We're planning to replace our roof later this summer and want to get some ballpark estimates from a number of pros. How can I estimate how much roofing we'll need?

Knowing how much roofing material you'll need will make it easier to compare bids from contractors.

Roofs are measured in 100-square-foot areas, or squares. A square of three-tab shingles is typically three bundles, while for laminated shingles a square is usually four bundles. Read our latest report on roofing, which includes ratings of more than two dozen products and expert buying advice (report and ratings available to subscribers).

To gauge your needs, multiply the overall length and width of each section of your roof to determine the square footage, adding 10 percent to allow for waste. Then divide that number by 100 to determine how many squares the job will require.

If you don’t want to go on your roof to get accurate measurements, you can estimate the roof surface area by using the length and width of your home as the basis. Increase each of these dimensions by the amount the roof overhangs the exterior walls on each side of the home. Since the roof might have distinct sections and because most roofs are pitched, you'll want to increase the number you calculate. Adjust the calculated area by about 5 percent for a low-sloped simple roof to about 40 percent for a complex roof design with a steep slope.

When you get bids, be sure the contractor includes the cost of the underlayment (roofing felt), which creates a moisture barrier for the wood sheathing and rafters underneath. And don't forget to factor in adding an ice-and-water shield, which runs along the eaves and the valleys where two wings of the roof intersect, as well as new drip edges and metal flashing, which are often needed around pipes, chimneys, and the like. e-mail | Twitter | Forums | Facebook