Main Structural Deck Components: RIM JOISTS Part VI of VI

Today we will discuss article VI, about rim joists, and close out our long and detailed series on the structural components of decks. This series highlighted the main structural components and there are other smaller details involved in the structural load path and uplift resistance of decks. today we conclude by taking a close look at rim joists which are the joist, similar to the other main flooring or deck joists, but the rim joists are at the perimeter of the deck and tie the rest of the joist together. The ledger, back against the exterior main building, creates a unifying bearing point, but at the outer edges, at the other three sides of the deck, the rim joists create that band which binds those joists to one another.

The outline of the series follows:

    1. Footings
    2. Columns
    3. Girders
    4. Joists
    5. Ledgers
    6. Rim Joists

outer rim joist

The concept of rim joists is pretty simple, they keep the whole deck’s joist frame together, bounded and essentially set the band at the outside of the frame system. Sometimes rim joists are also referred to as band joists, but it might depend on the particular context.

Joist, along with the associated rim joists are essentially long and thin boards set up on the thin edge in a horizontal position but with the thin edge at the top and bottom.  In this particular position, a joist has a much higher resistance to deflection and failure or subsidence over time. For example, if you lay a board flat and support it at two points along its run or length, the amount of weight or structural load required to bend that board to deflect any amount of measurable significance such as 1 inch over 12 ft is very low. If you take a 12-ft board and support it at one end and then support it at anywhere in the mid span, the amount of force or pressure it takes to deflect that one inch downward is significantly lower when that board is laid along it’s flat wide side, by comparison when the board is set up on its thin side it’s very hard to get that board to deflect.  Because of this structural characteristic, joists are set in a horizontal position on their thin edge and generally supported at each edge with a joist hanger and or cross blocking in the intermediate area of the span and then at the outer edge bounded together with a rim joist. That outer rim joist doesn’t provide nearly the same structural support that the ledger board provides because the ledger board, after all, is set and mounted back to the main building. The outer rim joist, though by comparison, is basically floating in the air and has nothing below it to support the rim joist itself. Instead, the rim joist is just connected to the other two rim joists on the sides, and the joist tails of the intermediary joists at the outer edge. That connection is very important, though, because it essentially ties all of the remaining joists together and stabilizes them and prevents them both from tilting (called: roll restraint).  The rim joints at the outer edge of the deck also prevent the boards from spreading or shifting inward as they are set in parallel.

outer rim joist hangers

Just as at the ledger board, at the outer rim joist joist hangers are also installed to add additional support to each of the intermediary joists, within the deck frame.  These joists are the backbone of the deck frame.  The intermediary joists will often be set on 16-inch centers, tighter spacing are often overkill or unnecessary depending on the load calculations, overall deck span, thickness of the lumber and associated rating.  

In some cases diagonal sway bracing or metal reinforcement tees may be installed at the upper or lower edge of the deck frame  system.   Diagonal bracing or diagonal ties are essential elements in a deck joist system to prevent specific types of structural failures. These components are designed to provide lateral stability and resist racking or twisting forces that can compromise the deck’s integrity. Even though we’re not right in the middle of hurricane or tornado alley, here in Washington DC, we still often have weather patterns that bring significantly high winds at different parts of the year.  

Lateral forces, such as wind loads or seismic activity, can exert significant stress on the deck structure, causing it to shift or rack sideways. Without sufficient bracing, these forces can lead to structural failures, including the separation of the deck from the main building or the collapse of the entire deck system. Diagonal bracing is installed to counteract these lateral forces and maintain the deck’s structural stability.

functions of rim joist

These diagonal elements act as tension and compression members, effectively triangulating the joist system and creating a rigid, interconnected frame. When lateral forces are applied, the diagonal braces resist the racking or twisting motion, transferring the loads through the deck frame and ultimately to the support posts or footings.

Diagonal ties, on the other hand, are typically metal connectors or straps that are installed in a similar diagonal pattern. These ties work in tension, resisting the lateral forces and preventing the joists or rim joists from separating or pulling apart. They provide additional reinforcement and ensure that the deck remains securely anchored to the main structure, preventing potential uplift or separation failures.

Installation of diagonal bracing and ties is often required, and other hardware described above such as joist hangers, framing anchors, or structural screws are almost always required.  However, each deck system is unique.  

rim joist

By incorporating diagonal bracing and ties into the deck joist system, you can significantly enhance the structural stability of the deck, increasing the frame’s ability to withstand lateral forces and reducing the risk of potential failures, such as racking, twisting, separation, or collapse.

Use a contractor who understands and cares about doing things right.  Always, feel free to reach out to us here at Dupont Decks and Patios.  You can call us at (202) 774-9128.  You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us 

 

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