I've got a tractor barn, well house, and carport at our weekend place in the country with open, metal roofing. Just metal roofing panels set on perlins, on top of rafters or pre-engineered trusses. Next year (hopefully), we will have a pole barn built with the same basic construction. Right now, I am having an open-sided boathouse built. A contractor has pounded the pipe, and I plan to add the roof and decking. He has recommended a plyroof underlayment for the metal roof, to add strength. I'm thinking that I don't really need it. The trusses will be set 2' O.C., with 2"x4" perlins also at 2' O.C. If no plywood decking is good enough for the tractor barn, carport, and pole barn, why is the contractor recommending a plywood deck for the boathouse roof? We will be using dark green R channel panels, that would transfer a lot of heat to the decking anyways, so perlins would probably be wise in any case. I would appreciate your thoughts.