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exerpt from "Common Failures in Wood Structures"
by Paul Fisette - © 2002
Wood Flooring Symptom: Floor boards have developed unsightly gaps 6 months after a home is occupied. Several individual boards appear to have shrunk significantly while large areas of flooring remain tightly arranged. Investigation: Several 1/2"-gaps separate large sections of the tongue-and-groove strip flooring. The occurrence seems random, but the separations run in a connecting zig-zag pattern across the room. The humidity in the room is 50%, a reasonable level. The manufacturer's and distributor's records show that the material was delivered at a moisture content between 6% - 9%, also an appropriate level. And you know the material was installed correctly. Could there have been a few wet boards mixed into the shipment? Possibly, but not likely since the separations are continuous across the room. Checking your construction records you realize that the basement slab was poured a week before the flooring was delivered, installed and finished.
Dry flooring was installed, but it took on moisture provided by the curing slab. Wood fibers at the edges of the boards were pressed against each other and compressed as the boards increased in size. As indoor humidity dropped, the boards shrunk to a size smaller than their installed size("compression set"). Polyurethane finish dripped between the floor boards during finishing effectively gluing the boards together. The wood flooring shrunk as a monolith, separating along a few planes of weakness. It only appeared that a few boards had shrunk. In reality, all the boards had swelled and then shrunk.
Prevention/Cure: Prevention in this case is the only cure. Relaying the floor is the fix. To avoid similar problems: Only install flooring that has equalized to its in-use moisture content. Control indoor humidity levels and do not introduce powerful sources of moisture into the home. Record and document humidity levels in the home and moisture content of the flooring during storage and installation. Good record-keeping helps assign liability when problems arise.
Contrary to conventional wisdom, I do not believe that leaving a 3/4" gap around the perimeter of the room solves the problem. Flooring nails would have to be sheared off or pulled out of the subfloor in order for flooring to fill the recommended 3/4" perimeter gap. It's simple, either you stabilize your floor or you have problems.
Source Contact Information:
Paul R. Fisette, Director
Building Materials and Wood Technology
126 Holdsworth Natural Resources Center
University of Massachusetts, Amherst, MA 01003
Tel: +1 (413) 545-1771
Building Materials and Wood Technology is part of the Department of Natural Resources Conservation in the College of Natural Resources and the Environment at the University of Massachusetts, Amherst
Copyright 2002 University of Massachusetts, Amherst, Massachusetts, 01003, USA.
The importance of this cannot be overemphasized. Very good results can be obtained by using a vacuum sweeper for this purpose. First, it should be run with the grain of the wood; then, a second pass should be made across the grain.
