The collectors don't don't mount to the roof, they mount to racks which mount to plates that are secured to the roof. There are products designed that can be screwed to the rafters and the screws pull butyl sealant pads into the holes that seals it up and are guarenteed to not leak.
Would it hurt to have a roofer on the job to install them? Probably not, but there are products out there that are proven that can be used. Of course all of this depends on your location and roofing type.
With that said, I do work with a roofer for all my penetrations. I do not like the idea of being responsible with the roof.
