How to Estimate Roofing Materials

When roofing a new home, or replacing the roof on your current house, obtaining accurate estimates of your roofing costs is vital. Roofing costs can be calculated in two ways: using the square footage of your home and the roof pitch, or, using the square footage of your home and roof angle. Both construction estimate methods are detailed below, or you can use the roofing calculator on the left.

The methods and the calculator will generate accurate estimates of roofing materials--regardless of whether you have a hip roof or gable roof. This is because a hip roof and a gable roof on the same house have exactly the same surface area! The roofing surface area is also independent of the number of gables, ells, or shape of the house. If you are curious, you can verify this mathematically using three-dimensional geometry.

Method 1

First compute the square footage of your house by examining the floor plan. If the house is a rectangle, the area is width times length. If the shape is more complex, divide it into rectangles, compute the area of the rectangles, and find the sum. Call the area of the house A.

Next, determine the pitch of your roof. In roofing, pitch is usually expressed as a fraction with a denominator of 12. For example, a pitch of 5/12 means the roof rises 5 inches for every 12 inches of length. Call the roof pitch P.

Now, compute the roof surface area with the equation

Roof surface area = A*sqrt(1+P²)

For example, if the area of your home is 1800 square feet and the roof pitch is 5/12, then the roof surface area is

(1800)sqrt(1 + 25/144)
= (1800)sqrt(169/144)
= (1800)(13/12)
= 1950 square feet.

To account for overhang, add about 4%-8% extra.

Method 2

Method 2 is the same as method 1, except instead of pitch, use the roof angle. Measure the slant angle of your roof from the horizontal and call the angle Z. Now use the formula

Roof surface area = A/cos(Z),

where cos is the cosine of angle Z. For example, if the area of your house is 2300 square feet and the roof has an angle of 25 degrees, then the roofing area is

2300/cos(25)
= 2300/0.9063
= 2538 square feet.

© Had2Know 2010