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.
Finally, if you have any questions about your estimate, ask a roofing contractor in your area. Most will be happy to give you a free estimate.
What's Your Slope?
You will also need to know the slope of your deck. In order to determine this, measure
the vertical rise of your deck in inches over a 12" horizontal distance. If this rise is
4", then your roof slope is 4 in 12. Roof slopes are always expressed with the vertical
rise mentioned first and the horizontal run (12") mentioned second.
What If You Have a Steep Roof?
To measure a steep roof, you will have to use an alternate method. To obtain the roof length, measure the exterior walls plus the overhang for the length of the house parallel to the ridge. Next, throw a rope over the ridge and mark it on each eave. This will give the width dimension to use in figuring your area. This should be done on each roof section containing a horizontal ridge.
Now How About Nails?
You will also need to know the amount of nails needed. Generally you should use four
nails per shingle. With regular three-tab shingles, that would require 320 nails per square.
For high wind areas or when shingles are being applied to a mansard, six nails per shingle are
required (480 nails per square). This is based on 80 shingles per square. Other styles of
shingles may require more or fewer nails per shingle and may have more or less than 80
shingles per square. Refer to the application instructions on your shingle wrappers
for the correct nailing pattern.
Nails are purchased by the pound, so ask your dealer for the correct amount of nails for your size roof in the length you specify.
Measure the lengths of your rakes and eaves to determine the amount of drip edge needed.
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