Truss Uplift and the Ceiling Crack That Comes Back

How do you tell truss uplift from settling?

Uplift follows the seasons. CMHC’s guide says cracks where the ceiling meets the partition walls may open during the winter and close in the summer, and that the problem is generally worse during the first year after construction, while the house is drying out. It needs a trussed attic directly overhead, so we look for it on the top floor, in the corner where an interior wall meets the ceiling, running level along the joint. The NRC digest adds two things that make it worse: more insulation over the bottom chords, which widens the moisture difference between top and bottom, and a lower roof slope, which arches more for the same difference.

The catch is that other movement can crack the same corner. In Simpson Strong-Tie’s engineering post on truss movement, the author points out that it is not always the trusses that move; the walls or floors beneath them can, and the possible causes include foundation settlement, expansive soils, shrinkage of the framing and trusses pulled down to a partition during construction. CMHC also treats settlement and racking of the structure as a separate cause of cracks at wall and ceiling intersections. A crack that opens every winter and closes every summer points at the trusses. One that never closes is the one we look at harder before anybody patches it. Cracks along the tape seams out in the middle of the ceiling are a different problem, covered in our post on nail pops and cracked seams.

• Opens in winter, closes in summer: points to uplift
• Level, in the corner where an interior wall meets the ceiling
• On the top floor, with a trussed attic above
• Often worst in the first year after the house was built
• Never closes, or arrives with other cracks: look further before patching

How is a truss uplift crack hidden?

The accepted fix is to let the ceiling float at the wall. The ceiling drywall is left unfastened to the trusses for a set distance on each side of the interior wall and is held at the wall by clips or blocking on the top plate, so when the truss lifts, the ceiling stays down at the wall and flexes over the unfastened span. CMHC gives minimum float distances of 12 inches for half-inch ceiling board and 16 inches for five-eighths, keeps the wall board’s fasteners 8 inches below the corner, doubles the first row of fasteners and notes that gypsum manufacturers advise against floating the ceiling more than 24 inches. Built that way, a floating corner can take 2 to 3 inches of uplift without cracking, by CMHC’s figures. In a finished house, the same guide’s remedy is to drive the ceiling fasteners near the corner through and retape the joint, which turns it into a floating corner. That is how we handle it as drywall repair: release the screws nearest the wall, retape and refinish the corner, then blend the ceiling around it.

Where that is not practical, a molding attached to the ceiling only, not to the wall, covers the joint and slides against the wall as it moves. CMHC’s drawing of this slip joint shows the trim nailed to the truss only and the drywall painted before the trim goes on, and the NRC digest describes the molding sliding up and down the wall with the seasonal movement of the trusses. It also suggests sealing the crack with adhesive tape first if the vapor barrier was torn, to keep house air from leaking into the attic. The gap keeps opening and closing behind the trim, where nobody sees it. If you are not sure which of the two suits the room, book a free estimate and we will tell you what we would do.