For those upgrading an old asphalt shingle roof to metal panels, there may be the option of leaving the old roof in place instead of having it removed. There are some benefits as well as some concerns when it comes to this installation method. 

Benefits

Materials and labor make up the bulk cost of a metal roof installation. Leaving the old shingles in place eradicates the need for installing new underlayment, as these shingles take the place of the underlayment. Less labor is also needed since there is no need to spend time tearing off the old roof and installing new underlayment. This can shave quite a bit off both the cost and installation time.

There are a few other benefits. One cost-saving benefit is that less material will need to be disposed of, so this offsets both hauling and landfill fees. One could also argue there are environmental benefits since the old roof will be recycled into the new roof's underlayment as opposed to ending up in the waste stream.

Concerns

Of course, there are a few concerns to consider before going forward with this option. Foremost, one must verify that it is allowed within the local building codes, as some municipalities will not approve an inspection on a roof overlay. Only a teardown and new underlayment are permissible. 

The old roof must also be in good condition. A few minor leaks or worn shingles are fine, as these can be patched quickly, but extensive damage or aged shingles that are deteriorating will compromise the quality of the new metal roof. The worst-case scenario is that you will have to take off the panels and rip off the old shingles in the future, which will negate any cost or time savings that were realized during the original metal roof installation.

Process

The process begins similar to a standard metal roof installation. The old roof and its supporting structures are inspected thoroughly. If no major damage is found and the current support structure is up to code, then small repairs will be made to the old roof to prepare it for the metal roof installation.

Ventilation is another concern, so most roofers will install furring strips over the old roof. The metal roof panels are installed onto these furring strips. This type of installation allows room for airflow, which aids in ventilation and prevents condensation issues beneath the metal roofing. 

Contact a metal roofing service if you need more guidance on installation methods.