You have OSB as your roof sheathing. Do you cheer or panic?

That depends on what grade the OSB panels are, and whether the right panels were installed.  OSB (which stands for Oriented Strand Board) is a variant of plywood, and is an engineered product, meaning that each type of OSB panel has defined end-use and method of application.  Information on the types of panels and their uses can be found here:  http://www.apawood.org/level_c.cfm?content=pub_osb_libmain

The panels are manufactured in two grades: Exterior and Exposure 1. “Exterior panels have bonds capable of withstanding long-term exposure to weather.”   The Exterior 1 grade “is intended to resist the effects of moisture due to constructions delays or conditions of similar severity”.  Which means that if the OSB panels have been installed as roof sheathing, then a few days exposure to the weather is permissible before the tar paper or other underlayment is installed.

If the roof panels  are Exterior grade, then you have nothing to worry about.  Most of the panels installed as roof sheathing, however,  are Exposure 1 grade,  which means they CANNOT be exposed to long-term moisture.  That is fine, as long as the roof doesn’t leak, or condensation doesn’t happen.  Leaks, when they do happen, generally start gradually, and it may be several years before a leak is detected.  By that time, the OSB has been exposed “long-term” to weather (which in this context, means water), and therefore the glue holding the wood chips (strands)  together starts to dissolve.  This leads to the weakening of the panel, as well as causing it to swell and buckle with absorbed moisture.  Persistent condensation will do the same.

In short, if you have Exterior grade OSB, you can change your roof when it’s due – the panels should not be affected.  If you have Exposure 1 grade, then you should carry out a regular program of attic inspection to ensure that no water is getting in.  Otherwise, the cost to reroof may have to include the cost to redeck the roof, and do whatever mold remediation that is necessary.

OSB damaged by persistent leakage, being removed.  Note the dark stains in the insulation.  It too will be removed.

OSB damaged by persistent leakage, being removed. Note the dark stains in the insulation. It too will be removed.

From another installation / repair, we had very localized leakage, but it ended up ruining that piece of OSB.

Damaged OSB.  Leakage was localized but still need to be redone.

Damaged OSB. Leakage was localized but still need to be redone.

If you’re not sure of what you have, or if you know you have OSB but aren’t use what state it is in, give us a call.  We’ll do an inspection and tell you our opinion.

(c) 2013 Paul Grizenko

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