Can Trusses Support Drywall Ceilings?

Engineered to Specs

• Roof trusses are engineered to suit the purpose of the structure. When the contractor orders the trusses, she will submit a set of blueprints to the truss manufacturer, who will use the blueprints to figure out the best truss configuration. For residential use, manufacturers design the truss system to comply with standard building specifications, meaning you can install drywall on the ceilings if you’re building a typical house. Truss specifications for commercial buildings and barns, however, may not accommodate drywall.

Truss Spacing

• Trusses are generally spaced wider than stick-framed rafters, which will affect the way you install the drywall. In stick framing, ceiling rafters are usually 16 inches apart. Because trusses transfer the roof load more efficiently, fewer are necessary. Trusses are often spaced 24 inches apart. Both of these spacing measurements match up with standard 4-foot-wide drywall panels, but when you’re using trusses, you’ll have only one rafter behind a panel for support, instead of two. To ensure a smooth ceiling, use 1/2- or 5/8-inch thick drywall instead of 1/4-inch thick panels, which may sag slightly between widely spaced rafters.

Truss Uplift

• Because you can’t attach trusses to the tops of interior walls, they may arch or lift upward when exposed to humidity or temperature fluctuations. This can result in cracks in the finished drywall where the wall meets the ceiling. To alleviate this problem, ask your contractor to use “floating corner” drywall installation, whereby the drywall panels attach to blocks instead of directly to the trusses at the point where the trusses cross an interior wall. When the trusses move, they can lift off the interior wall, but they won’t pull the drywall with them, since the panels attach only to the wall blocking at that point.

Nailing and Warranties

• Prefabricated trusses come with explicit installation instructions. Typically, the contractor will attach the bottom of the trusses to the outside walls plates. The warranty clearly states that if the trusses are nailed in places other than specified, the warranty on the trusses is void. This doesn’t mean, however, that you can’t attach drywall panels to the underside of the trusses. The nailing restrictions apply only to attaching the trusses indiscriminately to the top of interior walls, which reduces their engineered weight-bearing capability.