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Q Our 10-year-old single-level home has a full-span truss roof, and in the winter, a crack opens between the ceiling and the walls down the center of the house. It grows up to 1/2 in. wide and has pulled the drywall tape off the ceiling. How can we stop it?
Mike Horn, Iowa
A This problem occurs occasionally with roof trusses. Trusses are designed to carry the weight of the roof on the ends, that is, the outer walls. If a seasonal crack opens along a wall somewhere in the middle, one or both truss ends are rising or the truss is arching. The first case is a foundation problem. If the footings under the wall aren’t deep enough, the freeze/thaw cycles in your Iowa winters could lift the outer walls. Changing moisture conditions in expansive, heavy clay soil can also lift the walls, but I don’t think that’s the problem in your case. You’d probably see other wall/floor gaps or cracks as well.
The second cause, truss arching or truss uplift, is a slight bowing of the entire truss upward during the winter. Moisture that escapes into the attic in cold weather tends to be absorbed by the cold top chords (Figure A), which causes them to expand slightly. The bottom chord doesn’t expand because it remains buried in the insulation close to the warm ceiling. The result is a slight warp, just as a board warps when one side is damp and the other remains dry. The warp goes away once the weather warms and the moisture content in the wood equalizes.
One solution is to reduce the humidity in your attic by (1) closing up air leaks through the ceiling, and (2) making sure you have good, unobstructed attic ventilation (soffit and roof vents).
A Typical Truss
Another solution: Don’t nail the ceiling drywall to the trusses within about 16 in. of any wall. Instead, support those edges by attaching them to blocks nailed to the tops of the wall framing or to special drywall clips attached to the walls. That frees the trusses to flex slightly upward without pulling the drywall corners apart.
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