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?
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.
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.