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I plan on building a wood shed with a corrugated metal roof. My question is do I need some sort of CDX plywood or osb under the metal. Or can I attatch the metal right to the rafters.

I have seen it both ways.

Yanita

Jim, the only problem when you use bare steel is condensation. You wouldn't think that a sunny morning could cause the problem, but it does. My deck roof has a bare steel spot that will drip. Greg

Jim,,did You mean to say "wooden shed"? If not;

Generally purlons then steel on top.A woodshed is likely to be open and airy so dunno that condensation would be a problem. However leaks or condensation trapped between sheeting layers could be bad for either or both layers.

Personally I would prefer either type of water to drip on My cordwood than to be trapped between sheeting.

Dean

Dean a layer of the pink 1/4 inch dow fanfold Styrofoam over the purling and under the steel does a nice job of reducing and channeling the condensation to the bottom plus keeps steel from transmitting heat in summer and cold and winter to inside of shed I just overlapped bottom to top with it when done my roof and screwed steel down haven't had a leak make it in since as styro is water proof if steel leaks the water either has to go to bottom or is trappped between and summer heat evaporates it.

I did mean wood shed for fire wood, and yes it will be open.

Since it only sheds the rain then I would be more concerned with any water getting to the floor,,unless You just leave it dirt,use gravel or pour concrete..Gravel would drain great when the storms come and blow rain in,just gotta contain it round the edges.

Dean

Hello,

Even in an open type cover the bare steel can condensate under the right conditions.

Digital Dreams is spot on regarding the insulation. There are some companies who make a foil faced rolled product specifically for Metal roof under layment.

If you decide to install decking than just felt paper underneath is fine. But if you go open rafter you do need the purlins so that all your seams have backbone and it will make your screw pattern alignment easier. Purlins 2' oc should be fine. Unless its low pitch & your in a heavy snow area go 16" or sheathing.

Good Luck

Rod

I can see where a thin insulation wich breathes might be a help,especially if it breathes fast enough to counter condensation or small leaks..I've installed some fanfold under siding and it does have tiny holes in it and reportedly breathes like tyvek. As I understand it these holes are meant for water vapor to pass but not actual water.

I also know that,whatever is done,if moisture is trapped between 2 layers of anything,eventually it will do it's work,such as rusted metal or ruined wood sheething.

For a woodshed I would lay the steel on the purlons and pay close attention to the screw/nail gaskets(rubber washers)during installation and let the wind dry any condensate or eventual small leaks underneath,,in a tighter/heated enclosure I might well think differently.

I'm not a bonafide roofer tho,just an old drywaller. I bow to those more experienced and hope it all goes well in the longrun.

Dean