What causes cracks to open along the line between the ceiling and walls?
Monday, August 5, 2019
Cracks along the line where the ceiling and interior walls meet can be caused by settlement of the floor slab, but usually there is a separation between the baseboard and floor first, along with stress cracks emanating diagonally from the corners of doors, before any ceiling/wall crack lines appear.
If the cracks are just along the ceiling/wall line, and your roof structure is wood trusses, it is likely due to truss uplift. These cracks usually occur during the winter, and close back up during the summer, because they are thermally induced. Due to the mild winters in most of the southern two-thirds of Florida, it is more likely to occur in North Florida and during a harsh winter.
What happens is that the bottom chord of the truss bows upward during cold weather, pulling the ceiling up and away from the walls. To understand why, you first need to know that wood expands and contracts more in relation to moisture content than temperature. As a percentage, expansion/contraction is significantly higher perpendicular to the grain than along the the grain. But because the length of the truss lumber is many times the width, and the chord members more constrained in length than width by the truss plates, the movement with the grain (along the length of the lumber) is the more significant issue with trusses.
During the winter months the bottom chord of the truss in buried in ceiling insulation, warm and dry. The top chord and web members in a ventilated attic are exposed to the high relative humidity of cold air and possibly occasional condensation. So they have a high moisture content and expand, while the dry bottom chord contracts. Because the bottom chord is secured at both ends to the bearing walls and cannot move laterally, the result is that the expanding top chord and web members pull the bottom chord upward, making it bow, as shown by the dotted lines in the diagram above.
This is not a structural problem. It is only cosmetic, but nobody wants unsightly cracks at their wall/ceiling line. There are several solutions that do not stop truss uplift, but keep it from becoming a problem. Unfortunately, the various recommended fixes should be done at time of construction, and are difficult and expensive to retrofit.
- Install an inverted “L” angle clip, with the verical side nailed to the top plate, and the horizontal side resting on top of the ceiling drywall, with no ceiling nailing of the drywall close to the corner, so the corner stays intact and the drywall can flex slightly upward when the trusses bow.
- Secure drywall to the top of the wall, but not to the trusses for a distance of 18” away from the walls. Another way to let the corner flex a little.
- Install decorative cove molding where the walls meet the ceiling, and fasten it to the ceiling, but not the walls. So the crack between wall and ceiling is hidden by the sliding cove molding. This is the one fix that is comparatively easy to retrofit, but do not paint the walls in the summer, to avoid having an unpainted stripe around the room when the molding rises during the winter.
One last thing that can cause ceiling cracks at the wall is sagging drywall, usually due to high humidity and corroded fasteners. This is most likely to happen in a garage because it is an unconditioned space and, also, cracks along the seams of the drywall panels perpendicular to the wall are likely to appear before the cracks along the wall. To read more about it, see our blog post Why is my garage ceiling sagging?
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