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What do I do next on my roof?

user-6514688 | Posted in General Questions on

Question on roof insulation and venting.

I’ve read a plethora of articles on GBA and other sources and I’m still scratching my head on what to do next.

I have a tiny home that I’m building that’s 16’x36′ with the long sides facing east/west. The west side has walls at 10′ and the east are at 8′. All wall framing is 2×6 lumber and the roof is a flat 2x8x26′ low slope structure. The roof extends off the west side by 7′ and 3′ off the east side. All framing/roofing is 24″ OC. The roof trusses have blocking and have been tied to walls with hurricane strapping.

My question pertains to the roofing. First, I’m in the North Dallas (climate zone 3a) area. 2nd, I do not for see having any penetrations through the roof to avoid potential leaks. Venting for plumbing will go out through a wall.

Here’s what I’d like to do:

1st layer – radiant barrier OSB, all seams taped, drip edge installed.
2nd layer – vapor barrier
3rd layer – 2 layers of 2″ rigid foam, alternating seams and taped
4 layer – water proof peel and stick membrane. recommendations?
5th layer – 1×4 stringers @ 24″ OC
6th layer – hidden attachment 2″ standing seam metal roofing

Interior – between trusses will be open cell spay foam and then 1/2″ sheet rock.

Do I need to vent the soffets? I’d like to keep them unvented if possible. What am I missing? what should I do different? I’m a novice weekend warrior that’s been build and remodeling for 20+ years, but am now trying to do it ‘right’.

Please help me ask the questions I’m not. Thanks GBA team!

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  1. GBA Editor
    Martin Holladay | | #1

    It doesn't make any sense to spend extra money for radiant-barrier OSB unless the radiant barrier faces an air space. Since you plan to install open-cell spray foam on the underside of your roof sheathing, all you need is ordinary OSB. (Without the air space, there is no benefit whatsoever to the radiant barrier.)

    You don't need a vapor barrier (2nd layer). The two layers of rigid foam are already an adequate vapor retarder.

    At this location, you would ordinarily need an air barrier, not a vapor barrier. However, your open-cell spray foam is already an air barrier -- so the rigid foam can go directly on top of the OSB. For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

    For your 4th layer, peel-and-stick membrane is overkill. Ordinary roofing underlayment (for example, asphalt felt) is all that is necessary at this layer.

    There is no need for soffit vents or a ridge vent with this assembly.

  2. user-6514688 | | #2

    Thank you Martin. This is exactly the guidance I was hoping for.

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