Without proper ventilation your roof will “sweat”, causing moisture to form and possibly damage decking or lathing or in the worst scenario cause damage to the interior of a home.

If metal is exposed to standing water for a prolonged period of time it is possible for the paint to begin to bubble then peel thus causing the metal underneath to begin to rust.

Cutting metal roofing with a power saw, even with the blade turned backwards, heats up and sears the protective coating on the cut edge thus allowing rust to begin.

The best way to cut metal roofing is with an approved set of metal cutting shears. There are many types available including hand snips and power shears.

Weather changes from hot to cold and metal roofing expands and contracts accordingly. This will case the metal to rub against the asphalt surface of the shingle, eventually wearing away the paint on the underside of the panel allowing rust to form. Given enough time, it will rust a hole through the metal panel.

Treated lumber contains a chemical to “wolmanize” the wood which is corrosive to metal products.

Yes you can. Using an untreated wood 1×4 lathe would be fine. You will need to be mindful of only stepping where there is support under the metal to keep it from buckling.