To calculate the area of your roof, draw a sketch of the roof as seen from directly above. Measure and calculate the "footprint" of your roof. In other words, looking down from above your roof might have one area that is 30' x 40' and another area that is 20' x 20'. Therefore, the footprint of your roof would be 1200 ft. plus 400 ft., or 1600 ft. in total. Next, you must determine the pitch (slope) of the roof. The pitch or slope is normally expressed as a fraction such as 6/12. A slope of "6 in 12" means that the roof rises 6 inches for every 12 inches of horizontal run. In other words, if you placed one end of a 12 inch level against the roof, and held it level, the other end of the level would be 6 inches above the roof.
Now multiply the factor below times the footprint of your roof. Pitch Factor4/12 1.054 5/12 1.083 6/12 1.118 7/12 1.157 8/12 1.202 9/12 1.25 10/12 1.302 11/12 1.356 12/12 1.414 In our example, with a 6 in 12 roof, the roof area would equal 1600 x 1.118 = 1788.8 ft. Shingles are sold by the square, with a square being 100 ft.. Therefore you would need at least 18 squares of shingles. In addition to calculating the area of a roof, you must also determine how many shingles you will need for the ridges and hips. You will also need to know how much of valley flashing you need, eave protection, and so on. There are many roofing calculators on the Internet. For example go to