Metal roofing

[8NTX]

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.

[Argee]

I don't see how plywood would add any more lateral than the metal does once you have it fastened. The only thing you would be doing is adding weight to your roof system and losing weight from your wallet. If anything, put 1/2" dow board on prior to the metal panels, then you won't be faced with a sweating underside.

[chevdog]

If you are using 26 ga roofing panels, you don't need the plywood underlayment.
Once the panels are fastened, each roof section actually becomes a unit and has a lot of rigidity.
Most pre-engineered buildings here in the gulf coast use purlins and girts at 4-5 ft spacing with 26 ga R panels. Your setup with 2 ft spacing should be very stout. My barn has trusses at 5 ft, roof purlins at 2' - no plywood is needed.
Ditto on the Dow board, plus it will help knock down some of that radiated heat.

[Pat]

Once again Chevdog hits the nail square on the head.

I am awaiting delivery of a metal building with metal trusses. the manufacturer supplies 29 gauge roofing and walls as standard. I can buy 26 gauge locally just a little cheaper than they would sell me 29. My builder says that a 29 roof will dent if you step between the purlins (2 ft centers). He doesn't profit by switching me to 26 as I am buying the material.

I want the shop/garage/tractor/implement buillding to blend well with the new house so I will be sheeting the walls with OSB and using the same vinyl siding as the house. We are upgrading the roof to standing seam 26 ga with a 30 yr paint guarantee to match the house as well. Ditto the roof pitch (12:12).

Pat

[SteveB]

I've heard more than a few people say that metal siding provides very little lateral (shear) strength to a wall or roof. The argument is that the seams are not staggered like in a plywood roof and the fasteners are only really designed to hold the sheet down. Also, plywood (or OSB) has some significant thickness that prevents buckling. I would think thin metals sheets would be very susceptible to buckling if the wall or roof were subject to any shear loads.

I've noticed that some newer large metal pole buildings strap bracing for lateral bracing. 1" heavy gauge straps are place in large X's on both the sidewalls (over the girts) and on the roof (over the purlins). This provides much greater strength than what you get from the siding. I think strap bracing is really big where building need to be designed for ridiculously high wind loads (like 120MPH in FL). I'm definately going look into it for the next barn I build. Looks cheap and strong [img]/forums/images/icons/smile.gif[/img]

[chevdog]

To get the best of both worlds, add the x-bracing to the standard pole barn framing and you will have a pretty rigid structure.
Our barn had lateral bracing installed at all the corners, but I will be adding additional bracing as well as strapping from the trusses to the poles themselves.