Building intricate floor truss details may bedevil a robot, but they provide the inherent advantage of floor trusses over I-joists. There are also things that floor trusses can do that I-joists cannot. And they facilitate the most efficient use of materials. Visiting an I-joist cut-off yard tells the tale. Isn’t the handling of 60-foot-long packs of I-joists in a lumberyard just as difficult as handling 16-foot-long bunks of 2x10s on a jobsite? Both processes create expensive cut-offs that often can’t be reused. However, in a floor truss operation, the cut-offs can be repurposed into short blocks for floor details and web members, yielding much less volume of waste at a much lower cost. This savings in cost and waste is the essence of offsite construction.
The most distinguishing details of floor trusses are the openings that are built into them, pre-engineering pathways for mechanicals. Wiring, natural gas lines, and plumbing can be routed directly to fixtures, saving up to 30% in materials. The I-joist suppliers attempt to overcome this by cutting openings for ductwork, even though this neglects the penetrations required by other trades. This practice can create multiple unique joist configurations, complicating cutting, packaging, and installation. To mitigate some of these drawbacks, a large integrated builder has attempted to cut the maximum allowable number of large-diameter holes in his clear span joists. But, does this approach consider the following laborious steps that had to be taken to achieve that result? [For all photos, See PDF or View in Full Issue.]
- Hauling a 60-foot bundle of joists to a saw with a monster forklift.
- Setting joists on the saw’s live deck, breaking bands, and manhandling each joist onto the saw infeed.
- Operator wait-time while the infeed pauses multiple times to cut holes at the router station and then cut to length.
- Manually stacking cut joists, banding them, and forklifting to the loading area.
- Forklifting cut-offs to the stocking area.
Did he consider the opportunity cost of the very large yard space dedicated to the I-joist cut yard, and the cost of the waste inherent in outside storage of cut-offs?
Two recent developments in home HVAC and plumbing systems that were demonstrated at the International Builders Show (IBS) further underscore the importance of an open web system. The first is the Rheia system, now installed in over 20,000 homes across the country. Rheia uses mini-ducts that can fit inside 2x4 walls, and uses conventional furnaces, unlike the older high-velocity systems. As shown in the truss system, the diagonal duct runs impart considerably less friction than the 90-degree bends in the I-joist system, reducing energy consumption. Specially designed off-center duct chases are not required with trusses, and Rheia ductwork does not interfere with strong-backing. All Rheia’s simplified fittings can be installed by hand. For more information, see “The Last Word on Rheia HVAC Systems.”
A second system, the plumbing manifold, has migrated from modular to conventional construction, and was featured in the innovative The New American Home (TNAH) at IBS. In this system, each plumbing fixture is served by a separate continuous run of plastic pipe from a central manifold, thereby eliminating all special fittings and connections that are potential sources of leaks. Each connection to the manifold is identified, like in an electrical distribution panel, and each line can be turned on and off individually at the manifold. One of the drawbacks of the system, a longer wait-time for hot water, is mitigated by running pipe diagonally through the floor system, thereby shortening the distance (and the time) traveled. Although these diagonal runs can be achieved in an I-joist system by drilling many dozens of small holes, it is unlikely that the extra effort will be undertaken. Instead, as shown in the accompanying illustration, the installer may simply bundle all lines together and run them through the precut holes in the joists (not shown), lengthening the distance, and extending the wait-time.
Joe Hikel and many other CMs have found out years ago that cutting holes in I-joists renders them more costly than floor trusses. His calculation doesn’t consider the installation savings gained by the mechanical trades. And floor trusses are particularly beneficial in the new Rheia HVAC and the manifold plumbing systems because they expedite installation and ensure the least energy consumption.
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Inside Floor Truss Details
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