Homeowners who have two layers of shingles must remove them before adding new ones, as the International Residential Code prohibits a third layer, as of 2015. Homeowners with one layer of shingles can place more over them; however, they may realize more long-term cost savings by removing the old layer.
While a single shingle does not weigh much, when multiplied by the number on an entire roof, the weight is very heavy and can negatively affect the structure of homes. People who lay shingles over an existing layer risk them blowing off in high winds, as shingles stay more firmly attached to the roof deck than to other shingles. Removing a layer of shingles also gives homeowners the opportunity to examine roof structures before shingling again.