A.David Utterback responds: If your floor is framed with green lumber, shrinkage of the floor joists could contribute to the problem you describe. But the primary cause is probably truss uplift. Lower truss chords that touch the warm ceiling dry out and shrink during the heating season, while the cold upper chords gain moisture and expand. The whole truss then curves like a bow, rising at the center. If the truss is nailed to the wall, it pulls the wall with it. The moving truss can also create drywall finish cracks in the corners where walls and ceilings meet (see illustration, below).

Whether floor joist shrinkage or truss uplift is causing the problem, the way to prevent it is the same: Instead of nailing trusses to wall plates, use truss clips as shown in the photo below (available from Simpson Strong Tie, 4637 Chabot Dr., Suite 200, Pleasanton, CA 94588; 800/999-5099). The clips allow each truss to flex up and down freely,...

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