May. 27, 2024
Synopsis: In the past, floor framing choices were largely restricted to dimensional lumber, limiting architects and engineers in their designs. However, the advent of engineered floor-framing materials such as I-joists and floor trusses is challenging the traditional dominance of dimensional lumber. Current data shows that I-joists make up around 50% of the floor-joist market. Editor Chris Ermides dives into the design and construction of I-joists and floor trusses, discussing their load-bearing capabilities and installation methods, while also evaluating their respective advantages and disadvantages. While engineered joists typically carry a higher initial cost than traditional lumber, they can significantly enhance a building's structural integrity and design flexibility.
For additional insights, please explore our i joist beam section.
In earlier times, architects and designers faced limitations in floor framing options, with dimensional lumber being the sole choice available. The necessity for support beams or load-bearing walls varied by species and age of the lumber, restricting creativity and the feasibility of large, open spaces.
Engineered framing materials like wood I-joists and floor trusses, on the contrary, can span larger distances with fewer restrictions, simplifying the creation of open areas. I-joists and floor trusses also help avoid many potential engineering challenges. Even with a higher upfront investment, these engineered products may lead to savings due to reduced labor and material costs, in addition to being more stable and simple to install.
I-joists were specifically engineered to accommodate the demands of open floor layouts. Architects sought a material that could span longer distances than traditional lumber without the need for an intricate engineering design. When I-joists were introduced to the market, they consisted of a plywood web combined with laminated-veneer lumber (LVL) or solid lumber flanges. Today, the web material is predominantly oriented strand board (OSB), which enhances durability and performance.
One notable advantage of I-joists over dimensional lumber lies in their consistency and stability; these engineered products do not present the variability found in traditional lumber that can lead to issues like warping or cracking. The potential for shrinkage prompts concerns over cracked drywall and uneven ceilings, which are frequently observed with dimensional lumber.
The exceptional strength of I-joists is perhaps their most significant benefit. Despite their appearance, the combination of a 3/8-inch OSB web capped with wood-flange blocks results in remarkable load-bearing capabilities. When a load is applied, the top and bottom flanges experience compression and tension, respectively, while the center web remains largely unaffected, allowing for the drilling of holes along the web.
Builder Michael Chandler from Raleigh, N.C. appreciates I-joists not only for their structural strength but also for their sustainability. “I aim to construct homes that endure for over a century while minimizing environmental impact,” he explains, noting that I-joists utilize less wood than traditional options, which is a significant advantage for many builders.
I-joist manufacturers have honed their design to optimize load distribution, creating products that place materials where they are most effective and minimize unnecessary materials. This results in a stronger and lighter joist that rivals traditional dimensional lumber.
Installation of I-joists mirrors that of dimensional lumber in most cases, providing familiar flexibility for builders. Since span limits are established by manufacturers, they can adjust layouts on-site as needed. I-joists can be doubled for added strength or repositioned for plumbing and mechanical installations, and they also offer greater latitude for drilling.
Companies like iLevel provide complimentary software for builders and designers, enhancing customization of performance according to the building's specifications.
Wood floor trusses are becoming increasingly popular due to their strength, efficient material use, and design flexibility. Truss manufacturers can create custom designs tailored to specific building plans.
Trusses are advantageous not just for their strength but also for ease of installation when deployed accurately in accordance with placement diagrams. Construction manager Lisa Biggin of Habitat for Humanity in Newburgh, N.Y. values floor trusses for their ability to span distances without requiring additional structural support, thus reducing costs. They also simplify the installation process, making it accessible even to volunteers with limited framing experience.
Biggin stresses the importance of preparation and site management to maximize the benefits of trusses. Proper training of installers can help avoid costly errors, ensuring efficiency throughout the project.
While floor trusses may present a higher cost for shorter spans compared to I-joists, focusing solely on direct comparisons can be misleading. Builders favor trusses for their open webs that streamline utility installations.
Gary Striegler, a builder in Arkansas, notes that open webs allow subcontractors to run plumbing and wiring swiftly, eliminating the need for drilling which can slow down projects. This efficiency translates into time and cost savings for the entire operation.
In addition, Michael Chandler incorporates floor trusses into the second stories of his projects. By providing designated spaces for utility chases, he keeps work tidier and more organized. Most importantly, trusses allow HVAC systems to be well integrated within the conditioned space of the home, enhancing energy efficiency.
Previously, builders did not enjoy extensive options for customizing floor performance. Traditional lumber joists typically adhered to uniform grading and dimensions necessitated for the longest span. In contrast, I-joists and trusses offer tailored designs to enhance performance based on the specific usage of each area within a home.
