How to Measure Odd Angles on Roofs for Their Square Footage

Divide the odd roof area into a series of regular shapes. Any area with irregular angles at the corners can be divided into rectangles and triangles. Use a builder's square and a stick of chalk to mark out the edges of the regular shapes on the roof surface. For example, reduce a shape like a rocket to a triangle on top of a rectangle, and an "L" shape to two rectangles.

Measure the length and width of each rectangular area in inches. Determine the area of each rectangle by multiplying the length by the width. For example, a rectangle with a width of 48 inches and a length of 60 inches has a surface area of 2,880 square inches: 48 x 60 = 2,880.

Measure the width of the base of each triangle, and the height from the base to the apex. For example, find the height of a triangle shaped like a pyramid by measuring a line from the middle of the base to the apex. For a triangle that leans to one side, with an apex that isn't above the base, drop a line vertically from the apex to an extension of the base line, and measure that. Calculate the area of each triangle by multiplying the height by half the width of the base. This rule holds true for all triangles. For example, the area of a triangle with a height of 50 inches and width of 40 inches is 1,000 square inches: 50 x (40 / 2) = 1,000.

Combine the areas of all the rectangles and triangles to obtain the total surface area in square inches. For example, an oddly angled section of roof consisting of a rectangle with an area of 2,880 square inches and two triangles with areas of 1,000 and 800 square inches has a total surface area of 4,680 square inches: 2,880 + 1,000 + 800 = 4,680.

Divide the total area in square inches by 144, the number of square inches in one square foot. The result is the square footage of the odd-angled area. To conclude, the example roof section has a surface area of 32.5 square feet: 4,680 / 144 = 32.5.