My roofer is giving me the same cost for each, but suggest manually fastened for my application. I always thought fully adhered would be better. This is an image of the poorly installed metal roof he is replacing with PVC. He is also replacing the shingles around it. Manufacturer is IB Roof Systems
Here is what he says. Can I get your advice?
They are both equally the same to install, same wind uplift specs, same life span etc.
One pro for the fully adhered is it can look smoother since it is glued all the way down.
One pro for the mechanically fastened is their is no glue off gassing. The odor of the glue used for fully adhered can be a slight irritant to some people but the glue cures and off gasses within 1-2 days.
One pro for the mechanically fastened is it allows for some movement of the house structure. This will allow the areas such as the valleys between the low slope and the shingle a little movement move without pulling the membrane. This is the reason I would recommend the mechanically fastened for your application.