Structural Engineer Suggests Modifying Trusses with Collar Ties

Originally published by: Journal of Light ConstructionJanuary 1, 2009

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Q.My client wants to add a 16-inch-wide 4-inch-deep tray ceiling in a room where the drywall ceiling is attached to the bottom chord of roof trusses. To do this, I’d have to cut out and raise the bottom chords of the trusses. Is that possible?

A.Christopher DeBlois, a structural engineer with Palmer Engineering in Tucker, Ga., responds: Why not add a shallow soffit around the perimeter of the room to create the same effect? If there’s enough ceiling height, the loss of 4 to 6 inches of headroom around the edge may not be objectionable, and this approach has no structural implications for the trusses.

If that solution is not acceptable, it might be possible to cut out the bottom chord of the truss and add collar ties. But unless you know how to design trusses, you’ll need an engineer to help you. There are a few key issues.

The first problem is that the bottom chord of most gable trusses bears on the wall, with the top chord meeting the top edge of the bottom chord just above the bearing point on the wall. If you cut away the bottom chord, this joint will become unstable; you’ll need to reinforce it, most likely with a plywood gusset plate. The engineer’s design will specify material thickness, overlaps, and nailing.

Second, the truss top chord — often no bigger than a 2x4 or 2x6 — may not be strong or stiff enough to extend down from the new tray-ceiling collar tie to the top of the wall without being beefed up. Also, attaching the collar tie may require more nails than the top chord can handle without splitting, so you may have to add a plywood nailing plate or some other upgrade. An engineer can work out these details and provide a workable design.

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