Placing a layer of shingles on top of an existing layer can save you money. You can eliminate the time and labor involved in tearing off an old roof and avoid the necessity of having to dispose of the old shingles.
Although you can attain significant cost savings from an overlay, ensure that the original roofing has no serious defect before applying another layer. Covering up any roofing deficiencies, such as rotted floorboards, with new shingles simply leads to more costly repairs later.
A second layer of shingles adds to the weight of the roof and can increase the burden of the roofing materials, especially in areas subject to heavy snow. An overlay might also result in a shorter lifespan for the roof, according to the Minnesota Department of Labor and Industry, since older roofing materials are typically replaced when a tear down occurs.
Some types of shingles do not work well when placed over other types. For instance, an overlay with lightweight shingles on top of heavier shingles might look bumpy and unattractive.
Always check with your local building department before starting any roofing work. Most building codes allow for the overlay of one layer of shingles on top of the original layer, but do not allow additional layering after the second one.
Some areas have special situations that disallow any overlays. For instance, the state of Minnesota does not allow overlays in certain counties that are subject to severe hail storms.
Tearing off your old roofing gives you a chance to closely inspect the roof decking for any damage. A tear off can also indicate whether the attic has any moisture problems, enabling you to make needed repairs.
For example, look for sags or spongy areas on your roof. You do not want to put an overlay on a roof with structural problems.
Overlaying works best on roofs in excellent condition that have 3-tab asphalt shingles.