Truss uplift is a phenomenon where the bottom of the roof trusses actually lifts up in the winter months. Roof trusses are the wood framed members that are used to create the roof slope and also the ceiling of your home. Trusses can be fabricated to accommodate almost any roof or ceiling shape or configuration. Generally, the trusses have top members, top chords, that form the slope of the roof and bottom member, bottom chords, which form the ceiling of the home. The chords of each individual truss are connected to each other by members called webs. Homes these days are quite well insulated, especially in the attic, the insulation usually lays on top of the ceiling, where the bottom chords of the trusses are, as such the bottom chords are kept warm and dry in the winter. The top chords of the trusses however are not in the insulation, so they are cold. Also, because humidity accumulates in the attic the top chords will have a high moisture level than the bottom chords. When wood dries it tends to shrink as opposed to “wetter” wood that expands. Truss up lift is a winter phenomenon, the top chords of the trusses are getting longer, and the bottom chords are getting shorter and because the top and bottom chords are connected by the truss web members the shrinking bottom chords are lifted by the expanding top chords, thus the ceiling flexes upward. This up lift can be so small that you do not notice it or can be quite dramatic and cause the drywall of the ceiling to crack and even create a gap between the top of the walls and the ceiling. This cracking will be most noticeable at interior walls near the center of the home. In the summer time the truss uplift will be reversed, and the bottom chord of the trusses and the ceiling will flex back down. Truss uplift is not a structural concern, but it does cause cosmetic cracking in the home. Dealing with truss uplift and repairing the cracking can be a challenge. Patching the cracks is not a good option because the new patch will only crack again the next time the trusses flex up or down. In an existing home the only way to accommodate the flexing of the ceiling is to allow for the movement by installing ceiling moldings that are fastened to the ceiling but not to the walls. In this way the molding can ride up and down with the ceiling and slide up and down on the wall, therefore hiding the gaps that would normally be visible at the ceiling. In new construction truss up lift is handled by not fastening the drywall to the ceiling near any of the center walls in the home. The ceiling drywall is fastened to the walls only, so if the trusses lift up the drywall near the walls is allowed to flex a little and stay down with the walls, hopefully eliminating cracking.
Barry Chickloski, Owner and Licensed Inspector