Problem Signs for Roof-Truss Slippage

Truss Contraction

• Trusses include a bottom horizontal chord, two or more angled top chords, and various braces between the top and bottom chords. The top chords, which are subject to moisture in the air, may swell in humid conditions. The bottom chord is usually insulated and remains dry, which may cause it to contract. When the bottom chord contracts, it arches upward, away from the tops of interior walls.

Ceiling Cracks

• You can't see the bottom roof truss because it's generally covered by drywall, but if a chord moves where it attaches to the drywall, you’ll likely see cracks in the ceiling. If the cracking appears to originate along a line running in the same direction as the roof trusses, truss contraction may be the cause.

Gap at the Top of a Wall

• More common than cracks in the middle of the ceiling is the appearance of the ceiling pulling upward and away from an interior wall, which leaves a gap along the top of the wall. This is also caused by truss uplift, but is preventable by the use of “floating wall” drywall installation, in which the contractor installs the drywall panels to braces instead of attaching the panels to the underside of the bottom roof chord. When the chord lifts, the drywall, which is not attached, remains in place without cracking.

Roof Sag

• A sagging roof may be the result of roof truss movement, but it’s usually limited to improperly braced trusses or trusses that someone cut to install a skylight or ventilation. Since trusses come engineered to specifications, cutting even one will usually void the manufacturer’s warranty and put undesirable strain on the roof. If you must cut a roof truss, consult an engineer about bracing and transferring the weight load evenly to the other trusses.