Below is an apartment building we did in Lakewood. You will see an example of the nesting technique.
Residential roofing 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:
1. You cannot afford to remove the existing roofing.
2. The existing layer of roofing is extremely flat. If the existing roofing is not flat the bumps will show through the new roofing and will contribute to sideways water movement, which will result in a leak.
To install a second layer of asphalt shingle 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.