A new roof raises the value of your home and may be necessary if damage to the existing roof results in shingle loss or water leakage. In addition, old shingles may lose their ability to seal the roof against moisture. When it's time for a new roof, the current roof's condition is a big factor in determining the final cost.
The condition of your current roof figures prominently in the cost of a new roof. On older homes, the roof may include more than one layer of shingles, resulting in the need to tear off multiple layers. In addition, if the current roof leaks or has leaked in the past, the structure beneath the shingles may require repair before you can install the new shingles, adding to the cost.
Check with your local building authority to determine the maximum number of shingle layers allowed on a roof in your area. Some communities allow the homeowner to install one layer of shingles over an existing layer, thereby reducing the cost of your new roof because no tear-off is required. However, even if your building inspector allows this, a double layer of shingles increased the weight on your roof, which in some cases may be unsafe.
The removal of an existing roof adds to the cost of replacing the shingles. You may rent an industrial dumpster from your local waste authority or haul off the old shingles in the bed of a truck. Your county may charge an additional fee for dumping, since asphalt shingles are a petroleum product. Many communities monitor disposal and charge additional fees in order to comply with environmental regulations.
If the roof structure beneath the old shingles is in good condition, the only cost you will incur is for the purchase of the shingles, roofing felt and the nails needed to install the new shingles. You may rent a roofing stapler or use a regular hammer with roofing nails. If you are hiring someone else to install the roof, you will also have a labour charge.
Shingles are installed by the square. In roofing talk, that means 100 square ft. Measure the area of your roof and divide by 100 to determine how many squares you will need to install. Purchase extra shingles to allow for mistakes and trimming that will occur as you lap the shingles at the edge of the roof. A good rule of thumb is to purchase an additional 1/4 square for every 4 squares you install (see Resources below).
Unfortunately, until you remove the current shingles, you can't determine the extent of any damage that may be present beneath. If you have to repair structural beams or the wood sheeting that covers the roof, your cost will increase substantially.