Why Shingle Overlays Are Not Ideal

Placing a new layer of asphalt shingles over existing ones, particularly when there is notable wood rot, moisture damage, or mold on the underlying plywood, or when the existing shingles are dimensional, is not advisable for several reasons:

• Structural Concerns: Wood rot and moisture damage weaken the structural integrity of the roof. Adding a new layer of shingles adds weight without addressing the underlying issues, potentially leading to structural failure.
• Moisture Problems: Overlaying shingles over a damaged layer can trap moisture, exacerbating wood rot, mold, and mildew problems. This can lead to further damage and costly repairs AND potential health problems.
• Shortened Lifespan: The new shingles’ lifespan may be shortened as they cannot adhere properly to a damaged or uneven surface. Dimensional shingles already have a varied surface, making proper adhesion of a second layer more challenging. Only 50% of the new shingles will actually adhere to the underlying layer of shingles. You will have uneven nail application and the likelihood of nail pops is significant. Also, the original shingle layer will become super-heated and can “bake” the plywood below.
• Warranty Issues: Manufacturers often void the warranty on shingles if they are installed over an existing layer of shingles, especially if the underlying layer is compromised, deteriorating, dimensional, or covered in moss/algae.
• Inspection and Repair Issues: Overlaying shingles makes it difficult to inspect the roof deck and underlying structures for damage. Future repairs may require removing multiple layers of shingles, increasing labor and disposal costs.
• Building Code Violations: Many local building codes limit the number of shingle layers on a roof, typically to two. Installing more than this can violate codes and affect insurance or the sale of the property. 

For these reasons, it’s best practice to remove the old layer of shingles, repair any structural or moisture damage, and then proceed with a new roof installation. This approach ensures the longevity and performance of the roofing system.

If your desire to do an overlay is to save money, we definitely understand that concern but it will cost more money in the long run. We offer financing to homeowners who have these concerns. Information can be found here: mavennj.com/financing