Can you put a new roof over an old one, and should you?
In most cases a new layer of shingles can be installed over one existing layer, but a full tear-off is usually the better choice. A layover hides the decking, can shorten the life of the new shingles, and has to be removed at the next replacement, which adds to the cost of that job.
What a layover is
A layover, also called an overlay, is a new layer of shingles installed on top of the existing roof. Because nothing is removed, it costs less and takes less time than a tear-off.
Building codes generally allow no more than two layers of shingles on a roof.
The downsides of a layover
The decking cannot be inspected, so damaged or soft decking stays hidden under the new roof.
The new shingles follow the shape of the old ones, so the roof may not lie as flat.
Two layers of shingles hold more heat, which can shorten the life of both layers.
At the next replacement, both layers have to be removed, which increases labor and disposal costs.
How to tell if your roof has two layers
Look at the edge of the roof along a gable end. Two layers of shingles look noticeably thicker than one.
A roofing contractor can tell during an inspection. Because it affects the cost of a replacement, it should be noted in your estimate.
What a tear-off includes
A tear-off removes the old shingles and underlayment down to the roof deck. The decking is then checked, and any damaged sections are replaced before the new roof is installed.
A tear-off is the only time the decking can be fully inspected, which is one of the main reasons it is recommended.
Related questions
Is a layover ever a good idea?
It can make sense on a sound, single-layer roof when the decking is known to be in good condition and a lower-cost option is needed. It is best decided after a roof inspection.
How much does decking replacement add?
Decking is usually priced by the sheet, and the number of sheets is not known until the old roof is removed. Ask for the per-sheet price in your estimate.
Will insurance pay to remove a second layer?
It depends on your insurance company. Ask your insurer before the work starts.