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Community and Q&A

Will an uncovered OSB roof be okay for the winter?

duchenyj | Posted in General Questions on

I’m building a house in Northern Michigan currently that I was unable to roof before the snow came down. That was in December, so it’s been sitting for a month. There hasn’t been that much snow melt here and temperatures have mostly been below freezing. The roof has ice below so I can’t really scrape everything off to shingle it right now. Is there any danger of waiting until the major thaw and then scraping it all clean then? Do I risk wetting the OSB too much? Mold? Any other dangers? Also, how long should I let the wood dry before I attempt to roof it?

Thanks ahead of time.

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  1. GBA Editor
    Martin Holladay | | #1

    Normal procedure is that the same day that the framing crew drives the last nail on the last sheet of OSB roof sheathing, they begin installing roofing underlayment (usually asphalt felt, but sometimes synthetic roofing underlayment). The idea is that the OSB shouldn't be exposed to the weather for more than one night -- and ideally, for zero nights.

    Did you remember to install the roofing underlayment?

  2. duchenyj | | #2

    Hey, Martin thanks. It rained. It snowed. I couldn't get anything on it after I put the sheathing down. It was the last day of good weather. I left for Christmas and there was even more snow. My thought was that the snow is frozen so its not going to get into the wood until it melts. I was going scrape the snow off best I could before that point, so that I had as little as possible moisture going into the wood. Is it that bad?

  3. iLikeDirt | | #3

    Musing out loud hereā€¦ If a structural building material is so flimsy and vulnerable that ideally it can't be expected to survive even a single night exposed to the elements, maybe we should re-think whether it's a building material that's appropriate for the job. Your OSB is probably shot at this point, the victim of one of the many ways that OSB can deteriorate. For the cost to replace it with more OSB, you probably could have afforded durable board sheathing instead. What a shame.

  4. rocket190 | | #4

    Seriously this is getting blown out of proportion. I'm no big fan of osb, but if you have a ventilated roof, it's not like your sheathing will stay permanently wet. The ice will hopefully disappear from sublimation. You may need to replace the bottom course of osb if it has swelled too much, but your biggest problem is likely to be that the plywood seams may telegraph through your shingles if the edges have swelled enough.

  5. GBA Editor
    Martin Holladay | | #5

    At this point, it's hard to know how badly your OSB has been damaged. At the first opportunity -- once the ice and snow have melted, and the roof is safe to work on -- cover your OSB with roofing underlayment.

    I would wait for a while after that, until you have had two or three weeks of sunny weather, and then I would remove the roofing underlayment to see what the OSB looks like. You may have to replace 4 or 5 sheets of OSB, or in the worst case you may have to replace all of your OSB -- or your OSB may be OK, especially if you are willing to sand the swollen seams with a belt sander.

  6. duchenyj | | #6

    Nate: Thanks for the material shaming. If I had your hindsight and your budget, I probably would have gone with a more durable board.

    Rick and Martin: Thanks for your help. It's supposed to get warm this week, so I'll check out the seams and put on the roofing underlayment. Again, I really don't think it will be that bad, because the temperature this year was below freezing for most of the winter. But time will tell.

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