Warping of OSB won't always be apparent after just a short rain
This is very true, but sometimes it's amazing how fast it can start to deteriorate... With a few days of good rain the stuff can swell like crazy, and the dimensional thickness change can be very obvious... I have seen the 1/2 OSB swell almost 1/8" in thickness overnight with heavy rain with visible signs of 'flaking' on the surface when I did construction for a living...
I like saving money as much as the next guy, but for the floor of a coop I would use nothing short of pressure treated plywood, a few dollars extra now will be worth it in the years to come... And for the sides regular exterior grade plywood... OSB is simply not exposure rated, even with a liberal amount of paint it will fail much, much faster then traditional plywood... That is not to say OSB is useless, when protected from the elements like say under shingles or behind siding it works well, and it's also fine for quick and dirty builds that are not expected to last for long...
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