Overcoming Truss Installation Obstacles
Overcoming Truss Installation Obstacles
challenges of installing complex components.
Framers have access to better materials, tools and methods today than they did 25 years ago. The natural evolution of construction techniques over time is like any long-standing process; craftsmen learn to accomplish the same task in a more efficient way based on experiences, trial and error, and the technology available. The most prominent change in the construction industry in the last 45 years was the birth of component manufacturers. With an infinite number of possible truss designs, framers were able to build stronger structures at a faster rate without the need to assemble every member by hand. However, access to unlimited truss designs does present complications to framers who must install and secure trusses, regardless of how complex they may be.
One commonly overlooked question is: where can trusses be stacked and stored on a jobsite? The answer, with ease of access during installation in mind, is that it differs on each jobsite. In short, trusses need to be stored out of the way so they don’t obstruct framers working or risk being damaged (see Figure 1 below). However, they need to be near enough to be ready for installation and stored in the correct order to do so efficiently.
Positioning trusses proves difficult on many jobsites, but doing so on an urban, multi-story complex with trusses greater than 40 feet in length is its own kind of monster. The most important thing a crew can do before flying trusses is to study the plans. Nothing replaces knowing what the design plans call for and how to position framing members in the correct order on the jobsite. When the time comes to lift trusses into the waiting hands of framers, the process is safer and smoother when everyone is informed and materials are organized.
During erection, crews can run into problems with joint locations and truss plate size as it relates to lifting points. Many truss companies manufacture trusses with oversized truss plates, given that smaller plates can be damaged during the lifting process. Oversized plates give greater strength and stability to the truss member connection points during lifting. It’s important to remember trusses are designed to support roof loads, not loads applied by framers when they’re being lifted, dragged across top plates, and set into position. Crews should understand the best lifting points on different truss designs and stay away from putting undue stress on critical members, joints and plates. Above all, trusses should maintain their rigid form when lifted (see Figure 2 above). Trusses bent like noodles are at risk of being damaged or permanently broken (see Figure 3 photo above right).
Fortunately, due to the advancement in technology mentioned above, once trusses are raised into place, installation is straightforward (in theory). A framer’s task is to align one end of the truss onto the wall top plates, keeping it straight at the heel, tack it down, and repeat on the other side. When subsequent trusses have been set, pitches are aligned, sheathing ties all the chords together, and the task is done. Simple, right? Wrong. It can be simple, yes, but when complex truss designs are factored into the building equation, the simplicity soon disappears.
So then, what are some of the most difficult truss designs to install? In my experience, flat roofs with slight pitches can pose a real problem. Architects may call for a ¼:12 pitch, but it would be advantageous to encourage a ½:12 pitch to eliminate the possibility of standing water and other drainage issues. Although in this case it is more of a truss design issue, as framers, we must be cognizant of the intent of a truss design and the impacts of that design on our installation practices.
Another difficulty with flat roof designs is parapet configurations. Parapets can either be manufactured into the truss design (as shown in Figure 4 photo at right), or they can be manufactured as stand-alone pieces. In my opinion, it’s more difficult to set trusses when parapets are part of the truss design because you’re not only concerned with adjusting the roof line, but also ensuring the parapet is plumb. What’s worked well over the years is to install them after the main roof is complete, alleviating the difficulty of having to set competing angles.
Roof designs with multiple spans and hips can also be difficult to install. In most cases, the difficulty arises from access, stacking the trusses, and the order that the trusses come off the stack.
Accurate truss design and layout plans are very important in many cases, and it’s a good idea to show start points for layout so that the hip and valley points can be located. One major point of emphasis I’d like to stress from my years setting trusses is the utilization of drop-chord hip trusses. By dropping the top chord of a hip truss, a framer can can install lay-in gable trusses, which sit flush with the top chord of the trusses on the opposite side of the hip, giving framers an even surface to sheath. Additionally, the framing contractor can be more precise in his configuring hips, valleys and slopes.
Access to cantilevers or high-pitched vaults is another major obstacle for framing crews. Not only are these designs awkward to handle, but bracing techniques are generally more unconventional. On those designs with intricate webs, experience has proven that T-braces are a necessity to brace appropriately (see Figure 5 above). T-braces are used as an alternative to the combination of continuous lateral restraint and diagonal bracing when those techniques cannot be used. The T-bracing method generally uses more wood to brace, which increases cost, but in intricate web designs, it is the only way to properly resist buckling. (See Table 1 below to help determine what size T-bracing is required.)
Here are a few more examples of truss designs that are difficult to install, and key points to help framers:
1. Coffered: Framers must be attentive to align inside angles (ceiling) with outside pitches (hips) in order to set these correctly. Anytime you have to align interior ceilings and also manage complexities on the exterior roof, it becomes a difficult task.
2. Bowstring: The end goal here is to have a round roof deck even though the truss is built with straight chord members. It’s important to pay attention to the top chord angles, so when it comes time to brace and sheath, the sheathing sits correctly on the chords.
3. Attic: Align inside angles with outside angles, similar to Coffered trusses.
4. Scissors and Half Scissors: Stabilizing and bracing can be tricky because the bottom chord isn’t horizontal. Due to this, access is limited—framers can’t just walk up the truss. In many cases, scaffolding is the best route to position yourself for bracing between web members.
5. Vault and Studio Vault: In many cases, vault designs do not extend the entire length of a room and may transition into parallel chord trusses or other designs with horizontal bottom chords that transition back to a flat ceiling. In that case, stabilizing and aligning the walls is paramount to ensuring first, the trusses are set accurately at the correct height and angle, and second, the meeting point dimensions between the two truss designs is within those called for in the design plans.
To overcome difficulties presented by the designs discussed above and all other types of complex truss designs, framing crews should be familiar with the critical points: hips, offsets, valleys, hold dimensions for vaults, etc. Again, studying the plans and developing a redline set for critical dimensions and details is the best way to overcome obstacles. Above all, if the design is still unclear, clarify the plans with the component manufacturer/truss designer. Technology not only makes design capabilities endless, but also displays the answers with a click of a button. Waiting on installation in order to ask a simple question is far better than installing trusses incorrectly. A framing crew that follows these guidelines will be efficient yet maintain quality. In the long run, that means more opportunities for work, and we can all agree that’s good news.