4. Truss Uplift Truss uplift, or truss arching, is caused by wood reacting to changes in humidity, according to Rachel Smith, technical director of the Wood Truss Council of America in Madison, Wisconsin. The problem usually shows up in winter, when the bottom chords (the ceiling joist part of the truss), which are buried under ceiling insulation, stay warm and dry. Top chords, on the other hand, are exposed to moister attic air. As a result of the different moisture levels, the top chords expand and the bottom chords contract, which causes the bottoms of the trusses to bow, or lift, off of the wall. To confirm that this is the problem, use a handheld moisture meter on the top and bottom chords. But also ask a contractor to look for any settlement of bearing walls, headers, beams or floor members. In fact, truss arching is the cause of only about 20 percent of reported cases of partition separation, according to Smith. If the problem is moisture-related, installing attic ventilation can help correct the problem. Eave vents in combination with ridge vents are most effective.