How to Measure What Size Roof I Need

Measure the width and length of your home's exterior shape for a general estimate of the inner area. Multiply the two measurements together to get total square feet. If your home is not a perfect square or rectangle, mentally separate it into measurable sections and add the total area together. As an example, assume a home is a perfect rectangle and measures 50 feet long by 10 feet wide for a total area of 500 square feet.

Divide this number by 100 to convert the measurement to roofing squares. One roofing square is equivalent to 100 square feet. In the above example, there are five roofing squares.

Figure your roof's pitch. An example of a low pitch roof is one that rises three feet for every 12 feet horizontally. You can walk on a low pitch roof quite comfortably. A medium pitch roof is not ideal to walk on and may have a 6- to 9-foot rise for every 12 feet of horizontal length. The most difficult to install is the high-pitch roof, which has a 10-foot rise. The pitch determines the roof multiplier, which is essential for calculating roof size.

Multiply the roofing squares from the total area of the home by the multiplier for your roof pitch. Assume the roof from the example has a medium pitch. Multiply the five roofing squares by a base rate of 1.30 for a total of 6.5 roofing squares. Use anywhere from 1.15 to 1.25 for a low pitch, 1.26 to 1.4 for a medium pitch and 1.41 to 1.7 for a high pitch.