How to Determine a Roof's Pitch by Measuring

Use a tape measure, level and framing square to determine the pitch of an existing roof. Choose one of three methods, using a ladder to get to the roof or working inside an attic. Find the pitch, multiply the length times the width for the basic square footage, then use a roof calculator table to adjust for overhang at the eave and slope and find the actual footage for the material. Several websites offer roof calculator tables, among them APB Pole Barns, Universal Trusses. Roofer's World and Service Magic. You enter the figures for your roof and the table will calculate angles and other factors for trusses and roofing material.

Get on a ladder at one side of the roof with a 2-foot level and tape measure. Put one end of the level a foot or more up the roof. Hold it level, then measure from the 12-inch mark on the level to the roof directly below. Use a framing square as an alternate; set the end of the 24-inch blade against the roof, hold it tight and set a 48-inch level vertically against the square's tongue. Measure that distance and divide by 2. The vertical measurement is the rise; 5 inches makes a 5/12 pitch roof.

Use a ladder at the gable end of the roof if that is more convenient. Place one end of a 2-foot level against the barge rafter, the one on the roof overhang. Hold the 12-inch mark on the level on the rafter's bottom edge. Measure from the opposite end of the level to the bottom of the rafter. That distance in inches is the rise of the roof.

Measure in an attic with a similar technique. Put the level against a rafter at the 12-inch mark on the level. Hold it level and measure from the level to the bottom of the rafter. Use this same technique to find the rise at a soffit, the bottom of an overhang on the side of the roof that usually is enclosed with wood. Put one end of the level against the wall and measure from the 12-inch mark to the soffit covering.

Use a formula to estimate square footage. Multiply the length of the house by its width and add the amount of overhang at the sides, usually 2 feet. Multiply that by 1.08 for a 5/12 pitch roof and 1.12 for a 6/12, using a table, like one provided online by Service Magic or supplied by roofing material dealers. Get a more accurate figure with one of the online calculating programs mentioned in Step 1 that will figure the square footage based on the roof pitch, width and length.