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Re: OSB roof and shearwalls wet from these rain

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In response to the S.E. who has designed a commercial building w/ OSB.  The
A.P.A. literature makes a case for OSB as being as least the equivalent of
exterior grade plywood as long as reasonable precautions are taken when
storing the materials at the site.  I have had recent experience with OSB
shear walls which have been exposed to  been moisture within a wall cavitly
subject to repeated wetting and drying.  The result is the total recomposition
of the board at the locations where the moisture has affected the material.
The heads of 8d or 10d nails are sucked as much as 3/16" to 1/4" below the
surface by the swelling of the OSB.  One can easily drive a awl well into the
board with only hand pressure. 

Our office has concluded that the use of OSB should be limited to areas where
the likely hood of continued wetting is  very remote such as interior shear
walls, exterior shear walls with good roof overhangs and no water source such
bathroom showers, floor diaphragm but not under bathrooms with showers etc.  

Specific to the engineer with the OSB commercial building now being soaked by
the recent continuing rains,  It has to have a weakening effect by the time it
is covered to the point of keeping the material dry.  The real question is
only how much damage has been done. That may take further investigation.

Hope this helps,

Lynn W. Bockemohle, S.E.
Los Angeles