DIY Roofing

Determining How Many Shingles You Need

Your roof is a major structural component of your home. A successful re-roofing 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 list of Owens Corning Roofing Preferred Contractors can help you find the roofing professional that’s right for you. If you have decided to do the work on your own, check out our estimating guidelines below and these other Roofing resources:


To estimate how many shingles you’ll need, you need to know the total square footage of your roof’s surface. Find this number by measuring the length and width of each plane on the roof (including dormers) then multiplying length times width. To find your roof’s total square footage, simply add the square footage of each of the planes together.

For example, this shed roof has one roof plane. Multiply length (A) times 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) times width (B) and length (A) times width (C) to get the square footage for each plane. Then add the two planes together to derive the total square footage of the roof:

Roof surfaces are measured in “squares” – a “square” is equal to 100 square feet of the roof. To determine the number of squares on the gable roof above, divide its total of 24,000 square feet by 100 (24,000 ÷ 100 = 240). 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 need 240 squares of underlayment. Underlayment usually comes in rolls of 4 squares each (240 ÷ 4 = 60). So you need 60 rolls of underlayment. No underlayment is needed if you are applying shingles directly over an existing asphalt roof.

TIP: 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 Owens Corning Roofing Preferred Contractor locator.

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, use one of the following alternate methods:

Slope Correction Factor

Slope Plane Correction Factor
Pitch Approximate Angle Multiply by Projection Length
3:12 14 1.031
4:12 18 1.054
5:12 23 1.083
6:12 27 1.118
7:12 30 1.158
8:12 34 1.202
9:12 37 1.25
10:12 40 1.302
11:12 43 1.357
12:12 45 1.414
16:12 53 1.667
18:12 56 1.803
24:12 63 2.236

Now how about nails?

Generally, you should use four nails per shingle in the field of the roof, and 5 nails per starter shingle. For regular three-tab shingles, this would require 320 nails per square for field shingles. Multiply the number of starter shingles required by 5 to determine the number of nails required to fasten starter course. For high wind areas or when shingles are being applied to a mansard, six nails per shingle are required, or 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.

Always check what local building code dictates for fasteners. Ask your dealer for the correct amount of nails for your size roof, in the length you specify.