Step 1
Find the ground area covered by the roof. For example, if your home is 30 feet wide by 60 feet long, the ground area covered is 1,800 square feet, equal to the home's perimeter. Add any roof overhang. If the roof overhangs by 2 feet all around, the above measurements become 34 feet wide by 64 feet long and the ground coverage would total 2,176 square feet.
Step 2
Divide the ground coverage by 100 to determine the number of roofing "squares" needed for a flat roof without a peak. Roofing squares are 100-square-foot measurements used to simplify the task of measuring a roof. Typically, roofing materials such as shingles are sold in groups of 300-square-foot measurements or three squares per bundle. For simplicity, if your ground coverage equals 1,800 square feet, you would calculate that you need to replace eighteen roofing squares or six bundles.
Step 3
Factor the roof’s pitch. For a roof with a peak or gable ends, the amount of roofing squares will increase depending on the sloping grade. For a rough estimate, use the multiples of 1.15 to 1.25 for a low pitch roof, 1.25 to 1.4 for a medium pitch roof and 1.41 to 1.7 for a steep roof, suggests New England Metal Roof. For instance, if you have a fairly low pitch roof -- one that isn’t too steep to walk on -- and have determined that you need eighteen roofing squares for ground coverage, you might choose to calculate the slope at 1.25. By multiplying 18 by 1.25, the total estimation for roofing squares would now be 22.5.
Step 4
Calculate the determined number of roofing squares by the cost of tear off. If your roofer charges $150 per roofing square for tear off, cleanup and disposal of a basic, low pitch roof and you have determined a measurement of 22.5 squares, the cost estimation will be $3,375. Speak with your roofing expert regarding the cost per square depending on additional roof cuts, angles, valleys or architectural features.