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Re-roofing over old roof boards

Paul McKevitt | Posted in General Questions on

Hi,

I’m planning on re-roofing an older cabin (in zone 5) that has various sized roof boards with gaps under old layers of shingles.

My question is, should I put a layer of OSB (taped at seams) on top of the old roof boards to make it air tight, or can the membrane go directly on the old boards, or do I have to rip off all the old boards to the rafters and then put new sheeting?

The planned set up from top down, is as follows:

Standing seam metal roof
Structural OSB, or 1×4 purlins attached to 2×4’s on the flat which lays on top of two layers of 2″ rigid foam, offset and taped, (outside layer of XPS over layer of Poyiso) secured to the rafters below, to act as an exterior vent to ridge on a 6/12 pitch cathedral roof.

Airtight membrane on roof sheeting.

2×6 rafters below with Roxul Rock wool R23 24” spacing, then IntelloPlus as an air barrier.
Painted horizontal lap boarding on walls and ceiling.

Thanks,
Paul

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Paul,
    If you want to re-roof an old roof with existing board sheathing and existing asphalt shingles, the first step is to strip off the old shingles and inspect the boards. If the boards are sound, and the gaps aren't too bad, you can certainly reuse the boards -- perhaps after re-nailing some of them.

    Homes with a vented unconditioned attic don't need airtight roof sheathing. But in your case, you are planning to have an insulated sloped roof assembly (rather than a roof over a vented attic) -- an assembly that requires a tight air barrier at the sheathing layer. In theory, you could create this air barrier by installing a taped European membrane, synthetic roofing underlayment with taped seams, or a layer of OSB or plywood sheathing with taped seams.

    I think that it makes the most sense to install new plywood or OSB over the boards, taping the seams of the new sheathing, because you'll end up with a more durable air barrier and a nice new surface to work on.

    This new layer of sheathing has nothing to do with the condition of the boards; it's simply a durable air barrier. That approach makes more sense than trying to install a European membrane or synthetic roofing underlayment over the old boards, and hoping that the membrane is durable enough to be your air barrier.

  2. Steve Knapp CZ 3A Georgia | | #2

    Martin,

    It looks like Paul is planning a metal roof. For his climate, would it make sense to use nailbase to create a cold roof assembly?

  3. GBA Editor
    Martin Holladay | | #3

    Steve,
    As my article on this topic (How to Build an Insulated Cathedral Ceiling) explains, there are a great many ways to detail a sloped insulated roof assembly.

    The assembly can be vented or unvented.

    The assembly can include rigid foam above the roof sheathing if desired, or it can be built without any rigid foam.

    If the builder wants to use rigid foam, it's possible to install ordinary rigid foam (perhaps recycled foam), and it's also possible to use nailbase.

    If you use nailbase, you can choose unventilated nailbase, or nailbase with a ventilation channel built right in.

    All of these options, including the nailbase option, are mentioned in my article, How to Build an Insulated Cathedral Ceiling.

  4. Paul McKevitt | | #4

    Thanks Martin, that makes sense.

    I realize I have another roof problem, I was hoping you could shine some light on.

    There is a small "bump-out" (8' x 14") with low slope shed roof attached to the side of the cabin, which I was planning on just insulating with the rigid foam (vented same as on Gable roof) on the outside only and no insulation on the inside which would leave the existing rough pine roof boards exposed (rustic look).

    My problem is that unlike the insulated walls, I can't air seal that roof section on the inside because of the exposed boards and rafters. Is there any other way of sealing it, maybe from the outside before I put down the new roofing material?

    Thanks,
    Paul

  5. GBA Editor
    Martin Holladay | | #5

    Paul,
    The answer can be found in my Answer #1: "You could create this air barrier by installing a taped European membrane, synthetic roofing underlayment with taped seams, or a layer of OSB or plywood sheathing with taped seams." I didn't mention it, but you could use a peel-and-stick product like Grace Ice & Water Shield if you want (although some green builders are worried that peel-and-stick products might have an objectionable odor).

    For more information, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

  6. Paul McKevitt | | #6

    Thanks Martin.

    I understood your answer on the gable roof, which is sealed tight on the interior walls and cathedral ceiling, but I guess my worry was not having a membrane or air seal on the interior of just the "bump-out" section of roof, possibly letting warm air raise up through the exposed roof boards.

    So as long as I do the air seal on the exterior of this section as you described , I should be good?

  7. GBA Editor
    Martin Holladay | | #7

    Paul,
    Any air barrier needs to be continuous. The materials I describe will work, but of course your ceiling air barrier needs to connect to your wall air barrier. If you have cracks or leaks at the wall/ceiling intersection, all bets are off -- no matter what type of ceiling air barrier you choose.

  8. Paul McKevitt | | #8

    Got it.

    Thanks again Martin

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