The Roof Truss Is Slipping

Cause

• The phenomenon of truss uplift begins when the the top two sections, or chords, and the bottom section are subjected to different levels of moisture during the winter. The bottom chord, often lying beneath insulation, stays drier than the the top chords, which must contend with moist air at the upper reaches of the attic. The difference in the moisture levels makes the top chords expand and the bottom chord contract. As a result, the bottom chord lifts up, sometimes pulling its nails along with it.

Nonstructural

• Although the sight of a truss coming loose from its moorings might cause consternation on the homeowner's part, truss uplift does not compromise a home's structural integrity. The uplifting that occurs in winter ceases in the summer, as the moisture differential between the top and bottom chords recedes. The primary problem with truss uplift does not involve structural issues but the unappealing cracks in the ceiling drywall that it creates.

Clips

• Improper fastening of the truss can lead to cracks caused by the uplift. Trusses attached to the top plates of the walls with only nails have a greater susceptibility to slipping off their connections. To prevent the slippage, remove the nails and attach the trusses and plates with a metal fastener called a truss clip. One part of the L-shaped clip fastens to the chord, while the other part attaches to the top plate, making for a much stronger connection than what you can achieve with a nail.

Molding and Vents

• Another possible solution to the problem, if your primary concern is the visually unappealing cracks where the walls meet the ceilings, involves installing crown molding. This covers the cracks. Make sure you nail the molding into the ceiling and not the truss. Increasing the amount of attic ventilation may reduce moisture and limit truss uplift. Using both eave vents and ridge vents helps to alleviate potential problems.