How to Estimate Roofing

Calculate how much roofing material you'll need to stay within your budget

By Glyn Sheridan

Photo by iStockphoto.com

Before beginning any construction project, you need an accurate estimate of the costs involved. If you're planning to install a new roof or replace an existing roof, you'll need to know how much roofing material to purchase and how much underlayment and roofing accessories you'll need. Roofing shingles sell by the "square," a measurement equal to 100 square feet of finished roof surface.

• What You Need to Know
• If you're tearing off an old roof, add the cost of construction waste disposal to your estimate.
• Many roofing contractors offer free estimates. So don't just hop on top of the house and start measuring; see if your roofer will assume the risk for you.

Step 1:

Measure the height and length of all flat roof surfaces, in feet, and multiply the numbers to determine your square foot needs. Round the measurements up to the next foot. For instance, if the distance between the ridge of your roof and the edge is 12 feet 4 inches long, round up to 13 feet.

Step 2:

Allow two shingles per linear foot for roof valleys. Because shingles must be laced in the valley area of a roof, this spot requires twice as many shingles. If the valley is 16 feet lon, from ridge to edge, add 32 feet to your total shingle estimate number.

Step 3:

Divide the total number of square feet by 100 to find the number of squares you need. For example, if you figured 2300 square feet of roofing area, divide that by 100, and you will purchase 23 squares of shingles.

Step 4:

Add the length of the top edge of your roof and the top edges of all gables and dormers to determine how many ridge shingles you will use. Ridge shingles sell by the linear foot, typically in packs of 15 or 20 feet, although some ridge materials sell in different quantities, including shake, tile and metal roofing. You will also add the length of the bottom edge of the entire roof to find the amount of starter strip you will use.

Step 5:

Determine your roofing underlayment needs using the same technique you used to estimate your shingles. The most common form of underlayment is 15-pound roofing felt, and it sells in large rolls. Add an additional 5 percent of the total square footage when estimating roofing felt to allow for waste along the edges and the bottom of the roof.

Step 6:

Estimate the extra supplies and materials needed to complete your roof. In addition to the underlayment and the actual roofing material, you will need roof vents, vent covers and boot caps that fit around the base of every vent to prevent future leaks. You will also need roofing nails or staples and silicone caulking to seal around the boot caps.

• Tips & Warnings
• Consider the additional cost of buying or renting the tools and equipment needed to install the roofing, such as a roofing nailer or stapler, a saw, ladders or scaffolding and safety harnesses.
• For triangular parts of your roof, such as the sides of gables, measure the length of the bottom edge and multiply by the height; then divide by 2.
• Round up if your measurements aren't exact. It's always better to have more of something than less when it comes to order materials and completing home improvement projects.