How to measure your roof in squares (without climbing on it)

You can measure a roof well enough to check a quote without going on it: measure the house footprint including the overhangs, multiply by the pitch factor for your roof slope, add 10–15% waste (more for complex roofs), and divide by 100 to get roofing squares. For asphalt shingles, most products come in bundles of about one-third of a square, so squares × 3 gives an approximate bundle count. For example, a 40 × 30 ft house with 1 ft overhangs all round has a 42 × 32 = 1,344 sq ft footprint; at 6/12 pitch (factor 1.118) that is about 1,503 sq ft of roof; with 12% waste about 1,683 sq ft, or roughly 17 squares.

This guide is part of how to compare roofing quotes. Stay off the roof: falls from roofs are a leading cause of serious injury in construction, and a ladder and tape on the ground are enough.

What you need

A long tape measure or a laser measure.
The pitch, measured in the attic with a level and tape, or from your contractor's quote.
Optional: the house's plans, a survey or certificate of location, or a satellite view to check the shape (do not rely on satellite scale alone).

Step 1: measure the footprint with overhangs

Measure each exterior wall at ground level. Then estimate the overhang at the eaves and the gable ends (rakes), often 12 to 24 inches on Canadian houses, by measuring from the wall to the edge of the eavestrough or fascia with a ladder at the wall, not on the roof. Add the overhang to both ends of each dimension.

Example (rectangle): walls 40 × 30 ft, 1 ft overhang on all sides → 42 × 32 = 1,344 sq ft.

For L-shaped or more complex houses, split the footprint into rectangles, measure each and add them. Include attached garages, porches and bay roofs, and note separately any flat sections that will need a membrane.

Step 2: apply the pitch factor

Pitch	Factor
4/12	1.054
5/12	1.083
6/12	1.118
7/12	1.158
8/12	1.202
9/12	1.250
10/12	1.302
12/12	1.414

1,344 × 1.118 = 1,502.6 sq ft of sloped surface. If different sections have different pitches (a steep main roof and a shallow porch), apply the factor to each section. The pitch guide explains where these numbers come from.

Hip roofs: for a roof where all planes have the same pitch, the pitch factor applied to the footprint still gives the total area, because each plane's horizontal projection adds up to the footprint. Hips and valleys affect waste, not the base area.

Step 3: add waste

Waste covers cut-offs at rakes, hips and valleys, starter strips and ridge cap. Typical allowances:

Roof shape	Typical waste allowance
Simple gable, few penetrations	10%
Hip roof or several dormers	12–15%
Many valleys, turrets, complex cut-up roofs	15–20%

1,502.6 × 1.12 = 1,682.9 sq ft → 16.8 squares, rounded to 17.

Ridge caps and starter strips are sometimes counted separately by linear foot. Ask your contractor whether their square count includes them.

Step 4: count squares and bundles

Squares = area ÷ 100 = 1,683 ÷ 100 ≈ 17.
Bundles of typical architectural shingles: about 3 per square → 17 × 3 = 51 bundles (product coverage varies; check the bundle label).
Ridge length for ridge vent or cap: measure along the ridge from the ground by the length of the house plus overhangs. For a simple gable 42 ft long, about 42 linear feet.
Eave length for ice and water protection and drip edge: both long sides, 2 × 42 = 84 ft.
Rake length for drip edge on the gable ends: each rake is the half-width multiplied by the pitch factor: 16 ft × 1.118 = 17.9 ft, four rakes = about 72 ft.

Step 5: check against the quotes

If your estimate is 17 squares and quotes say 17, 18 and 23, ask the third contractor how they measured. Differences of one or two squares are normal (waste assumptions, rounding up to whole bundles). Large differences can mean an included garage or porch roof, a measurement error, or padding.

Then multiply by price. With an illustrative $500–$700 per square for architectural asphalt: 17 × $500 = $8,500; 17 × $700 = $11,900. Add tear-off, decking and other extras, and tax. The roof replacement cost estimator does this per square foot: enter 1,683 sq ft. A full example with tax is in the asphalt re-roof worked example.

Common measuring mistakes

Forgetting overhangs. A 1 ft overhang on a 40 × 30 house adds 144 sq ft of footprint (1,344 − 1,200), about 1.6 squares of roof at 6/12.
Using floor area instead of footprint. A two-storey house's floor area is roughly double its footprint.
Ignoring different pitches. Mixing a 12/12 front gable with a 4/12 back section changes the total.
Rounding the pitch down. 7/12 measured as 6/12 understates the area by about 3.6% (1.158 ÷ 1.118).
Counting a flat roof section as shingled. Low-slope areas need a different system; see flat roof replacement costs.

Measuring for siding

Siding uses wall area instead: length × height for each wall, plus gable triangles, minus large openings, plus waste. See the siding estimate worked example and the siding cost estimator.

FAQ

Do contractors use satellite measurements?

Many use aerial measurement reports or drone images, which can be accurate. Ask which method they used and whether it included all roof sections.

How many bundles are in a square?

Commonly three for architectural asphalt shingles, but some heavier products need four or more. Check the coverage printed on the product packaging or the manufacturer's data sheet.

Is it worth paying for a measurement report?

For checking a quote, a careful ground measurement is usually enough. Contractors typically measure as part of quoting.

Sources

National Research Council Canada — National Building Code of Canada (roofing and slopes)
Canadian Centre for Occupational Health and Safety — ladder and fall safety
Canada Mortgage and Housing Corporation — homeowner maintenance guidance