Before undertaking something as big as a roofing job, it is important to be able to accurately calculate the materials that will be needed for the job. The major material concern for a roofing job are the shingles and corresponding ridge caps. There is a method to calculate how many you will need so that project goes smoothly and without shortages or waste.
Get up on the roof and measure each rectangular area of the roof. While you are up there, also measure the length of all roof lines that will need ridge caps and save this cumulative linear measurement for Step 4. Simple gable roofs will only have one large rectangle on each side of the roof. More complex roofs can be thought of as many different rectangles. Calculate the area in square feet of each rectangle by multiplying its length times its width. Add all of the individual areas for the rectangles together to get the total are of the roof in square feet.
Find out how many bundles there are per square for the type of shingles you will be using. A square is the amount of shingles necessary to cover 100 square feet. Most shingles come three bundles per square, and have 29 shingles per bundle.
Take the total roof area you came up with in Step 1 and divide it by 100. This will give you the number of squares of shingles you will need. Multiply this number by the number of bundles per square for your shingle (from Step 2). You now have the number of bundles of standard shingles you need.
Calculate the number of ridge caps needed based on the following conversion factor: 1 bundle of shingles = 35 linear feet of ridge caps. Take the linear ridge cap measurement you have from Step 1 and divide it by 35. This is the number of bundles you will need for ridge caps.
Add the bundle numbers calculated for standard shingles in Step 3 and ridge cap shingles in Step 4. This is the total number of bundles you will need for your roofing job.