Roof Measurement and Estimating

Also known as roof estimating · roof takeoff · roof area calculation

Roof estimating converts plan dimensions into true roof area using pitch multipliers, expresses it in squares or square metres and adds waste allowances.

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Roof estimating is the process of measuring a roof and calculating the materials and labour needed to cover it. Because a pitched roof is larger than the floor area beneath it, measurements taken from plans or the ground must be adjusted for slope. Estimators then add allowances for waste, starter and ridge materials, flashings and accessories.

Units

In the United States and Canada roofing is priced and ordered by the square, equal to 100 sq ft (about 9.3 m²) of roof surface. Most other countries, including the UK, Europe and Australia, use square metres. Shingles are packaged in bundles, and the number of bundles per square is printed on each product's packaging.

Measuring

On the roof. Measure each plane along the eave and up the slope, multiply to get its area and add the planes together.
From plans or the ground. Measure the horizontal (plan) area including overhangs, then multiply by a pitch multiplier.
Aerial and drone measurement. Satellite or drone imagery with software can produce full roof reports, widely used for quotes and insurance claims.

Pitch multipliers

The multiplier converts plan area to sloped area and equals the square root of 1 + (rise/run)². Examples:

Pitch	Multiplier	Angle
4:12	1.054	18.4 degrees
6:12	1.118	26.6 degrees
8:12	1.202	33.7 degrees
12:12	1.414	45.0 degrees

A 150 m² (1615 sq ft) plan area at 6:12 therefore has a roof area of about 168 m² (about 18 squares).

Waste and extras

Estimators add a waste factor for cutting at hips, valleys, dormers and edges. Simple gable roofs need the least; cut-up roofs with many hips and valleys need more. Canadian guidance for wood shakes and shingles suggests a 5 to 10 percent allowance, plus extra material for valleys. Separate quantities are calculated for starter strips (eave and rake length), ridge and hip caps (linear length), underlayment, ice barrier, drip edge, flashing and fasteners.

Labour factors

Steep pitches, height, access, number of layers to remove and weather all affect labour time and cost.