Longitudinal Moisture Response in Truss Lumber–A Potential Cause of Ceiling-Partition Or Floor-Partition Separation

In recent years we have seen an increase in a phenomenon referred to as ceiling-floor partition separation (CFPS). Generally, CFPS can be described as cracking or breaking of the ceiling drywall joints at the juncture of a roof truss and an interior partition during the heating season. This report discusses five projects designed to determine the causes of CFPS. The first is a summary of reported information, field observations, and measurements. The next three projects include a brief discussion of full-scale truss experiments to investigate moisture and temperature relationships of insulated truss lumber. The fifth project, discussed in detail, is an investigation of longitudinal shrinkage of juvenile and compression wood sometimes found in light-frame construction lumber. Two sets of lumber specimens were selected; one having detectable amounts of juvenile and compression wood, and the other having neither of these detectable characteristics. Length measurements were taken at three moisture content (MC) levels. Seven of the specimens containing juvenile and compression wood shrunk between 0.13 and 0.30 percent during a 5 percent MC loss. If this lumber were installed in the lower chord of a truss, this amount of shrinkage, relative to the upper chords, could cause the truss to arch upward a significant amount. Several other reasons which can cause cracking at the ceiling or floor are also briefly discussed.

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