New Roof on Top of Old Shingles
You should never install new shingles over the old shingle roof system. Your roof system will not be eligible for a warranty if you do not remove the existing shingle roof system before installing the new system. Installing over the old shingles can also cause unusual shapes to form in your new roof system and the roof system will not look smooth and level.Rot, Moisture, and Accelerated Roof Aging
If the old roof system is not removed it will not be possible to inspect the plywood deck sheathing for rot or deterioration that can become a much larger problem in the future. Moisture from the previous roof system can be trapped under the new system and the increased surface temperatures from having two layers of shingles can accelerate roof aging.
Avoiding The Inevitable Cost
While at first, it may seem like a more cost effective approach to overlay new shingles on top of old ones. It will be more costly in the long run. Replacing a roof is inevitable, it will have to get done at some point. You aren’t avoiding the cost of a roof replacement. Instead, you’re delaying it and possibly even ignoring foundation issues which could end up costing more when left unsolved.Added Weight
Adding another layer of shingles to your roof also increases the weight on your roof. This puts unnecessary strain on the roofing structure. Chances are, your roof was designed to only carry the weight of one layer of shingles plus extra for snow and safety.
