Calculating the roofing materials needed for your home or business requires an understanding of the geometry and the ability to complete some basic math problems. For basic roof designs, this is a fairly easy task to accomplish. For roofing designs that are more complex, it is a bit more challenging and requires you to complete several calculations to make your determination. These calculations will give you the total surface area of the roof in square feet.
Things You'll Need
 Ladder
 Tape measure or measuring wheel
 Calculator
 Pencil
 Paper

On a note pad, sketch out the various geometric shapes that comprise the overall shape of your various roof surfaces. For simple roofs such as a ranch style home, you will likely have two rectangles with which to deal. For more complex roofs, you will likely have several different geometric shapes including squares, rectangles, and triangles.

If you can safely get onto your roof with your pencil, pad, and a measuring device, you begin measuring the outside dimensions of each geometric shape that you have sketched onto your pad. Be sure to write down the dimensions for each side of your shape as you measure them. You will use those measurements to calculate the square footage for each shape. Once you've made those calculations, you'll add them together to determine the total surface area of your roof. See the references below for the formulas to accomplish this task.

If you are unable to safely get onto the roof to do the handson measurements, there is another way to determine the square footage of your roof with reasonable accuracy. You can calculate your roof pitch in several ways, the easiest is to go into the attic. After measuring the outside dimensions of your home, enter the attic. Holding a level parallel to the floor so that the 12inch mark is against the rafter; make sure the bubble indicates level condition. Using your tape measure, find the distance from the front edge of your level up to the roof rafter to determine the rise. The Service Magic Roof Calculator at servicemagic.com shows a diagram and performs the calculations based upon your measurements.

Once you have determined the overall square footage of your roofing system, a simple division problem determines how many "squares" makes up your roof. A square in roofing terminology is 100 square feet (a 10foot by 10foot section). You will typically order your roofing shingles in "squares." Divide your overall square footage calculation by 100 to determine how many squares of materials you'll need to complete the roof.
Tips & Warnings
 Don't forget to consider the other materials necessary to install the roof. These items include roofing nails, felt paper, flashing, and if conditions dictate, ice and water shield. Recommendations from roofing experts indicate that you should order between five and 15 percent of extra materials to allow for waste, trimming and mistakes.
 Be extremely careful when you attempt to get up on your roof and move about on it. Wear work boots with skid resistant soles and work gloves in case you slip and need to grab hold of something.