Shingle Removal
New Shingle Installation
Roof with multiple layers of shingle
Are two layers of shingles acceptable?
As a Home Inspector, I am often asked “if there are more than one layer of shingles on the roof and are multiple layers okay? Sometimes it is not possible to determine if two layers of shingles are in place because there are ways to install the second layer and conceal the layer below. The layer below can be cut back in layers around the edge of the roof or the drip edge can be installed over the covered layer/s.
Some reasons your home should not have multiple layers are: Shingle manufacturers will not provide a warranty if their product is applied over old roofing material. Shingles have an adhesive on them that seals the shingles together, and the manufacturer requires designated materials to be in place below the shingles to accomplish this, as sun and heat take effect. If shingles do not seal adequately they can be susceptible to high winds that will disengage the shingles and tear them off. Contractors applying over existing layer may take other short cuts such as not installing long enough nails thus reducing holding power. Multiple layers will allow additional heat to build up and cause premature breakdown of materials.
Obviously the additional expense of removing existing shingles and properly prepping the surface is important. Shingles may adequately perform their intended function in some cases when a second layer is installed, however, one layer is best and three layers is never recommended by Building Sciences 16:3. If you have multiple layers on your roof now and the structure is supporting the additional weight it may be best to allow the existing shingles to stand and replace them when they are at the end of their life expectancy. Oh FYI, let your contractor know he will need to remove two layers of roofing. Inspection and Direction 4 a Happy Home by Building Sciences 16:3