I want to get a good definition on what causes “truss lift,” when the trusses will actually raise off the top plates of interior walls (even when nailed), causing the drywall to crack. Is it from drastically different temperatures in the attic and living area?
Truss arching, sometimes called truss uplift, is caused by wood’s natural response to changes in moisture levels. The problem usually shows up in winter climates, when the bottom chords of trusses are warm and dry, buried under ceiling insulation. The top chords are usually exposed to cool, damp outside air. The result is high moisture top chords wanting to expand and low moisture bottom chords wanting to contract. When they both try to go their own ways, the bottom chord arches or lifts up. Once moisture levels in the lumber start to equalize, the problem goes away.
Truss arching can be an issue if the truss is over interior partition walls. The arching may be severe enough to develop an opening between the top of the partition wall and the truss, called partition separation. Many contractors and homeowners are aware of truss arching and any time there is a partition separation problem, they assume it’s the truss. There can be many factors at work to produce partition separation. In fact, truss arching is the cause of only about 20% of reported cases of partition separation.