Things You'll Need:
- Carpenter's square
- Measuring tape
- Calculator
- Safety harness
- Fall-arrest system
- Ladder
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Step 1
Draw a detailed sketch of the roof layout. Some surfaces will be rectangles or triangles. Others will be a combination of both. Subdivide these combination sections into rectangles and triangles as well.
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Step 2
Put safety first. Never try to work on a roof when it's raining or there is a high wind or chance of lightning. Double check to be sure your ladder is in good condition and securely positioned. Use a fall-arrest and safety harness system. You can buy or rent these at most building supply stores. Finally, stay aware of the time. Don't try to keep working when it starts getting dark in the evening.
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Step 3
Use a measuring tape to find the length and width of each rectangular section. Record these and all your measurements on your sketch. To measure a triangular area, locate the largest angle. Measure a line from that angle to the opposite side--called the base of the triangle. Use a carpenter's square to make sure the line intersects the base at a 90 degree angle. This line is called the height of the triangle. Next, measure the length of the base.
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Step 4
Calculate the square feet in each section. For rectangular areas, simply multiply the length times the width. For triangles use the geometric principle that the area of any triangle is equal to one-half the product of the height times the length of the base. Thus, you multiply the height of each triangle by the length of the base and divide by two.
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Step 5
Total the areas of each section to find the square footage of the roof. Roofing is sold in lots of 100 square feet. Roofers simply call these lots "squares." For example, if you measured a total of 2850 square feet, you'll need to buy 29 squares (2,900 square feet).
