Still defaulting to rim boards at every floor edge? In modern wood-framed construction, it’s worth taking a closer look at several detailing options including ribbon boards or top chord bearing detailing. These options can simplify construction and reduce blocking material while still being able to offer a reliable, flexible, and code-compliant alternative for lateral shear transfer, gravity transfer, and floor-to-wall transitions. [For all images, See PDF or View in Full Issue.]
Understanding Lateral and Gravity Load Paths
In any wood structure, gravity and lateral loads must be continuously transferred through structural members to the foundation. Lateral loads from wind or seismic events are picked up first by the horizontal diaphragm (typically floor sheathing), transferred to the walls, and ultimately down to the ground. Similarly, vertical (gravity) loads from roof, walls, and floors must be supported and channeled to structural bearing points.
Rim Boards
Rim boards are a common and reliable method for floor-to-wall transitions. They sit directly on the wall plate and act as full depth framing members, providing a direct bearing surface for both vertical and lateral loads. Floor sheathing is fastened directly to the rim board, and vertical loads from above—including stacked walls or posts—transfer cleanly into the supporting wall below. Rim boards are typically specified as Engineered Lumber such as LVL or LSL and widely used in conventional wood framing due to their simplicity, familiarity, and predictable load path behavior.
However, while rim boards offer clear advantages, they are not the only option available. Designers should consider whether alternative details—such as ribbon board framing with sheathing splices or top chord bearing trusses—could provide similar structural performance with greater flexibility or cost efficiency. In projects where load demands are modest or where floor framing depth and fire-rating conditions are critical, alternative detailing strategies may reduce material use, simplify coordination, or improve constructability.
Ribbon Boards
Ribbon boards are a practical solution for light-frame construction, especially in 1- to 2-story buildings where vertical loads are relatively low and don’t accumulate significantly from above. This makes them an ideal fit for single-family homes, garages, and small accessory structures.
Ribbon boards are set within the depth of a floor truss and do not typically align directly over the wall plate. When properly designed and detailed for gravity loading, ribbon boards can be used to transfer vertical loads. In most standard installations, ribbon boards serve as alignment and lateral support elements. If gravity loads are not specifically designed to bear on the ribbon, supplemental blocking should be added to transfer those loads to appropriate bearing supports.
Figure 2 shows a separate sheathing panel to bridge the gap between the upper and lower levels. The gravity load path begins at the studs in the upper wall and is transferred through the sill plate to the ribbon board or blocking. From there, the load is redirected into the adjacent floor trusses, through the truss bearing, and ultimately down to the top plate of the supporting wall below, completing the vertical load path to the foundation.
To ensure lateral load continuity, the additional sheathing panel is connected to the sill plate of the upper wall and the top plate of the lower wall or floor framing using proper shear wall edge nailing. This splice allows lateral loads—such as wind or seismic forces—to transfer across the floor assembly, preserving diaphragm continuity and maintaining the effectiveness of the overall shear wall system.
Edge nailing at both upper and lower wall plates is essential to maintain the integrity of this lateral force-resisting path. Lateral loads from the diaphragm are first transferred into the ribbon board through diaphragm edge nailing, then into the sheathing via the fasteners connecting the sheathing panel to the ribbon. From there, the sheathing transfers those loads into the adjacent shear wall system below. This layered connection ensures a continuous and code-compliant lateral load path.
This approach maintains the integrity of the gravity and lateral load path and is a well-established method in wood-framed construction. Though it involves an additional sheathing step compared to using a rim board, ribbon detailing offers greater flexibility in truss layout and material usage—key factors in both design efficiency and cost control.
Top Chord Bearing Conditions
Top chord bearing trusses make more sense in multi-story or multi-family buildings, where loads are greater due to accumulated loading. At exterior walls, code requirements for fire resistance add another layer of complexity. In many jurisdictions, blocking within trusses at exterior walls must be fire-treated, which can increase material costs and complicate detailing. By contrast, top chord bearing trusses may avoid fire treatment requirements in the floor assembly, as clarified in resources like DCA-3 (Design by Code Acceptance by the American Wood Council) and the International Building Code 2024, particularly when the top chord is fully embedded and shielded by rated assemblies.
In multi-story construction, top chord bearing trusses provide a highly effective floor-to-wall transition. Here, the top chord of the truss sits directly on the wall plate, allowing the floor sheathing to span continuously across the truss. This creates a clean and direct gravity load path and often eliminates the need for a separate rim or ribbon board.
Figure 3 illustrates a simplified gravity load path where vertical loads from the wall studs above transfer directly through 2x blocking and top chord bearing trusses into the supporting wall below. Because the top chord rests directly on the wall plate, this detail eliminates the need for intermediate framing elements like ribbon or rim boards, streamlining vertical load transfer.
The lateral load path is also simplified in this configuration. Lateral forces from the wall above are transmitted through the top chords and 2x blocking using either nailed shear transfer connections or mechanical hardware. This direct path enables clean diaphragm continuity and reduces detailing complexity, making top chord bearing conditions especially effective for multi-story and fire-rated assemblies.
Design Recommendations
Figures 4 and 5 are detailing examples for both ribbon board detailing and top chord bearing detailing. To ensure performance in either lateral or gravity load transfer, we recommend:
- Include notes for ‘diaphragm nailing per plan’ at all critical interfaces.
- Ensure ribbon boards are properly designed if they are to support gravity loads.
- Provide sheathing splices or extended sheathing where diaphragm continuity is needed.
- Plan for blocking where vertical loads align with ribbon boards.
Conclusion
Selecting the right truss end detail—whether it’s a rim board, ribbon board, or top chord bearing condition—should be a deliberate decision informed by structural needs, code requirements, construction priorities, and cost considerations. At MiTek, we understand that good detailing isn’t just about following tradition; it’s about critically evaluating each project’s unique demands and identifying opportunities to simplify framing and reduce material use while delivering improved building performance. Early coordination between designers, engineers, truss manufacturers, and contractors creates more options for smarter, more efficient solutions—especially when fire ratings, load paths, and framing layouts intersect.
By engaging with MiTek during the development of your construction drawings, you will be able to unlock the power of Offsite Construction through our collaborative network of Design Make Build partners and achieve optimized results. We aim to come alongside the engineering community to enable a more thoughtful and holistic approach to design, considering all detailing strategies, to deliver better results for wood framed construction. Choosing the right detail isn’t just good engineering—it’s good building. The best solutions come from understanding how systems work together, asking the right questions at the right time, and pushing framing practices forward, one thoughtful decision at a time.
Interested in collaborating with MiTek and our partners on your next project? Email DesignEngagement@mii.com.
Sources
American Wood Council. WFCM: Wood Frame Construction Manual for One- and Two-Family Dwellings, 2021 Edition. American Wood Council, 2021. https://awc.org/publications/wfcm-2021.
WoodWorks – Wood Products Council. Detailing Floor-to-Exterior Wall Conditions in Type III Projects. WoodWorks, 2023, https://www.woodworks.org/resources/detailing-floor-to-exterior-wall-conditions-in-type-iii-projects/. Accessed 21 Apr 2025.
American Wood Council. DCA 3: Fire-Resistance-Rated Wood-Frame Wall and Floor/Ceiling Assemblies. 4th ed., American Wood Council, 2021. https://awc.org/publications/dca3.
International Code Council. “Section 705.7.1: Projections.” International Building Code 2024. International Code Council, 2023.