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I'm making a carport that is about 10 feet wide with 2x6's for the "roof frame" and 4x6 posts. I was going to use a medal roof using tin I think. Do I first need to put down wood sheathing? Or can I attach the medal directly to the 2x6's?

I think the UNDERSIDE would look better seeing the medal but I also don't want to have it blow away.

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I would not add plywood under the metal. The only thing that will come with this is the possibility of rot and the loosening of your metal sheet connections. However plywood does offer structural support to your roof so you may have to add a few crosses and make sure you have a good screw schedule on the tin sheets.

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Unless you mean something out of the ordinary, you probably need at least "purlins" to support the metal between the 2x6 rafters. If you could provide a sketch of your proposed framing scheme it might help. With typical rafter spacing that might mean a 2x4 laid flat every 1-3 feet or so (depending on what the roofing metal can "self-support" across, roof loadings, etc.)

Normal metal roofing has no structural strength to speak of if simply attached to rafters in the direction where water drains properly. If attached to rafters in the direction it has strength, it tends to leak because water pools in-between the ribs.

Plywood sheathing can serve the same function as purlins do, but normally costs a bit more.

If, in fact, your proposed framing is one where you are using 2x6 as purlins, then you should not need additional wood.

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You can go with or without wood substrate. If it's a car port and you are using R-Panel, putting metal on purlins is sufficient.

For heated spaces it is hightly recommended to use wood sheeting, ventilate the attic space, and use breathable synthetic underlayment.

Do not use vapor barrier under metal, as it will trap moisture and cause rot.

Good luck

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