Effective bracing is key to safety on a roof truss job. I had worked out a general concept of bracing for that day’s job, and at the meeting I made sure everyone understood it. Not only is it important for workers to know their own tasks; I want each carpenter to understand how his assignment relates to the overall concept. If everybody understands what we are trying to achieve, the whole crew can recognize and prevent any problems that might arise.
Any truss roof needs permanent bracing as part of the structure — bracing that will remain in place in service. For the top surface of the roof, the sheathing serves that purpose, and for the lower chords that make up the ceiling of the room below, the drywall may suffice (although there is usually some 2x4 bracing installed across those lower chords for added strength). In addition, most truss roofs have bracing nailed across the web chords at specified locations. All of that is part of the engineered strength of the truss roof — the system that allows trusses to serve as a strong structure, using less material than a rafter-framed roof to achieve equivalent or greater structural capacity.
But during construction, before all those stiffening elements are in place, the trusses are unstable. OSHA’s concern is that while workers are up on the trusses, applying force with their weight and their movement, something may cause the whole stack to collapse like a row of dominoes — and that can kill or injure several people at once. Those accidents are more common than you might think. It has never happened on my job, but I’m aware that it’s a major risk.
So we try to get temporary bracing in place as soon as possible, to stabilize the whole system in the same way that the permanent bracing will stabilize it later. Until we get that temporary bracing in place, though, we’re vulnerable. Our goal is to shrink this window of vulnerability. It’s my job to plan the whole temporary bracing system in advance and to teach it to my supervisor and crew.
I based our system on guidelines in a 2003 pamphlet from the Wood Truss Council of America, “Building Component Safety Information” (BCSI 1-03), which costs $10 at amazon.com. You can also get an updated version, in print or as a PDF file, from the Structural Building Components Association, at sbcindustry.com/bcsi.php.
The first step is bracing the gable-end truss; unless the gable trusses are firmly secured, the braces on the rest of the trusses won’t be fully effective. We first install vertical struts aligned with the locations where the rooftop bracing will be set, then run long 2x4 braces from the tops of the verticals diagonally back to stakes in the ground. These diagonals are themselves cross-braced and braced back to the foundation with additional diagonals. All of this takes several hours, so we do it the day before setting the trusses. On the day we set the trusses, we set the gable-end truss first and secure it to the uprights in the bracing system. That anchors the entire roof system back to solid ground.
As soon as four to six trusses are in place, the crew runs temporary 2x4 bracing in three planes: one set of braces across the top chords, where the sheathing will go later; one set along the horizontal plane of the bottom chords; and one along the interior web chords. Once the bracing is in place, the roof system is rock-solid and it’s safe for workers to get on the roof to install the plywood sheathing.
Procedures for Working at the Peak
Two carpenters have to move up and down along the top chords of the trusses, setting spacers, installing bracing, and unhooking the crane. And they have to do this work before the whole system is fully braced. They’re in a risky position, especially at the beginning; there is no way to tie them off to anything, or to give them a conventional scaffold to work from.
To make this work safer, we stabilize the trusses as much as possible before the men climb on them. As each truss is placed, the crew attaches 2-foot spacers to set the truss spacing. But those spacers are not bracing — they’re not strong enough to provide adequate structural stability. Still, they add some stability, and we get them in place before unhooking the crane. As soon as enough trusses are in place to provide room for the temporary structural braces, the crew sets those braces, working from the bottom up.
Also, on this job, we set up rolling pipe staging with plywood and plank platforms below the work area. The plan was to move this staging along as the work progressed, so that it would remain below the leading edge and provide a midheight platform to break the fall of anybody who slipped. Since it was only 6 feet off the ground, the rules did not require railings for it — which is good, because railings would have gotten in the way of the trusses. The rolling staging also provided a midheight work platform for Eric, so he could pass materials to the guys at the peak and assist them in any other way they needed.