Roof sheathing got caught in the rain?

Question

blog-ol-sage-wet-roof-osb-smMy contractor is using Norbord’s Trubord to sheath the roof of our renovation. It has been left uncovered for the last three days and it has rained consistently for those three days. Does this exposure undermine the integrity of the product? I was told by another contractor that it should have been covered.
Joan M, Port Orange Florida

Answer

Thank you for the e-mail Joan and your question. Our OSB is rated “Exposure 1” and that means that it will withstand normal delays in construction. Normal delays and the rain you indicated will not affect the structural properties of the panels. Certainly it would have been better to have covered the roof with a tarp or roofing felt before it rained. OSB, like all other wood, will expand when exposed to moisture and I assume the contractor spaced the panels the required 1/8″ along the 4′ edges and installed H clips along the 8′.

The OSB (and in fact any wood panels) must be allowed to dry completely (depending on how wet the panels are this could be a few days) before any roofing material is applied to avoid trapping moisture under the roof material that could cause edge-swell and buckling. If your contractor has access to a moisture meter he can take multiple readings in various areas and compare the average of those readings to some taken from a piece of OSB that is known to be dry (i.e. in a store). Also, with that amount of rain, plywood and OSB may have some swelling at the edges and a thorough inspection of the panels should be done after the panels have completely dried. Address any edges that are of concern to make the roof level and flat before installing the finished roof material but this should not be done until the panels have had time to dry.

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