The codes allow your roof to have a grand total of two layers of roofing. Sometimes going over the top of your existing roofing is a cost saving alternative compared to removing the existing roofing and then installing new roofing. Installing a second layer is only advisable if, one, you cannot afford to remove the existing roofing, and two, the existing one layer of roofing is extremely flat. If the existing roofing is not flat, the bumps will show through the new roofing and will dramatically reduce the life of it.

To install a second layer of roofing a special technique called "nesting" must be employed. "Nesting" is when you install the tops of the new shingles in line with the bottom of the old shingles.