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2018 National Home Improvement Estimator, All Rights Reserved Page 128

Modified bitumen membrane roofing comes in 100-square-foot rolls. Use one, two, or three plies to
create a finished roof. Or, combine modified bitumen membrane with fiberglass felt to form a hybrid roof
system.

The minimum roof slope should be not less than 1/4" of rise per 12" of horizontal run. The surface can
be finished with mineral granules or an aggregate, such as gravel or slag.

How to Measure Roof Area
Flat roofs are easy to measure. Just divide the area into rectangles, calculate the area of each, and add
up the total. You can probably do that from the ground. Sloping roofs are a little more complex. The
steeper the slope, the more the roof area differs from the horizontal (plan) area. To calculate the roof
surface accurately, you have to multiply the rafter lengths by the width of each pitch. However, if the roof
pitch is no more than 5 in 12, a simple rule of thumb will yield nearly the same result in a fraction of the
time: Multiply the length and width of the building, including eaves and overhang. Divide by 100 to find
the number of roofing "squares." Then add 10 percent for a gable roof, 15 percent for a hip roof and 20
percent for a roof with dormers.