How to Calculate the Square Feet of a Roof

Updated February 21, 2017

Whether you are thinking about getting a new roof, or having repairs done to your existing roof, calculating the square footage of your roof is an important part of the roofing procedure. Roofers refer to the square area of a roof (in square feet), to accurately estimate a job and to order materials. With a little basic geometry, you can calculate the square footage of any roof.

Draw a rough sketch of your roof. This does not have to be to scale, but it should be as accurate as you can make it.

Make measurements of eaves (outer edges), ridges (the "top" of the roof), valleys (the "dip" in the roof where two right-angled segments of roof meet), and hips (the intersection, or junction, between two roof slopes). Jot the measurements down on your rough sketch.

Identify any triangular sections in your roof. Figure out the square footage of those individual sections by multiplying the length by the width and dividing by half. For example, if the triangular section is 10 feet x 10 feet, the section's square footage would be: 10 feet x 10 feet / 2 = 50 square feet.

Find the square footage of any squares, rectangles or parallelograms by multiplying the length by the width. For example, if you have a rectangle that is 10 feet x 14 feet, then the area is: 10 feet x 14 feet = 140 square feet.

Add the square footage of each individual piece together. In the above example, 50 square feet + 140 square feet = 190 square feet.


Nearly all roofs can be divided up into basic geometrical shapes for calculating square footage. Consider dividing an area into multiple, smaller shapes and calculating the square footage of the individual pieces instead of trying to "guess" at a tricky area.

Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Stephanie Ellen teaches mathematics and statistics at the university and college level. She coauthored a statistics textbook published by Houghton-Mifflin. She has been writing professionally since 2008. Ellen holds a Bachelor of Science in health science from State University New York, a master's degree in math education from Jacksonville University and a Master of Arts in creative writing from National University.