# DIY Roofing

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 roofing contractor.

Search our list of independent roofing contractors in the Owens Corning Roofing Contractor Network to find the roofing professional that’s right for you.

## Determining How Many Roof Shingles You Need

If you have decided to do the work on your own, check out our estimating guidelines below and these other Roofing resources:

### Estimating

#### Total Square Footage

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:

• Plane 1 (A x B): 120′ x 100′ = 12,000 sq. ft.
• Plane 2 (A x C): 120′ x 100′ = 12,000 sq. ft.
• Plane 1 + Plane 2 = 24,000 sq. ft. for the total square footage of the roof.

#### Roof Squares

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.

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:

#### Method 1

Calculate the roof length by measuring 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 where it meets 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.

#### Method 2

Determine the roof area by using a mathematical formula that accounts for the roof length, total span, and roof pitch:

1. Determine your roof pitch by using a pitch gauge (available at most home improvement stores) or a smartphone app (available free through any app store).
2. Measure the length of the roof surface, including overhangs.
3. Measure the span of the roof, including overhangs.
4. Multiply the length x the span. The result will be the plane area. Use the chart below to calculate the total area of the roof.
5. Calculate the total roof area – Plane Area x Correction Factor = Roof Area