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For Homeowners


How Much Do I Need?

Your roof is a major structural component of your home, and a successful reroofing project requires a large investment in both time and money. Since you cannot afford mistakes during installation, we highly recommend you hire a professional contractor. Our Find a Roofing Professional locator tool can help you find the roofing professional that's right for you.

If you have decided to do the work on your own, we'd like to help. Check out our estimating guidelines below and visit Choosing the Right Roof and Things You Should Know for additional information.


To estimate how many shingles you'll need, first estimate the total square footage of your roof's surface. To do this, 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. Finally, calculate your roof's total square footage by simply adding the square footage of each of the planes together.

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, simply divide its total of 24,000 square feet by 100. The result is 240, and this means you would need 240 squares of shingles to cover that roof. The most common type of shingle, called a three-tab or strip shingle, is generally packaged three bundles per square.

For a new roof, you will also need the same amount of underlayment. So, in the gable roof example above, you would need 240 squares of underlayment. Underlayment usually comes in rolls of 4 squares each. So, covering 240 squares would require 60 rolls of underlayment. No underlayment is needed if you are applying shingles directly over an existing asphalt roof.

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. For a listing of roofing contractors in your area, use our Find a Roofing Professional locator tool.

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 calculate 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 determine the amount of nails you'll need. Generally, you should use four nails per shingle. For regular three-tab shingles, this 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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