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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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.
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.
OSB 3 is classified as resistant to humid conditions. This is a liability choice to avoid misunderstandings.
We have had OSB 3, also in horizontal orientation, exposed to rain for at least 4 months. Even puddles were forming and deliberately ignored by us.
Our 1100 m3 project has survived 3 heavy storms thus far and shows no signs of giving up soon.
The reasoning by our structural engineer was as follows: OSB3 retains it's structural properties if it does not let loose layers more than 1 or 2. If deeper strands go loose then it is time to replace because water has found its way in and was not able to breathe out quick enough for what ever reason. Or if the osb 3 panel expands by more than 10 % then caution is needed, 15% expansion and the panel should be deemed to have no more contribution to structural calculations.
Always consider the guidelines from the manufacturer to be on the cautious side when looking for warranty enabling advise. However a well seasond structural engineer probably knows how far one can stress wel produced osb3