How to Calculate the Cost of a Flat Roof

Calculating the cost of a flat roof varies according to the material, size of the roof, the removal of the existing roof, the time of year you are replacing or adding the roof and in what region of the country you're working. Several online calculators can help you calculate the cost of a roof, but you can also work through the steps using simple math.

Learn how to estimate the cost to add a flat roof.
1. Calculate the square footage of the roof. Since the roof is flat, you do not have to worry about adjusting the number for the pitch of the roof. To calculate the square footage, measure the width and the length of the roof and multiply these two numbers together.

2. Identify the type of roofing material you want to use. Some roofing materials include asphalt, steel, aluminum, Spanish tile or copper. Go to your local home improvement store or roof material retailer to price the roof covering material and adhesive for attaching the materials to the roof. Use the square footage of the roof to determine how much roofing material you need to complete the job. You should also consider the cost of tools, such as hammers, rollers for spreading the tar and a saw for cutting metal or tile roof shingles.

3. Verify if you will remove the old roof yourself or hire a contractor to remove it for you. Contact local roofers for estimates on tearing off the roof. Also, verify that they haul away the debris, and include the price of the trash receptacle in the demolition. If you do the work yourself, contact local trash haulers to determine the rental, delivery and takeaway fee for the trash receptacle.

4. Identify who will install the flat roof. Determine if you are going to install the roof yourself or hire a roofing contractor to install it for you. If you decide to do the work then you do not need to consider labor costs. If you need to hire a roofing contractor, then contact local roofers to obtain an estimate on installing the roof. Have them send two estimates: One that includes the cost for materials, and one that only includes the cost of labor, so you can compare the contractor's cost of materials with what you can buy the materials for directly.

5. Add up the costs of the material and labor for a total cost estimate of a flat roof.