Q. We see ourselves as quality builders. We frame everything 16 inches on-center, including roof trusses. Most visitors to our job sites feel that this is overkill and that we're wasting the customer's money. Is putting trusses 16 inches on-center with 5/8-inch sheathing and 1/2-inch drywall on the ceiling a thing of the past?
A. Frank Woeste, P.E., responds: I am not aware of any performance issues regarding 1/2-inch drywall installed on trusses at 16-inch versus 24-inch centers. If interior moisture were improperly managed — for example, if a clothes dryer were vented into a finished garage area instead of to the outside, a 16-inch on-center truss spacing would be more forgiving of the bad practice.
But in general, it doesn't matter if the truss spacing is at 12, 16, or 24 inches. What dictates the truss design is the roof design snow load — typically 20 or 30 psf in most of the U.S. Once the builder or architect specifies the loads (for example, 20-10-0-10, for top chord live, top chord dead, bottom chord live, and bottom chord dead loads), the truss spacing, and the desired shape of the truss, then truss engineering takes over.
The engineering design — usually computer generated — dictates the size and grade of lumber required in the chords and webs. There are ten grades of 2x4 southern pine, for example. If the builder requests a 16-inch on-center spacing, a lower grade will be used that meets the structural requirements. Conversely, builders who request the 24-inch on-center design should get a higher grade of lumber relative to the 16-inch on-center design. Higher grades of lumber obviously cost more than lower grades, so it becomes a trade-off between spacing and grade. In general, the wider 24-inch spacing is the most economical for residential construction.
