Crafty ........... when, a "conventional" roof is designed. Allowance is always made for windload, snowload, self-weight, everything. I must say that these guys do tend to just bung them up, put in a few fixings to make sure it stays there, and go away.
Before fitting it, they should have made a proper assessment of where the things going, especially whether it's going to be able to take it. They rely on the fact that most customers are lucky, and it stays there. They really should carry the can, as any decent builder would.