Used for centuries in many different ways, cantilevers as a construction device were first recognized in 19th Century bridge designs. But they gained significant notice in architectural use when Frank Lloyd Wright unveiled his iconic “Fallingwater” commissioned residence. There the cantilever was not just a structural means to an end but also a dramatic design feature.
Today, cantilevers are commonly used for balconies and decks but are also effective in extending upper story living space beyond the boundaries of bearing walls. “Bargain” space results and architectural interest is often created through the use of cantilevers.
Let’s look at how best to frame for cantilevers when using an open web floor joist like TRIFORCE®.
Establish the Situation
Any time a cantilever is being considered as part of a floor system design, loading conditions related to the cantilever must be recognized and analyzed. Design software will dictate specific framing practices but for our purposes here, general approaches to cantilever installation will be discussed.
Cantilevers Parallel to Joist Direction
In this situation, while the open web joist stops at an exterior bearing, a 2X extension from it extends beyond the bearing to become the cantilever joist (see following illustration). This 2X is fastened to the side of a TRIFORCE® joist so that it “snugs” against the joist webs and under the top chord of the joist. 1-1/2” dimension lumber is attached to the top of this 2X so that the cantilever can achieve the same top plane as the open joist. Where this 2X crosses the bearing, a solid wood filler is needed to transfer load. If the cantilever is being extended from the trimmable I-Joist end of a TRIFORCE® joist, a 5/8” wood filler is required on the OSB panel.
Cantilevers Perpendicular to Joist Direction
With TRIFORCE®, it is possible to construct a cantilever perpendicular to the joists by using 2X joists inserted through the open webs of the joists. In this situation, the following illustration shows proper framing to transfer load correctly to the bearing wall.
The recommended maximum extension for a cantilevered exterior deck is four feet. In this situation, 2X joists should extend back into the floor at least eight feet, using a recommended 2:3 ratio. It is possible to extend beyond four feet but each situation must be examined and analyzed for structural capacity.
In all cases where dimension lumber is used for framing an exterior deck, it is recommended that treated lumber be used.