Q: In the winter, the ceiling in my upstairs hallway—directly under and parallel to the roof ridge—separates from the walls, rising nearly ½ inch. When the weather warms up, however, the ceiling settles down and the gap closes up. The house was built in 1967, and has brick-veneer siding and a prefabricated-truss roof. An architect said that a thermostat-controlled attic fan might help. A contractor told me I should climb up in the attic and nail the trusses' bottom chords to the top of both the hallway walls. Does either suggestion make sense to you?

—Oly, Chesterfield, MO

A: Tom Silva replies: The trusses are causing this problem, but nailing them to the top of the walls would be the worst thing you could do. If you did, those trusses could lift the walls right off the subfloor.

What's causing this is truss uplift, or partition separation. It happens when the trusses' chords—the horizontal pieces at the bottom of the truss—arch upward, taking the ceiling drywall along with it. The reason you don't see any gaps in summer is that then all the truss members are warm and moist, so everything is in equilibrium. But as soon as the heating season starts, the bottom chords tend to dry out faster than the sections of the truss above the ceiling insulation. The difference in shrinkage between these parts causes the bottom chord to bend upward.

Ventilating the attic is always a good idea, but it's no guarantee against uplift. The better solution is to wait until winter, when the gap is widest, and remove any drywall screws in the the ceiling that are within 18 inches of the wall. Now, go into the attic and screw lengths of 2xs to the wall plates between the truss chords. This "backing" should be wide enough to overlap the plate on both sides by an inch or more. Now screw the ceiling drywall into the 2x. You'll have some drywall patching to do, but with the ceiling corner effectively disconnected from the truss, that gap shouldn't come back.
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