When you want to calculate a roof replacement cost you will need to know several pieces of information. The first one is the size of the roof. Asphalt shingle, the most common roofing material in the United States, are sold by the square. For a roof a square is 100 square feet which is an area 10 x 10. Asphalt roof shingles are usually sold in bundles of about 25 shingles and it takes about three bundles to equal a square.
When you know the area of the roof you can figure out how many squares of asphalt shingle are going to be needed to figure the roof replacement cost. This is probably the easiest part of figuring out the roof replacement cost. But you also have to consider the quality of the asphalt shingle your roofer is going to use.
There are different time periods roofing material is guaranteed for and different weights of shingles also. The cheaper the materials the less time the roof will last.
Included in the rest of the replacement roof cost will be the teasing off and haul away from the old roof. This will be in the labor costs.
You will need to have new roofing felt and flashing. The roofing felt comes in different weights so we are using 30 # felt for the base.
When the contractor inspected the roof before he made up a proposal or bid he would have walked the roof. Also you will tell him how long you think the existing roof has been on the structure. He would know if you will require new vents and pipe jacks. If your roof currently has skylights then you may have to consider replacing them.
They also age and if you are having a roof re-roofed then you may as well replace the skylights at the same time. They are not a high dollar item and they will have the flashing installed at the same time as the roof instead of finding out they leak at some future point and have to then replace them.
They may have to replace some gutter or install a new down spout or fix some dry rot or even replace some of the roof decking. All of these add to the cost of the actual asphalt shingle. There are many components in roof replacement cost calculations that cant be known unless you are inspecting the roof.