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Community and Q&A

Paint OSB subfloor to protect from rain during slow construction?

Jeff Cooper | Posted in General Questions on

The house I’m building in central Arizona has a subfloor of 23/32″ Norbord Truflor OSB on i-joists over a ventilated crawlspace. I’m building by myself and very slowly, so it might take me three months before I have walls and a roof. In the meantime, I’ve considered protecting the OSB from weather by painting it with Rust-Oleum Porch and Floor, which is designed to withstand sun and occasional puddles of water. On average, my location will get 6 days/1.2 inches of rain in March, 3 days/0.7 inches in April, and 2.1 days/0.3 inches in May. The Truflor has an Exposure 1 rating to withstand normal delays in construction without affecting its structural properties. I tried tarps to keep the subfloor dry, but they failed miserably.

Does my dry climate compensate enough for my slowness in building that the OSB should endure the time it will take me to get it out of the weather?

Would painting the OSB create any problems because it will only be able to dry from the underside when it absorbs moisture from the crawlspace air until I seal the undersides of the i-joists with foil-faced polyiso?

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  1. GBA Editor
    Martin Holladay | | #1

    Painting the subfloor won't cause any problems, but it's a waste of paint. Your OSB panels absorb water from the edges, and the paint won't slow that down.

    You'll probably end up with OSB that has swollen edges, creating ridges like picture frames. The usual fix is to attack your subfloor with a belt sander after the roof is on, and to replace the worst of the OSB panels with new ones.

    Lesson learned. Next time, invest in Advantech.

    -- Martin Holladay

  2. Jeff Cooper | | #2

    Thank you, Martin. It's great to know that the paint wouldn't cause any problems. Are you saying that some panels may absorb so much water from the edges that they will need to be replaced? The edges on the Truflor come painted, I assume to resist water. Would it make sense to paint the edges of those that I've cut? My panels are glued down, so I'm willing to go to some lengths to avoid replacing any, and I found an extraordinary deal to get enough paint to cover the entire upper surface (and edges) for around $80.

  3. GBA Editor
    Martin Holladay | | #3

    I've never used Truflor, so I don't know how resistant to moisture the product is. Ordinary brands of OSB will get swollen edges when exposed to rain for several weeks.

    Q. "Would it make sense to paint the edges of those that I've cut?"

    A. It can't hurt.

    -- Martin Holladay

  4. Expert Member
    Malcolm Taylor | | #4

    It's unlikely you will need to replace any of the OSB subfloor for structural reasons. What commonly happens is that the edges swell and telegraph the seams through any finished flooring or underlayment you will be installing.

  5. Nate G | | #5

    Wood in Arizona (especially moist wood) is going to be a termite nightmare. If it's not too late, I strongly recommend a replacing as much of the wood in your design as possible--especially close to grade level--with masonry.

  6. Jeff Cooper | | #6

    Thank you, all

    Re termites, I chose a masonry stem wall over a slab so that I could see whether or not termites were invading. With a slab, they would be able to come up through a crack underneath a wall and eat away at the wall unseen. If they want to reach wood above my stem wall, they will have to build mud tubes up the stem wall, which I will be able to see and address.

    I assume any deterioration in the OSB will occur gradually. Just in case the answer isn't as obvious as it might seem, what should I look for to tell whether the OSB is deteriorating to the point where I might have to replace a panel?

    I know drying is an important topic at GBA, so I hope this will be an interesting question even if I won't actually paint the top surface of the OSB. Someone just told me that if I paint the top of the OSB, moisture will get trapped inside it, because if the dry, top side facing the sun is sealed, and the side facing the moist air of the crawlspace is unsealed, moisture will try to migrate from the moister, crawlspace side to the drier, top side, but get trapped underneath the paint. I'm thinking that the surface of the OSB facing the crawlspace will fluctuate between being moister and drier than the interior of the OSB so that moisture will migrate in and out through the underside, with an overall moisture level inside the OSB roughly equal overall to the moisture level of the crawlspace air--until I seal the undersides of the floor joists. If I'm correct, would the atmospheric moisture within an amply ventilated crawlspace in my dry climate be low enough not to harm the OSB?

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