# How to Calculate Gravel for a Driveway

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Roof repairs are a big project. However, finding the square footage of a roof isn't hard as long as you keep safety foremost in your mind. Carelessness can lead to a serious fall and injury. Roofs are designed in all sorts of ways, but nearly all are composed of a combination of rectangles and triangles. Once you know how to identify the sections of a particular roof, you'll be able to find the square footage without much difficulty.

### Things You'll Need

• Safety harness
• Fall-arrest system
• Measuring tape
• Carpenter's square
• Calculator
• Take the proper safety precautions. Make sure your ladder is in good condition and position it so it is resting securely on a firm surface. Buy or rent a safety harness and fall-arrest device and follow the instructions for proper use. Work only in full daylight and never try to do any work on a roof in rainy or windy weather or if there is a chance of lightning.

• Make a sketch of the roof showing each section. You'll see the roof is composed of triangles, rectangles and sections that are combinations of the two. Divide the combination sections into triangles and rectangles.

• Measure the length and width of each rectangular section. For triangular sections, start your measurement at the largest angle and measure to the facing side of the triangle (the side opposite the largest angle). Use a carpenter's square to make sure you measure a line that intersects the facing side at a 90-degree angle. Finally, measure the length of his facing side.

• Multiply the length and width of each rectangular section to find the square feet in that section. The area of a triangle is always equal to one-half of the length of the facing side multiplied by the distance from the facing side to the angle--called the height of the triangle. So multiply the length of the facing side times the height and divide by 2 to get the square footage of the triangle.

• Add up the square feet in each section to find the total square footage of the roof. Next, convert the square footage into roofer's squares. Roofers use a standard unit called a square that is equal to 100 square feet. This is also how roofing material is sold. To convert square feet to squares, just round up to the nearest 100, then divide by 100. For example, if you measured a roof at 2,520 square feet, round up to 2600. Divide by 100 for the number of squares (26 in this example).

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