Truss Uplift

I recently visited a client with concerns about cracks in their ceiling that were thought to be a foundation issue.  A problem with the foundation can lead to all sorts of interior cracks, but if the problem is below – the foundation, and the crack is above – the ceiling, there will be some evidence in the walls of the shifting support. 

Alternately, in homes with manufactured trusses, a seasonal distortion of the truss can occur.  This is referred to as truss uplift.  Not to be confused with the wind driven lifting that the trusses must be designed to resist, this lifting is due to the seasonal temperature and humidity extremes that cause distortion to the shape of the truss.

This homeowner recently noted several ceiling cracks.  These three or four 20-foot cracks extended the width of living area and also in the adjacent master bedroom.  Looking closely, the cracks are along seams in the ceiling drywall and are also perpendicular to the direction of the roof trusses. 

The cracks are not the result of foundation settling, or a structural failing.  The cracks are a result of distortion of the truss.  Trusses are designed to meet certain stiffness standards, expressed as the deflection per length of span.  The roof trusses in this home have a large span of 56 feet, and large enough that even deflections of the ceiling within the allowable limits for the truss may still exceed the limits of the drywall.  But factors other than the deflection from wind, snow, and building loads are at work here.  The upper portion of a truss is exposed to temperature and humidity extremes, whereas the bottom cord where the ceiling is attached is insulated and at room temperature.  This difference in temperature and humidity can cause the bottom cord of the truss to seasonally arch up bringing the ceiling with it.  In this home the cracks appeared a year after construction.  Humidity changes throughout the year along initial drying in the local desert climate are also major sources for truss distortion.

In this case, the arching forced the drywall sheets together causing minor crushing of the material at the edges, and possibly a subsequent relaxing of the truss help reveal the crack. 

Cacks due to uplift of the bottom cord of a truss will often be found where the truss crosses a wall.  The truss should not attach to the wall unless a slotted bracket is used allowing the truss to move vertically.  In turn, special attention is required for the drywall installation.  The drywall should not be screwed to the truss within 12 to 18 inches of the wall thus allowing the drywall to flex.

The resulting crack in this home may be difficult to control.  The large, uninterrupted span of ceiling presents a challenge.  Additional or stronger ceiling tape will not resist the compressive force.  A sealant between the drywall sheets that remains pliable, such as caulking, may be a solution.