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In the UK it rains, as you know. a lot.

Is it OK for OSB3 boarding to get wet now and then during the construction of a building? or will it be completely destroyed like chip board or MDF (which absorbs liquid, expands then falls apart)

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For your temporary outbuilding, you might want to consider 4x8 siding since it is designed for 24/7/365 weather exposure, and would be less laborious than sheathing+siding. In the US, T-111 is common. –  mike Oct 16 '13 at 21:27
    
Washington and Oregon maritime climates are equivalent. –  Fiasco Labs Oct 17 '13 at 1:45
    
Thanks for the answers - So I'm going for, "it's not terminal, but very much to be avoided" Don't understand the -1 though...... –  Darkcat Studios Oct 17 '13 at 18:53
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2 Answers

up vote 1 down vote accepted

OSB won't be completely destroyed by water, unlike MDF or similar interior laminate materials; however, like any wood product, it will swell and shrink as it absorbs and releases water, so you should typically avoid more than casual contact with water.

If you watch homebuilders putting up a typical light-frame construction home, you'll notice that the framing lumber and OSB are typically delivered on pallets wrapped with Tyvek or similar "moisture barrier"; the wood is allowed to "breathe" and so to acclimate to ambient humidity, but the covering is impervious to liquid water like rain. You'll also notice that the roof of the house is sheathed, papered and shingled as soon as the frame is developed enough to support it, and if the crew is caught in a sudden downpour while the roof is going up, they'll drape and tie tarps over any section of the roof that hasn't at least gotten a layer of tar paper. This is all to protect the OSB; the framing lumber, while it doesn't benefit from getting rained on, is much more water-tolerant, as long as the lumber is allowed to dry out completely after a good soaking before it's closed in by walls and vapor barriers. Once the roof is up, the next priority is the outer side walls, again primarily OSB, and which nowadays also get a layer of Tyvek as soon as they're up even if the final exterior siding isn't put in place for days or weeks after.

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Good answer - thank you! –  Darkcat Studios Oct 17 '13 at 18:55
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The brochures I've read suggest that OSB3 is designed for humid conditions rather than wet conditions. OSB3 should be covered and fully protected from the elements.

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Heh, Oregon weather tells me from actual experience that you're right. It gets kind of pillowy even though intact when subjected to too much moisture. Doesn't melt and fall apart like chipboard, but has problems retaining its thickness <grin>. During the Econolypse several framed up houses were left to the rain, one had a roof, the other was bare roof sheeting for the whole winter. Since OSB is just compressed chipped wood, you have black mold leaking out of every crevice in the material. Cutting through it shows it to be shot through with the stuff. Not a good idea. –  Fiasco Labs Oct 17 '13 at 1:40
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