Roofing might be expensive if you don't know how to calculate roof square footage. An inaccurate estimate may cost you a lot and expose your roof to elements.

How to Calculate Roof Square Footage

To get the total square footage of your roof:

Calculate the total area by multiplying the length by the width of each of the roof's planes, including dormers.

Add up the total square footage of each plane.

>>Related post:>>Related post:What Roofing Squares Are and How to Calculate Them

Example #1

One-planed shed roof

Take, for instance, a shed roof with a 40' x 30' dimension.

A x B = 40′ x 30′ = 1200 sq. ft. for the total square footage of the roof.

Example #2

Two-planed gable roof

Suppose you have a gable roof that is 40' x 30' in dimension and 30' in width.

So, to get the roof's total square footage, you'd:

Use length A times width B and length A times width C to get the total square footage of each plane. Take the sum of the two planes.

For instance:

Plane 1 (A x B): 40′ x 30′ = 1200 sq. ft.

Plane 2 (A x C): 40′ x 30′ = 1200 sq. ft.

Plane 1 + Plane 2 = 2400 sq. ft. for the total square footage of the roof.

What Exactly is a "Roofing Square?"

"Squares" are used to measure the area of roofs.

A roofing square equals 100 square feet of the roof area in the roofing industry.

Divide 2400 square feet by 100 to get the number of squares on the gable roof example in this article.

This translates to a total of 24 shingles needed to complete the roof installation. To account for trimming, round all your material costs up to 10%-15%. (waste factor).

>>Related post:What Roofing Squares Are and How to Calculate Them

What Exactly is a Shingle Bundle?

A bundle of roofing shingles is referred to as a package.

How many bundles of shingles per square?

The most prevalent form of roofing shingles, laminate or architectural shingles, come in three bundles per square:

3 bundles = 1 roofing square

It is reasonable to assume that in order to cover the 2400 square feet covered by the gable roof, you would require 72 shingle bundles or three bundles each square.

What About Waste Factor? How Much Additional Roofing Material Should I Purchase?

A range of 10% to 15% is a decent starting point for the waste factor, although your results may vary. Waste is more likely to accumulate on more intricate roofs due to the presence of more chopped shingles at corners, walls, and edges. It's OK if you have a few extra shingles on hand. They may be kept for future repairs or if the roof has to be repaired.

This means that for a 24-square roof, you'd need to add 2.4 squares, which is equal to 7 or 8 bundles.