I had a call for a potential structural issue where the drywall gap formed at the top of an interior wall, see photo below. Typically, this is due to an engineered truss bowing up in the winter months in a ventilated attic. The bottom chord is usually buried in the insulation and stays warmer than the remainder of the truss members which are exposed to cold dry winter air. The exposed truss members are colder and dry out which combined causes the bottom chord to deflect upward lifting any drywall attached to it.
In this case the attic was a conditioned attic where the underside of the roof was spray foamed, including the top chord of the trusses. In this situation the top chord temperature and moisture level was different than the exposed web members and bottom chord. The warmer and dryer attic air causes the exposed truss member to expand. Plugging this information into my engineering software causes the bottom chord to bow up, similar to a typical vented attic. Note: The changes in wood temperature is the driving force behind truss uplift, changes in wood moisture content only affects the width and thickness of sawn lumber, the changes in length is negligible.
Afternoon, Randy. I was under the impression truss uplift was caused when there is a significant difference in moisture and/or temperature than the rest of manufactured wood roof trusses. With the attic conditioned, what lead to the lower chord bowing?
Robert, The truss top chord was encased in foam insulation and attached to the roof deck. The top chord would quickly reach the outdoor temperature. So, there is a temperature differential. The configuration of the truss and the locations of the supports all determine if it moves up or down.
