Estimating roofing jobs is fairly easy and straightforward compared to many others in construction. Because most roofs are visible and accessible, many times all one must do to estimate the cost of a roofing job is to take measurements and apply some simple formulas.
Measure the area of all roof sections on the building that will be roofed. Write these area measurements down and save them.
Investigate the condition of any existing roof materials. Although it is possible to add layers of asphalt shingles, for example, over existing shingles, many building codes do not allow more than two layers. Investigate local codes and determine whether the existing roof materials will need to be removed. If they are to stay, ensure that their condition is such that a new layer of roofing can be added on top of them.
Estimate the cost of roofing materials by the "square," or 100 square foot block. Determine the per square cost of the materials you will be using. Using your measurements of the roof area, determine the number of square feet you will be roofing, and divide by 100. This is the number of squares. Multiply that number by the per square cost of all necessary materials.
Add the cost of your labor to the cost of the roofing materials. Add any overhead, taxes, or markups to that number. Consider any factors that could make your project more difficult, such as very steep pitches, height off the ground, the cost to remove existing roofing materials, etc, and include any necessary charges. Your final estimate is the total of these numbers.