If you are starting a new construction project, you may be wondering what the best option for your floor system will be: floor trusses, I-joists, or dimensional lumber floor joists. Each type has its own strengths and limitations. Continue reading this buying guide for some guidance on what may work best for your project.
If you are starting a new construction project, you may be wondering what the best option for your floor system will be: floor trusses, I-joists, or dimensional lumber floor joists. Each type has its own strengths and limitations. Continue reading this buying guide for some guidance on what may work best for your project.
Floor trusses can span farther between bearing points than I-joists, allowing for larger open rooms. This also reduces the need and cost for extra bearing posts, beams, and footings.
Floor trusses have built-in openings, which can be used to install HVAC ductwork, plumbing lines, and electrical wiring.
Custom-built floor trusses allow building designers to add extra loading for in-floor heat, heavy tile floors, heavy kitchen islands, quartz countertops, and high-traffic areas. I-joists are limited when it comes to accommodating extra loading.
Floor trusses offer four different bearing conditions for a wider variety of uses: bottom chord bearing, bottom chord bearing trimmable ends, top chord bearing, and mid-block bearing are all available. The multiple bearing conditions make matching existing floor heights much easier when using floor trusses. I-joists offer only one bearing condition.
Why Choose Floor Trusses?
• Floor trusses can span farther between bearing points than I-joists, allowing for larger open rooms. This also reduces the need and cost for extra bearing posts, beams, and footings.
• Floor trusses have built-in openings, which can be used to install HVAC ductwork, plumbing lines, and electrical wiring.
• Custom-built floor trusses allow building designers to add extra loading for in-floor heat, heavy tile floors, heavy kitchen islands, quartz countertops, and high-traffic areas. I-joists are limited when it comes to accommodating extra loading.
• Floor trusses offer four different bearing conditions for a wider variety of uses: bottom chord bearing, bottom chord bearing trimmable ends, top chord bearing, and mid-block bearing are all available. The multiple bearing conditions make matching existing floor heights much easier when using floor trusses. I-joists offer only one bottom chord bearing condition.
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Duct ChartWhen it comes to installing plumbing, wiring, and ductwork, the setup of a truss (especially an open-web truss) makes it relatively simple to get the job done. In contrast, I-joists are subject to building code restrictions that make it much harder to run wiring, ductwork, and pipes through them. For example, there are some restrictions as to how big of a hole can be drilled or cut into an I-joist. This can limit the size and placement of the ductwork and pipes.Ductwork is usually installed under I-joists, requiring dropped ceilings or boxed soffits if the basement is to be finished. This in turn requires 9-foot high basement walls to allow for an 8-foot finished ceiling. Using floor trusses allows you to utilize 8-foot basement walls, as the ductwork is installed within the openings of the floor truss. Additionally, having the mechanicals and ductwork hidden within the web openings of the floor truss allows for the easy installation of drywall to the bottom chord of the floor truss to finish off a ceiling.
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With low lumber prices, many developers are moving towards wood-framed construction, where you will come across two main floor framing systems: I-Joists & Wood Trusses. Each floor system has its unique differences. The system you choose will significantly impact your project’s overall design and pricing. Both framing systems offer benefits and drawbacks. Read on to find out more!
I-joists
I-joists are strong, lightweight, “I” shaped engineered wood structural members that meet demanding performance standards. I-Joists could be manufactured in a wide range of depths ranging from 9 ½″ to 24″.
Wood Truss
A wood truss is a structural frame that relies on a triangular arrangement of shorter lengths of lumber, transferring the loads to reaction points. Wood Trusses are typically spaced at 12″, 16″, 19.2″, and 24″ on center, with 16″ being the most common and 12″ achieving the most significant span.
Elements to consider are:
MEP Coordination:
• Wood Floor Trusses are generally better (than I-Joists) for running MEP components such as HVAC ducts, electrical cables/conduits, domestic water, sanitary, and sprinkler piping through the truss openings.
Floor To Floor Heights:
• Open web floor trusses can add height to the building because they are typically used for longer spans, thus resulting in a deeper depth. However, this can be mitigated if ductwork can be run between the joists versus below, which is required with I-joists.
Loads:
• Open web floor trusses typically allow for footings and bearing walls to be along the long exterior wall and corridor walls, resulting in fewer footings and bearing walls overall, but this depends on the project design.
• Open web trusses with longer spans can result in less structural steel over open areas than I-joists.
Insulation:
• Trusses must be compartmentalized, filled with insulation, or sprinkled, affecting the project’s overall cost. Most have sprinklers now and still require insulation per IBC 718.2.
• I-Joists require less insulation due to less depth.
IIC Ratings/Sound Insulation:
• In almost all conditions, resilient channels will be required on the underside of the Trusses and I-Joists. Due to the increased air cavity in the trusses, the IIC performance is better.
Spans:
• Trusses enable the construction of longer spans compared to I-Joists.
Site Considerations:
• I-Joists are much easier for the contractor to handle and install, as Trusses require a crane to lift them into place.
Cost:
• The price of these systems varies depending on the depth, width, wood species, length, and transportation costs to the building site. The selection of which of the two to use is a decision that has far-reaching cost implications, including insulation, steel framing, and foundation costs. Therefore, more than a simple A vs. B cost comparison would be required. Furthermore, it’s important to note that these costs vary by location and are influenced by factors such as suppliers and manufacturers.
• In the last 24 months, 70% of our projects went with Wood Trusses vs. 30% with I-Joists.
One last thing to consider is the framing direction. We’ll touch on this next week.
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Blair Hildahl
BASE4 Principal
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BlairH@base-4.com
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