How to Figure Roof Area Without the Pitch

Anchor the tape measure at the corner of your house's exterior. Run it to the adjacent corner and note the measurement.

Repeat for the adjacent wall. You should now have the ground length and width measurements of your home. For example, let's assume your home's ground dimensions are 100 feet by 40 feet.

Place the ladder next to the roof, climb up and measure the distance from the lip of the overhang to the exterior wall. Note this measurement and repeat for the adjacent corner. Let's assume you have overhangs of 6 inches on each side. This leaves you with measurements of 101 feet and 41 feet. Multiply these numbers to get 4,141.

Climb to the peak of the roof and measure from the peak vertically to the base. Assume 10 feet of height; this gives you one side of a right triangle.

Divide 41 by 2. Most houses have the peak running along the length, so you're halving the shorter side. You should get 20.5. This is the base of a right triangle and 10 is the height.

Use the Pythagorean Theorem -- a^2 + b^2 = c^2 -- to calculate the length of the roof's slope: 10^2 plus 20.5^2 equals 520.25. Taking the square root gives you 22.81.

Multiply this by the length of your roof, 101 feet. This gives you 2,303, the surface area of one side of the roof. Double it to get the total area, 4,606 square feet.