The amount of siding that a garage will need is measured in square feet. It is not a difficult task to accomplish for a simple garage. The best way to work up the square footage of siding needed is to work on one wall at a time. Be sure to write down your area for each wall before measuring the next wall. Once you have all of the walls done then just add up your calculations.
Measure the length of one side of the garage. Write this dimension down on a piece of paper. Even if your garage is approximately 24 feet on each side, measure the exact length to accurately calculate how much siding you will need.
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Measure the height of the same wall of the garage. Write this down next to the length.
Multiply the two dimensions together to determine the area of the wall. For example, a 24-feet-long side that is 8 feet high equals 192 square feet of siding for that wall.
Subtract any openings that are in the wall you measured, such as windows and side doors. A typical doorway is 3 feet by 7 feet, and that would subtract 21 square feet from the total amount of siding needed for that wall from 192 to 171 square feet.
Measure the remaining walls. Add up the total to determine how much siding you will need.
If the garage has gable ends, measure each one to determine how much siding you will need. Measure from the very peak down to the ground. Subtract the wall height that you already measured for the other walls. In our example that was 8 feedt. So if the total height to the top of the peak was 18 feet, you would start with a gable height of 10 feet.
Calculate the area of the gable using the formula for measuring the area of a triangle: multiply the base by the height and divide it in half. For our example, multiply 24 feet (the base) by 10 feet (height), to get 120 square feet, and divide by two to get 60 square feet. Add the gable areas to your total.
Be sure to check with the material supplier to determine any waste factor needed. A waste factor is a percentage of additional material that will need to be added for cutoffs, trim work, mistakes, etc. For example, if you buy 10-feet-long pieces of siding, you will need to cut pieces to fit your 24-feet-wide walls. Straight cuts can be reused elsewhere, but angled cuts to fit gable lines may need to be discarded.
A typical waste factor is 10 percent. Take your total wall area, multiply by 0.1, and add that number to your total.