Is an old asphalt roof normally torn off before replacing with a metal roof?

I have a cabin and am considering replacing the old worn asphalt roof with a metal one. Is it normally necessary to tear off the old roofing to install a metal roof? And is this within the scope of work you do? Has anyone installed metal roofing in this area, and have references/addresses of jobs like this I could see? Thank you.

Yes, It would need to be

You absolutely have to remove the asphalt shingles; It should be in the scope of the work.
I’m not from PA, but i would recommend you first find the desired product, and use the company’s website to find a certified applicator in your area.

best regards,

Sam