A detachment of the ceiling from the wall is an irritating, but all too common problem. Its principal cause is a natural event called Truss Uplift.
Wikipedia defines truss uplift as an occurrence common in new houses built with roof trusses as opposed to rafters. If a house undergoes truss uplift, the top floor ceilings lift off the interior walls.
Wood is mysterious. When it shrinks, it shrinks more at right angles to the grain, than along the grain. Wood studs in the walls of a new home don’t get shorter; they get thinner in thickness and width.
However, when 2″x4″ studs are connected (butt-ended to each other) in long spans, as in top or bottom chords in conventional roof trusses, they may contract and expand at the rate of 1/2″ to 3/4″ per 40-foot length, depending on its moisture content.
Since the bottom chord has no other direction to move but up, it creates the break at the joints between the wall and the ceiling as it takes drywall with it.
If for some reason, the interior walls are nailed more securely to the truss than the floor, the truss could lift the entire wall. This accounts for gaps you might see at the bottom of your baseboard trim.
Why is it happening?
Most roof trusses consist of a flat horizontal 2″x 4″ dimensional wood member called a bottom chord. The angled portions of the truss are named the top chords.
Field tests show that differential shrinkage between upper and lower members causes the whole truss to bow upward, and if the drywall is attached to the bottom chord too close to the partition, this upwards truss movement will crack the drywall,
The lower chord in your home and many others like yours is probably enclosed with insulation. During the winter, the warmth from the heated ceiling below allows this portion of the truss to stay nice and dry.
Meanwhile, the upper part of the truss is exposed to much more moist and air together with the snow load.
Moisture in the attic condenses on the cold top chords, which act like sponges and soak it up to reach an equilibrium with the surrounding air. The idea is that the top chords expand while bottom chords shrink thus causing trusses to arch upward.
How to fix truss uplift:
- Some people use a crown moulding where the walls meet the ceilings. Molding is fastened to the ceilings but not to the walls. As the ceilings move up, the mouldings move up with them hiding the gap.
- Some people float the truss by using brackets with a vertical control slot rather than toe-nailing the truss to the top plate.
- Attach clips as a backup for all drywall comer joints involving interior partitions DO NOT reduce the amount of insulation to expose the bottom chord.
- Ensure that adequate airflow is maintained at the eaves.
- Float the drywall corners. It is vital that the recommended ceiling float distances of 12 inches and 16 inches (for 1/2″ and 5/8″ drywall respectively) be maintained. For walls, the proper distance is 8″ from the ceiling.
- Careful material handling before construction can also prevent or minimize truss movement. Arrange for the trusses to be delivered to the site just before they are needed so you don’t have to store them on site.
- If on site storage is unavoidable, store the trusses upright with bracing or horizontally with adequate blocking.
- First, specify the use of dry lumber. S-Dry graded lumber has a maximum of 19% moisture content and will shrink less.