Truss Uplift

Separation of the ceiling from the wall is an annoying but all-too common problem. Its main cause is a normal phenomenon called Truss Uplift.

It cannot be prevented but its nasty side effects can be controlled by:

• Providing adequate ventilation
• Floating the trusses and the drywall
• Good design

As prefabricated roof trusses dry, they shrink. Since the bottom chord is surrounded by attic insulation, it's warmer and drier. Therefore it may shrink more than the other chords. The top chords may actually lengthen in some circumstances, if they absorb moisture.

Field tests show that differential shrinkage between top and bottom members causes the entire truss to bow upwards and if the drywall is attached to the bottom chord too close to the partition, this upward truss movement will crack the drywall. Therefore, cracks at the intersection of the ceiling and interior partition may open during the winter and close in the summer, because the truss is never completely stationary. Such problems are worse during the first year after construction, when most drying of the structure takes place.

You can recognize truss uplift by the characteristic separation of the drywall at the ceiling wall ange. The ceiling lifts with the truss, away from the interior partition. If the truss is secured too tightly to the partition, it lifts the entire wall and separation appears at the floor level