Hip Roof Calculations

What Is A Hip Roof?

• A hip roof is a roof constructed of four sloped planes all of the same pitch. The sides of a hip roof are sometimes triangular and other times trapezoidal. To calculate the area of a hip roof, you'll need to get on the roof and take some measurements. On sides of the roof that are triangular, you'll need to measure the width of the bottom and the height from the center of the triangle. For trapezoidal sides of the roof, you'll need to measure the length of the two sides running parallel to the ground, and the height.

Calculating the Area

• After you've taken the measurements, the only thing you need to calculate the square footage of your roof is a basic understanding of geometry, a pencil and paper, and maybe a calculator. For triangular sides of your roof, multiply the height by the width, and divide by two. For trapezoidal sides, add together the length of the two sides running parallel to the ground, then multiply this measurement by the height and divide by two. Once you've determined the square footage for each roof face, add them together. This is the total square footage of your roof. To help you catch mistakes, do all your calculations twice.

Squares

• Roofing materials use a metric called "squares." There are 100 square feet in one square. To find the total number of squares of your roof, start with the total square footage and divide by 100. Shingles are sold by the bundle, and on average three bundles of shingles will cover one square. The exact number of bundles per square will vary according to the thickness of the shingles, so once you've picked out the type of shingles you want, check the manufacturer's details to find out how many bundles of shingles will equal one square.

Minimum Number of Bundles and Rolls

• To determine the minimum number of bundles required to cover your roof, multiply the total number of squares of your roof by the number of bundles required to cover a single square. In addition to shingles, you will need to purchase felt paper to cover the deck of the roof. Use a similar calculation to determine the minimum rolls of felt paper that you will need; multiply the total number of squares on the roof by the number of rolls of felt paper required to cover a single square. The number of rolls of felt paper required to cover a single square will depend on the weight of the felt paper. Check the manufacturer's details for this information.

Overage

• You will make mistakes during your roofing project. To ensure that you have enough material to cover your roof and accommodate material loss as the result of mistakes, plan to purchase 10 percent more materials than the minimum necessary to cover your roof. To calculate this amount, multiply the minimum number of bundles needed and multiply by 0.1. Then, add the product of this equation to the minimum number of bundles. For example, if you have determined that the minimum number of bundles you will need to cover your roof is 20, then multiply 20 by 0.1. The product of this equation is 2. Add 20 bundles plus 2 extra bundles, for a total number of 22 bundles. This equation would look something like: (minimum number of bundles x 0.1) + minimum number of bundles = total number of bundles. Use this same method to calculate how much felt paper you should purchase.

Determining Pitch

• At some point, you will probably need to determine the pitch of your roof. The pitch, or slope, is the angle at which the roof rises over the house. The pitch is represented with a pair of numbers that might look like this: 4/12. This number means that the roof rises 4 inches into the air for every horizontal foot. The higher the first number in a pitch, the steeper the slope of the roof. For the most accurate measurement, you can purchase tools at home improvement centers specifically designed to measure roof pitch. If you have no such tool, hold a T-square on its side with the horizontal end of the square resting on the roof and the leg of the square rising vertically into the air. Hold a level against the leg of the square, and rest the bottom end of the level against the roof. Move the T-square and level around the roof together until the level reads plumb. Use a marker to record the spot on the level where the joint of the T-square meets the level, then measure from this point to the end of the level. Divide this number by two -- this is the first number of the pitch.