Why brace some truss webs, but not all?

Ron Strohbeck, a truss expert in Carlinville, Ill., replies: The permanent braces sometimes required on rooftruss webs are called continuous lateral braces, or CLBs. They’re typically required on a web that is in compression. The brace is intended to keep the truss web from buckling.

To explain why they’re sometimes necessary, imagine pushing down on a yardstick that extends vertically to the floor from the palm of your hand. It doesn’t take much pressure for the yardstick to buckle.

Now imagine taking your other hand and restraining the yardstick halfway up from the floor. If you press down now, it takes a lot more pressure to make the yardstick buckle. In effect, you’ve added a CLB to the yardstick.

The web of a truss is much like a yardstick. It can withstand a certain amount of compression without bracing. The amount of compression a web can withstand depends partly on its size, species, and grade. But the biggest factor in determining CLB requirements is the overall length of the web.

In some cases, webs can require two rows of bracing rather than one. You’ll see this design in tall trusses (12 ft. or more).

If one CLB is required on a web, it should be roughly in the center of the web. If two CLBs are called for, they should be at one-third points on the web.

Continuous lateral bracing won’t do any good if it’s not anchored to something solid. The CLB will just transfer the buckling, and the whole set of webs will buckle in the same direction. Typically, CLBs are anchored with diagonal braces to rigid points such as the top chords of trusses.

To determine if you need CLBs, look at the drawings accompanying the trusses; they should have the locations of any CLBs on them. Many truss manufacturers also put tags on webs that require braces. For more information on trusses, visit the Wood Truss Council of America’s Web site at www.sbcindustry.com.