Step 1
Determine where each measurable section of the hipped roof begins and ends. You want to split the into simple three-dimensional shapes whose volumes are easy to calculate. A typical hip roof, for example, contains a central triangular prism and a half-pyramid shape on each end of the roof.
Step 2
Measure the width, length and height of each shape in feet. You can also refer to your blueprints for exact lengths.
Step 3
Determine the volume of the central triangular prism by multiplying 1/2 width times height times length. Note that the formula "1/2 width times height" represents the area of a vertical cross section of the triangular prism. If you're working with a prism that is 8 by 10 by 20, your total would be 800 cubic feet.
Step 4
Multiply the length of the half-pyramid by the width. Double the figure if the pyramids are the same size so that you can find the total volume for both ends of the roof at once. That's the area of the base of a full pyramid. So if the length of your full pyramid is 5 and the width is 8, the total area is 40.
Step 5
Find the volume of the two pyramids at once by using the base area you just calculated in the formula "1/3 times base area times height." If the height is 10, then multiply 1/3 times 10 times 40 to get your total volume of 133.3 cubic feet.
Step 6
Add the totals for both shapes together to find the total volume. In the example, the total volume would be 933.3 cubic feet.