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I'm building a 14x16 tiny house and doing a metal roof. It's a slanted/skillion roof and the rafters are running down from the tall end to the lower end on 16" centers with no cross braces. I'm questioning whether I need to add 2x4s in between all of the rafters or not. The OSB won't be a problem because 4' will always break in the middle of a rafter, but I don't think the metal will always break directly on top of the rafters where I need to screw. Is it okay if some of the screws going into the metal just go through the OSB? Will it have adequate holding power? And should I be concerned about leaking? The rubber for the screws should obviously stop leaking but I'm curious if it's normal to do it this way or not.

Thanks.

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No, you do not need to screw into solid lumber (joists, stripping,etc.).

You’ll want to follow the manufacturer’s installation instructions, but here is a major metal roofing manufacturer’ installation chart for screws into plywood, OSB board, and 2 x lumber for wind uplift, which is on page 14:

http://pdf.lowes.com/installationguides/716702706866_install.pdf

As you can see, the thinnest material they recommend is 1/2” (7/16”) OSB board at 9” on center up to 24” on center.

Be sure to follow their instructions in order to obtain the warranty.

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  • Lee, OSB is commonly sold in both 7/16" and 1/2" thicknesses. I'd recommend the 1/2" for any roof. I mention it because you seem to imply that 7/16 is nominally half inch, but it's a separate size more commonly used for walls (and it's odd that they're not both listed in that chart). Half inch is noticeably thicker. – isherwood Mar 12 at 3:18
  • @isherwood I’ve seen 1/2” particleboard and 1/2” MDF, but I’ve never heard of 1/2” OSB. Who sells that? It’s not available where I live. – Lee Sam Mar 12 at 5:08
  • There are side-by-side stacks at every HD, Lowe's, and Menard's in my entire region. Preferences vary, I guess. No one uses 7/16 around here for roofs, possibly due to snow loads. After walking on 7/16 when building walls I'd never use it on a roof regardless of snow potential. – isherwood Mar 12 at 12:57
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Metal roofing screws need to go into framing.

I guess at 14x16 you won't be towing this one, but the wind still blows, and OSB does not have adequate holding power to resist the wind trying to rip the metal roof sheeting off.

A potentially easier option is to use purlins (on top of the rafters) rather than framing between the rafters. That's common for metal roofing; it does create some additional work just at the edges to close between the purlins, but that's much less than cutting cross bits between every rafter.

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  • What if I screw the metal to all of the rafters and around the outside perimeter of the roof? I'm just worried about where the metal breaks and how to fasten that down without adding the potential for leaks. But as far as purlins go, I would put those over the OSB right? – Mark Mar 12 at 0:55
  • Metal roof screws have very aggressive threads, and OSB holds a screw as well as solid lumber (though obviously it's thinner). Any wind that pulls them out will do much more than that. I wouldn't hesitate to do this. – isherwood Mar 12 at 3:15

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