In order to estimate how much roof paint you'll need, first you need to estimate the total square footage of your roofs surface. Just measure the length and width of each plane on the roof, including dormers. Then multiply length o width to pet the square footage of each plane. Simply add the square footage of each of the planes together to derive the total square footage of your root.

For example, this flat root has one roof plane.

Simply measure length (A) times the width (B): (include overhangs if any)

Example:
A = 120'
B = 100'
A x B = 120' x 100' = 12,000 sq. ft. for the total square footage of the roof.

This gable roof has two planes.

So, multiply length (A) times the width (B) to get the square footage for each side, then add the two sides together to determine the total square footage of the roof.

Example:
Side 1: 120' x 100' = 12,000 sq. ft.
Side 2: 120' x 100' = 12,000 sq. ft.
Side 1 + Side 2 = 24,000 sq. ft. for the total square footage of the roof.

Be sure to add 10% to all of your material totals for waste and spillage.

What it you have a steep roof? To measure a steep roof, you will have to use an alternate method.

Steeper Roofs

Measure your house at ground level, then add in the roofs overhang for greater accuracy. As an example, if the roof overhang is 12 inches, you'll add 2 feet to the overall length and 2 feet to the overall width of the house. This nominal square footage amount is then multiplied by the factor listed to the right.

Pitch

4 in 12

5 in 12

6 in 12

7 in 12

8 in 12

9 in 12

10 in 12

11 in 12

12 in 12

Multiply by

1.06

1.08

1.12

1.16

1.20

1.25

1.30

1.36

1.42

Determining the pitch of your roof:

There are many methods of determining the pitch of your roof; the simplest is to measure it directly.

To Determine Roof Pitch Place a level against the roof as shown.

From the point where the level touches the roof, measure out 12 inches. From this point, measure the distance in inches from the bottom of the level to the roof. This will be the first number in the roof pitch. The second number will always be 12.