Recurring cracks in a ceiling with full-span trusses may be caused by various types of moisture problems—sometimes in foundations, but more often in the attic.
By the DIY experts of The Family Handyman Magazine
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Likely causes of recurring ceiling cracks in newer houses
Figure A: Typical truss
Warm, moisture-laden air leaking into the attic gets absorbed by cold parts of the truss, expanding the wood and causing the truss to warp slightly. If the drywall cant move enough to accommodate the warp, it cracks away from the wall.
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 arent deep enough, the freeze/thaw cycles in cold winters could lift the outer walls. Changing moisture conditions in expansive, heavy clay soil can also lift the walls, but thats probably not the problem unless youre seeing other wall/ floor gaps or cracks.
Truss lift or truss uplift
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 doesnt 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.
How to fix truss lift
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).
Another solution: Dont 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.