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What makes plywood sheathing delaminate?

What makes plywood sheathing delaminate?

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One of the implications of high moisture levels, from either condensation or leakage, is delamination of plywood sheathing. This is sometimes accompanied by buckling. Panel-type roof sheathing may lift off the rafters in a regular pattern.

High moisture levels break down the glue in the plywood and allow the plies to delaminate. Buckling is usually the result of elevated moisture levels in the sheathing, combined with inadequate room for expansion in adjacent panels. H-clips are commonly used with panel-type roof sheathing to allow for some expansion and contraction of adjacent panels. Where these are not used, buckling is more likely.

Delaminated plywood is weakened and may not be able to carry its intended loads. Ultimately the plywood may give way under the weight of foot traffic, for example. Buckling can weaken the roof by separating the sheathing from the rafters or trusses. If fasteners pull through the sheathing, the entire roof covering and sheathing becomes loose and may be blown off during high winds.

Check the plywood sheathing, particularly at the panel edges, for evidence of delamination. In many cases, you'll need to make a judgment call as to how severe the delamination is. Where you are unsure, get a specialist for further investigation. Check that the sheathing sits tightly on the top of rafters or trusses over its entire length. Where the sheathing has clearly buckled, make recommendations to reduce moisture levels in the attic and possibly refasten the sheathing.
by Manage My Life
April 26th, 2007
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