For example, hallways do not demand the same deflection ratings as bedrooms. Ensuring minimal noise and structural integrity is critical, as variability in joist size and strength is possible throughout different sections of a house. Open-web floor trusses enhance this customization, as builders can demand specific deflection ratings and installation layouts directly from truss manufacturers.
Custom builders such as Mike Guertin in Rhode Island appreciate this tailoring and the confidence it brings. “I prefer handing over my plans and receiving designs without needing to calculate spans myself,” he says.
However, estimated costs for trusses and I-joists depend on numerous factors, including location and regional pricing variances. It's noteworthy that, contrary to traditional lumber, deeper trusses do not inherently equate to higher costs; depending on design, a shallower truss might incur higher overall expenses due to increased material and labor needs.
Ultimately, both I-joists and trusses may carry a premium compared to dimensional lumber, but they deliver stability, customization, and efficiency that can make installation easier and faster.
Today, I-joists occupy over 50% of the market share for floor joists. Their lightweight and strong design outperforms traditional dimensional lumber, offering builders various options in terms of spans, depths, and flange widths. They are readily available from manufacturers across the country, with prices varying due to regional differences.
The shift from plywood to OSB as the web material provides many manufacturers with strength and quality assurance. Each manufacturer has proprietary OSB formulations designed for moisture resistance, which helps maintain flange-to-web bonds over time.
The flanges can either use solid lumber or LVL material, with LVLs composed of several thin wood layers, enhancing stability. While wood flanges are comparable in strength to LVL but may be more prone to dimension fluctuations, ESBs provide significant advantages in maintaining structure and integrity.
Pros
Cons
Open-web floor trusses are increasingly favored, particularly for custom home applications. Their lightweight and robust nature enable significant spans while allowing for extensive customization based on structural engineering needs. Builders appreciate that they can design trusses with designated load points, potentially eliminating the need for additional bearing walls.
The unique advantages of open webs make it easier to run plumbing and electrical work. By designing customized utility chases within trusses, builders like Chandler enjoy cleaner installations, optimizing energy efficiency and maintaining an organized project site.
These trusses can be sourced from localized manufacturers, with pricing fluctuating based on design specifications and regional availability.
Using stress-rated 2x lumber ensures the quality of top and bottom chords while minimizing costs through selective material grading, with heavier gauge steel plates securing web members to chords firmly.
Advantages
Cons
Hybrid Joist Options Available Companies like Georgia-Pacific and Universal Forest Products offer hybrid joists that combine features of both I-joists and custom-designed floor trusses.
Georgia-Pacific's XJ 85 is an I-joist with pre-cut holes for utilities while maintaining structural integrity as additional drilling is restricted. They are available in a range of lengths and depths, integrating utility chases seamlessly.
Meanwhile, United Forest Products’ Open Joist and Trim Fit’s SpaceJoist utilize alternative web structures, providing valuable options for builders who need both efficiency and strength.
The spans achievable by joists correspond to their strength and rigidity, as well as the loads they need to support. Key factors like deflection—how much a joist bends under pressure—play a critical role in floor performance. This includes calculations for both live loads (furniture and occupants) and dead loads (building materials).
According to the International Residential Code (IRC), deflection for floor joists is capped at span/360 for living spaces. A higher denominator indicates less deflection and typically better performance. However, deflection isn't the only vital parameter; additional factors come into play, influencing perceived performance under load.
As Tim Debelius, an iLevel representative explains, floors can possess a seemingly high deflection rating yet still feel bouncy—contrarily, lower-rated ones may feel solid. The depth of the joist is intertwined with such performance, as deeper joists yield less flex.
Ceiling materials and structural components also contribute to a floor’s performance. These elements provide lateral stability, minimizing movement while factors like weight and material elasticity can impact how a floor reacts under loads.
Exercise Caution When Cutting Joists Each I-joist component—both flanges and webs—plays a role in distributing loads effectively. Any modifications, especially to flanges, could lead to structural integrity failures. Thus, it is critical to adhere to manufacturer installation guidelines for hole sizes and placement to avoid potential failures.
Do Not Alter Trusses Each truss component is engineered for a specific role; hence, even minor modifications can compromise overall structure. It is crucial to refrain from alterations such as trimming ends or adding loads without consulting an engineer first.
I-Joists
Floor Trusses
For further reading on best practices for I-Joist installation, click the View PDF button below for more detailed information.
Previous: Is I-joist stronger than 2x10?
If you are interested in sending in a Guest Blogger Submission,welcome to write for us!
All Comments ( 0 )