Metal roof panel (corragated) over a shingled roof

Curious for other opinions. I’ve seen a couple roofs recently where a metal roof has been installed directly on top of an old shingled roof. Does this in anyway affect the performance or life span of the metal?

Obviously shingle over shingle drastically affects the new shingle’s performance and and life span but metal over shingle?

OK ! Done all the time.

Yep, actually a good idea

Better doing it that way then in reverse .


I would think it would depend on how many layers of shingles there are under the metal roofing and how long the screws/nails are that are used and how good the sub-straight, being attached to, is at the bottom of all that roofing.

Most jurisdictions here in California,a reroof requires a permit and inspection.

In Ontario any cowboy with a roofing hammer can install roofs. A word of caution about metal roofs. The so called life time warranty applies only to installations completed by factory warranted installers. Ordinary roofers and do-it-yourselfers are not convered.

Roy, that is awesome.

Thanks …
No matter how long you are at this industry some thing different comes along .

Always follow the metal roofing manufacturer’s installation instructions.

Most that we inspected where not installed correctly

It has been said that as the metal expands and contracts the asphalt will wear the protective coating under the metal panel causing panels to starts to rust from underneath.

It is done all the time, but not recommended for several reasons

  • Expansion/contraction can wear at the underside of the metal (not an issue if installed over strapping)
  • Weight
  • Improper underlay/flashing (existing flashing are typically removed)
  • Unknown condition of decking (deterioration due to leaks)

Discussed here; :slight_smile: