Drywall Cracks: Is There a Problem Here?

Most buildings will experience drywall cracking over the course of their lifetime. They are easy enough to fix, but may have an important story to tell. What causes these cracks? Are they an outward sign of something more insidious behind the surface?

Drywall cracks are indicative of movement within the structure, which typically manifests itself initially in brittle materials, such as drywall or ceramic tile. Some movement in a building is normal: for example, if wet lumber was used during construction, the wood will shrink as it dries out, pulling the drywall along with it. Buildings can also experience some shifting and racking with wind pressures; wood is flexible enough to absorb this movement, but it might show up in the drywall.

Movement related to settlement or frost heave is more of a cause for concern. If you see diagonal cracking or unusually wide cracks, this can indicate more serious structural problems that should be investigated by a Professional Engineer.

If you experience sudden or seasonal cracking, particularly at wall-to-ceiling interfaces, truss uplift may be the culprit. Truss uplift is a phenomenon which is caused by a difference in moisture content between the top and bottom chords of a truss.

Truss uplift occurs in the winter when the temperature differential (and relative humidity differential) is the greatest. The top chord of the truss is cold and the surrounding air is at a higher relative humidity, with a lower dew point temperature causing condensation to wet the top chord. The bottom chord is kept warm from insulation, and is at a lower relative humidity; water vapour in the air will tend to condense on colder surfaces, away from the bottom chord.

As the top chord absorbs moisture, it expands. The bottom chord dries out and contracts. This movement causes the bottom chord of the truss to deform, lifting the drywall ceiling away from partition walls with it, as shown in the figure below:

(http://www.carsondunlop.com/resources/articles/truss-uplift/)

Truss uplift is normal and is not a structural concern, although it can be a pain when drywall cracks reappear each year as temperatures change, causing old cracks to reopen after repairs are completed.

Certain drywall hanging techniques can allow drywall to move independently from the wood behind it. This allows the wood to expands and contract naturally, without constraining the drywall. Seasonal cracks can also be hidden by fastening corner trim to the ceiling but not to the wall, so the trim moves freely with the ceiling.

Truss uplift is also exacerbated by humid attics, so it is important to ensure that there is adequate ventilation and the vapour barrier is continuous: sealing bathroom vent fans and other penetrations through the ceiling.