Some homeowners or contractors will install a new layer of shingles over the old one to save time or money. Most roofing contractors do not recommend this for several reasons. In some areas building codes don’t allow this. The homeowner or contractor should always check with the city, township, etc. before installing shingles over old ones. By not removing the old shingles, the contractor cannot see if there are other problems underneath such as rotten or missing wood. Removing the old layer also allows access to install new protective membranes to keep the roof watertight. Shingles installed on each other often tend to buckle.
Doubling the roofing layer adds significant weight that the roof structure may not be designed to withstand. While not removing the old layer may save time and money initially, the lifespan of the roof is drastically reduced and will need to be replaced sooner. Most roofing manufacturers will not cover warranty claims when their materials are installed over old.
Craig Gouker Roofing has been installing quality residential roofing in South East Pittsburgh since 1991 and is the expert in shingle roof replacement.