Installing a new roof over shingles takes less time than tearing off the old roof.
How to install a roof over existing shingles.
Manufacturers will still.
Some roofers install a layer of waterproof shingle underlayment wsu over the existing shingles at the eave end as would be done for a roof laid on bare sheathing.
This provides added protection against ice dams.
Installing a new roof over an existing one also called a layover or nailover is often possible although not always the best option a properly installed layover roof can look and function as well.