Wall crack mystery.
Q I put up drywall ill my freestanding garage last summer. I installed it vertically, screwing the edges over the studs, then taped the joints. This winter when the temperature dropped into the teens, a big crack opened up at one of the vertical seams. What's going on and how can I fix it?
Mike Magee, Chicago
A It looks like a classic case of frost heave. You can see from the painted design on the drywall that the adjacent sheets of drywall didn't pull apart horizontally. The movement was up and down, which means that the foundation is shifting. The fact that the crack opened at low temperatures surely indicates that frost is the cause.
Frost heaves occur when moisture collects in one zone in the soil and freezes there during cold weather. Water expands about 9 percent when it freezes. It's that expansion that causes the heaves. Your freestanding garage was probably built on a concrete slab, which "floats" on the soil, rather than on frost footings, which would extend down to below the potential freezing level (about 3 ft. in the Chicago area). So the heave lifted one part of the slab slightly. It doesn't take much movement to crack a wall, even as little as a quarter inch. You'll probably find a crack in the concrete slab somewhere near the crack in the wall.
There's no simple sure fix. Frost heaves are most common in poorly drained soil, so first try regrading around the exterior to keep rainwater away from the slab. Running a layer of 1/2-in. plywood horizontally across the wall might stiffen it enough to stop small movements. However, you'd have to tear off the drywall to do this. Or you can simply hide the crack with a thin wood strip, and let the wall move at will.