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Community and Q&A

Does tarped roof sheathing get wet and need to be dried out before applying membrane?

FixitAgainSam | Posted in General Questions on

Hi.  I am reroofing a low-slope (2.5:1) shed or lean-to roof over an addition, generally following the principles laid out in Martin’s articles on unvented cathedral ceilings.  Although I did decide to cut and cobble, as the inspector wanted a vapor barrier anyway, and I’ll use a smart barrier.  I put new 1/2 inch exposure 1 plywood sheathing on, tarped it with a polyethylene tarp clamped at the edges and loose at the eave, but with a few boards on it to hold it down.  My plan now is to cover the plywood with ice & water barrier, then 1/2 or 3/4 inch of EPS, then another layer of plywood, and use a self-adhering 2-ply asphalt system such as GAF Liberty.

So here’s the thing – I have been working on the inside for a couple months — that cut and cobble is taking forever, and I don’t get to work on this full time.  I’m in climate zone 5, running out of season.  We have had a lot of rain.  No leaks, but will the plywood be wet from moisture trapped under the tarp?  If so, I assume it must be dried out, since once the ice and water barrier is on, it won’t be able to dry inwardly or outwardly.  Probably a dumb mistake, should have put the membrane on right away.   If it’s got to dry out, how long would it take by just being uncovered on dry days?  Should I get a moisture meter, and if so, what moisture levels would be safe?
Thanks for your thoughts.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Ideally, you wouldn't have cut all that perfectly good rigid foam into narrow rectangles for your cut-and-cobble job. All of the rigid foam should have been installed as a continuous layer on the exterior side of the roof sheathing.

    In Climate Zone 5, if you are installing rigid foam above the roof sheathing, you need the rigid foam layer to have a minimum R-value of R-20 -- not a measly 1/2 inch or 3/4 inch of EPS. And the cut-and-cobble approach, as you have evidently discovered, is not the way to go.

    So this job isn't starting out well.

    As this point, you have no choice but to wait for good weather -- ideally, 2 or 3 sunny dry days in a row, to dry out your roof sheathing.

  2. Expert Member

    This isn't meant as a criticism of the OP, but as a general comment.

    it's important to choose building assemblies with an eye to the practicality of installation, and the way they integrate into the sequence of construction. Roof assemblies that rely on specific site conditions are probably unsuitable for any but the driest climates, or jobs with very flexible timelines.

  3. FixitAgainSam | | #3

    Hi Martin -- I'm not sure what it is about this site and names -- my profile name is set but is not reflecting in my posts. This is FixitAgainSam (again).
    Thanks for the suggestion of 2-3 days of sunny weather; we are supposed to have that this week.
    As to the other issues, I think this project is extremely odd. Very little additional height can be added to the roof, because it runs into the gable end soffit as it is. A couple inches max. So I chose to put the insulation on the inside. But two framing issues were working against me -- the roof had almost no overhang at the eave, so I extended the rafters by scabbing extensions on. And there are ceiling joists (actually old rafters from long ago when the roof was flatter) right next to the rafters. So there has been a bunch of framing in my way, and endless notching and fitting of the rigid foam.
    The purpose of the thin layer of EPS and overroof is to create a replaceable layer on top for future reroofing. I don't want to have to tear off the primary sheathing again and disturb the insulation when it comes time to replace the asphalt. I am hoping the minimal R-value above the primary deck will not cause any problems. I'll have R-49 or better below, except in the first couple feet in from the eave wall where space does not permit enough polyiso.
    If I were starting this over, I think I would drop the cash on a spray foam contractor and have them spray in R-25 or so, and then add fiberglass below it. I work slowly by most people's standards, but with this clutter of framing (and not much of it square), it is taking more time than it's probably worth.

    1. Trevor_Lambert | | #4

      The website uses the first and last name fields as your display name on posts. If those are blank, you get assigned the user number. As far as I can tell, the screenname field isn't used for anything. I don't know why the webmasters don't address this problem, as it comes up almost daily.

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