A roofing square equals 100 square feet, or about 9.3 square metres.
How can I find my roof pitch without going up?
Look at the gable of the house from the ground and compare it with the pitches in the calculator. The house plans sometimes show it. Our quote confirms the measured pitch.
Is the roof area the amount of shingles to buy?
No. You have to add cutting waste, starter courses and ridge caps. The number of bundles per square is printed on the manufacturer's packaging.
Source: Ville de Montréal, « Rénover un toit plat ou à faible pente », consulted October 3, 2026.