Measuring Your Roof

In order to estimate how many shingles you'll need, first you need to estimate the total square footage of your roof's surface. Just measure the length and width of each plane on the roof, including dormers. Then, multiply length x width to get the square footage of each plane. Simply add the square footage of each of the planes together to derive the total square footage of your roof.
For example, this shed roof has one roof plane.
Simply measure length (A) x width (B): A x B = 120' x 100' = 12,000 sq. ft. for the total square footage of the roof.
This gable roof has two planes.
So, multiply length (A) x width (B) to get the square footage for each plane,
then add the two planes together to derive the total square footage of the roof:
Plane 1: 120' x 100' = 12,000 sq. ft.
Plane 2: 120' x 100' = 12,000 sq. ft.
Plane 1 + Plane 2 = 24,000 sq. ft. for the total square footage of the roof.
Roof surfaces are measured in "squares."
A "square" is an area of roof which measures 100 square feet.
To determine the number of squares on the gable roof above, for example,
simply divide its total of 24,000 square feet by 100.
Which means you will need 240 "squares" of shingles to cover it.
The most common type of shingle, called a three-tab or strip shingle,
is generally packaged three bundles per square.
If this is a new roof, you will also need 240 squares of underlayment
(no underlayment is needed if you are applying
shingles directly over an existing asphalt roof).
Underlayment usually comes in rolls of four squares each,
so covering 240 squares would require 60 rolls of underlayment.
Be sure to add 10% to all of your material totals for trim allowance.

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