· by Michael Blair, Nashville's Drywall Repair
A gap that opens along the top of an interior wall in winter, in the corner where the wall meets the ceiling, and closes again by summer, is one of the most misdiagnosed cracks in a house. It is blamed on settling, on a foundation, on the drywall finisher. In a house with a trussed roof it is almost always truss uplift, a seasonal bending of the roof structure that lifts the ceiling off the interior walls by a fraction of an inch.
What is moving
A roof truss is a triangle of lumber: two top chords that carry the roof and a bottom chord that carries the ceiling, with webs between them. In winter, the attic is cold and dry at the bottom chord, which sits buried in insulation and stays close to the temperature and humidity of the house, and colder and wetter at the top chords, which are exposed to the attic air. The top chords take on moisture and lengthen slightly; the bottom chord does not.
When the top of the triangle gets longer and the bottom does not, the bottom chord bows upward in the middle. It is fastened at the outside walls, so the lift happens at the interior walls near the center of the house. The ceiling drywall, which is screwed to the bottom chord, goes up with it, and the interior wall, which sits on the floor, does not. The corner joint tears.
How to tell it from settling
Truss uplift is seasonal, opening in mid to late winter and closing by summer, and it repeats every year. It appears at the top of interior walls, often several at once along the center of the house, and it is worst on the top floor under the attic. The crack is horizontal, right in the corner, and a crack from uplift may be a quarter inch or more at its widest.
Settlement and foundation movement produce cracks that grow and do not close, that run diagonally from door and window corners, and that come with sticking doors and sloping floors. A crack that closes on its own in July is not a foundation problem. Well-insulated attics with high energy-efficiency insulation levels, common in newer Nashville subdivisions, can show it more, because the bottom chord is kept warmer and the differential is larger.
- Opens in winter, closes in summer, every year: uplift
- Horizontal, at the top of interior walls near the center of the house
- No sticking doors, no sloping floors, no diagonal cracks
- A house with roof trusses rather than stick-framed rafters
Why filling it never works
The joint is going to move again next winter, by the same amount. Compound, caulk, tape and paint are rigid at the scale of that movement, and they tear again as the ceiling lifts. Some flexible caulks last a season or two. None of them stop the crack, because none of them stop the truss from moving.
Screwing the ceiling down harder to the wall top plate does not stop it either. It transfers the force to the wall, and the result is a wall that is pulled up at the top, or fasteners that pop through the ceiling surface instead.
The repair that hides it
The accepted fix is to let the ceiling float at the wall. The ceiling drywall is not fastened to the trusses for the last foot or so on either side of the interior wall; it is held by clips or blocking attached to the wall top plate instead, so when the truss lifts, the ceiling stays down at the wall and flexes gently over that foot of unfastened span. In a finished house that means removing the screws nearest the wall and adding drywall clips, then refinishing the corner.
Where that is not practical, a crown molding attached to the ceiling only, not to the wall, covers the joint and slides against the wall as it moves. Paint the wall behind it before fitting, and the seasonal gap opens and closes behind the trim where nobody sees it. It is the pragmatic answer in most existing houses, and it is permanent.