Proper installation of roof truss systems

This entry was posted in Featured Magazine, Horse Barns, Industry Experts, Low Rise Construction, Post Frame, Post-Frame Technique, RB December 2011, Rural Builder Magazine and tagged roof truss installation, roof truss system, sbca, sean shields, structural building components association. Bookmark the permalink.

A builder can invest a lot of money in safety lines, guardrails and hard hats to keep workers safe, but another element to the equation should not be missed: proper installation practices. One area in particular where education can help save losses, both in terms of people and property, is the roof truss system.

Sean Shields, who addresses fall protection for the Structural Building Components Association (formerly known as the Wood Truss Council of America), believes proper installation of wood trusses is a simple matter of understanding and respecting basic physics.

Roof truss installation

SBCA photo

“Even though a truss is a three-dimensional object, it’s helpful to think of it in 2-D, because it’s engineered to essentially handle loads in one direction – downwards,” Shields explains.

“Further, trusses are designed for their end use,” he adds. “They are designed to hold up a roof and as part of a roof system, resist all the live and dead loads that are going to be applied to them, whether it’s wind, snow or rain or the occasional person walking on top of it and to hold up the sheathing and roofing materials. The entire roof system is designed to then transfer all of those loads vertically down onto the walls and ultimately to the foundation.”

A properly installed truss, once in a completed system, is exceptionally sturdy. During construction, however, prior to sheathing, there is a critical period of time when trusses are vulnerable to instability and possible damage. Builders need to pay special attention to the proper handling of trusses once they are delivered to the jobsite.

Avoid lateral bending of trusses:
“Many times when trusses are delivered to a jobsite, they’re laid flat like a deck of cards,” Shields says. “It’s very easy when you’re lifting single trusses from that deck of cards to bend them out of plane. If you hold a truss as it’s oriented in the roof, in that triangular pattern, with the peak pointed straight up towards the sky, that truss has a tremendous amount of strength, but any time you tip that peak towards horizontal that truss loses much of its stability and strength.”

Damage can be quick, especially on long span trusses and it isn’t always readily visible. “When the truss bends out of plane, you put a great deal of stress on the truss plates that hold the wood members together at the joints. You can potentially bend the plates, peal the plates away from the wood and even have the teeth rip through the wood fiber and loosen the joints. This kind of damage effects the overall strength and stability of the truss even after it is part of the completed roof system,” Shields notes.

There are additional challenges once the trusses have been lifted into place.

Provide proper restraint and bracing:
“What’s unusual about the erection process,” Shields points out, “is you have a situation, because there’s no sheathing on the roof, where wind loading can be applied to the trusses from every side and single trusses aren’t designed for that.”

Fortunately, the answer isn’t rocket science. “If builders did nothing else different, if they could install full and proper restraint and bracing to the trusses as they’re installing them, including top diagonal bracing, that would eliminate most of the potential for damage or collapse,” Shields offers. “That bracing provides the necessary resistance to lateral forces like gusting winds.”

Specific how-to’s are provided by most truss suppliers and may include a packet of installation guidelines called the Building Component Safety Information, produced by the SBCA and the Truss Plate Institute. Rural builders specifically need documents BCSI-B1, -B2 and -B3.

Additional post-frame installation guidelines are provided in the BCSI-B10 document.

The SBCA works primarily with residential contractors, but truss logic is basically the same for commercial and agricultural. In fact, because commercial and agriculture buildings often call for longer spans, the rules are even more critical to follow.

“Many of the roof collapses we have witnessed during the erection process can be attributed to truss systems that weren’t properly braced,” Shields says, “this is particularly true in rural agricultural applications. Like commercial applications, you have longer spans, so they’re more challenging to work with and easier to twist out of plane. Even light winds, winds that don’t seem to be that big of deal, when you’re talking about these larger span trusses, can make them unstable very quickly.”

There is a saying at the SBCA to “sheath early and sheath often”. Shields explains that there is no hard-fast rule of when to sheath, but he offers this suggestion: “As soon as it’s possible, start to apply your sheathing and stagger your sheathing so it can overlap. With that approach you’re going to get much greater lateral restraint. It may be a slightly different approach than how they’ve done it in the past. If they’d prefer to install the whole roof system first before sheathing, they need to make sure they’re constantly applying adequate bracing and lateral restraint throughout the system as they install it.”

Use good fall protection practices:
Even with proper installation, there are hazards associated with working more than eight feet off the ground. Even the most sure-footed framer can fall at an ill-timed moment. That’s why good fall protection equipment is critical and equally important is knowing where to attach it. Shields warns that tying off to the easiest, most convenient point — the top of a single truss — is a mistake that can spell disaster.

“We’ve been trying to address this approach more in the residential area, where residential builders want to install one or two trusses, then tie off to them and use that as a fall protection anchorage point. But having just a few trusses up there isn’t strong enough to withstand the lateral loading that’s associated with those falls. If someone actually falls, it’s going to jerk that truss laterally as opposed to vertically. Trusses are not designed to handle that kind of lateral load, so it’s possible for them to pull the truss system down when the framer falls.”

OSHA has implemented a new rule for the residential construction market that makes fall protection equipment use mandatory. While good in principle, the SBCA is concerned the new rule may have unintended consequences. “We’ve had this discussion many times with OSHA,” Shield explains. “In the past, experienced framers, they know what they’re doing, and they haven’t been tying off at all … OSHA now comes along and mandates that framers have to have some way to arrest their fall. So framers are saying ‘okay, I’ll tie off to the trusses.’ So we have seen some isolated instances where they’ve tied off to the trusses and when they fall, they’re pulling the roof down with them. Our concern is that this may become a more common incident without proper education..”

Bottom line, the SBCA recommends: Have a good lateral restraint to the system and once a more rigid system is in place, tie off to that. RB

 

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