The Role of Cross Blocking Infills in Deck Construction Without a Rim Joist
Recently, we looked at a relatively interesting deck on top of an old historic stone garage. The garage walls are built with a rubble type stone masonry, but some of the stones have been dressed to be near rectilinear in format. The stones though are randomized so they do not set in even cources. These details are nearly irrelevant in terms of the garage itself, but they matter in terms of the capacity of the historic garage walls’ ability to support a rooftop deck. Most of the details of the deck though, so far, seem pretty typical, especially for tight urban areas like Washington DC where rooftop decks are relatively common because we don’t all have large backyards.
In the next picture below, you can see the edge of the deck at the area above the rear termination of the garage roof. The deck spans from the side to side, from exterior wall parapet to exterior wall parapet. Essentially the deck itself makes no contact with the majority of the roof membrane, instead it just bears on top of the coping of the parapet.
So far, when we were checking out this deck, all of these details seemed relatively normal. Obviously, the deck boards are in the middle of their span, or older, and need at least some upkeep and restaining and sealing. When we look at the deck from the side though, we can see something that was an aberration or anomaly and different than we expected. Each of the floor support joist of the deck, where they span from side to side, we’re built without a connection to a rim joist.
A rim joist basically serves as the binding structural perimeter that ties together the ends of the joists, providing stability and support to the overall deck structure. The rim joist is always positioned along the outer edge of the deck (hence the term “rim”), and the rim joist helps distribute the load of the deck and any live-load placed on it, such as furniture or foot traffic. By transferring these forces to the supporting posts and beams. Here, in this somewhat unique context, with each individual joist bearing on the parapet of the stone wall below, maybe the function of distributing the load was deemed less critical.
The rim joist also helps maintain the alignment and spacing of the joists, which helps the deck remain level with planar consistency. The rim joist can also serve as a mounting point for the deck’s railing system, which can help in the safety and aesthetics of the finished deck. (Here, in this unique and different type of circumstance, many of the deck railing supports are connected to the cross blocking members which are then connected to the joist, but this configuration makes the guardrail weaker because it’s connected to a piece which is supported by another piece of the deck.)
In most normal cases, the rim joist is needed for the structural continuity and joining / trying of the deck together which prevents lateral movement and provides a firm, stable foundation for the deck boards. Here though, the rim joist was omitted and instead substituted for individual pieces of cross-blocking. The big trade off for downside to this type of methodology is that in this unique case, they have missed or omitted the rim joists’ help in keeping the deck joists uniformly supported and aligned in a lateral direction.
The next picture below shows each of those pocketed type of cross blocking pieces set between the tails or ends of each of the field joist.
The last and final picture below shows a close-up view of the cross blocking set between the ends of the joist. Even after years of age, the deck looks relatively uniform at these edges. You can see that when this deck was originally built, instead of setting each joist directly on top of the parapet wall coping, they instead installed a 2x board, run perpendicular to the joist system, in-line with the parapet wall.
In general, it’s often better to install cross blocking in a staggered type of fashion. The typical minimum span of deck joists that require cross-blocking installed in the middle of the span is usually around 8 to 10 feet, depending on the joist size, spacing, and load requirements of the specific deck. Cross-blocking is often recommended or required when the span exceeds this distance to prevent joist twisting or warping and to enhance the overall stability of the deck and rigidity of the deck frame. In the construction process, when you walk across a deck without blocking and then later once the blocking is installed, you can often notice a major difference because the intermediary blocking significantly reduces bounciness in the deck.
In most scenarios, cross-blocking is installed in a staggered configuration rather than directly opposite each other. Here, where they substituted a rim joist with cross blocking, they aligned each cross blocking piece to the next one because they all run consistently at the outer edge of the joist system. Staggered cross-blocking is usually preferred by most builders because it allows for easier installation; because staggered blocks don’t compete for space with the adjacent blocks, making it easier to nail or screw them into place. Additionally, staggered blocking provides more effective lateral stability by slightly spreading or spacing the mounting / connection locations. By staggering the blocks, the deck joists are slightly less likely to twist under load, as the blocking interrupts any continuous weak points along the length of the joist, reducing the potential for movement and improving the deck’s overall rigidity and structural stability.
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. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . 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
