Overloaded Cantilevers

Most framers are familiar with the 1-out/2-in rule for cantilevers. This rule of thumb says that the distance you can cantilever a joist is equal to one-third its total length. In other words, an 8-foot joist can be cantilevered 32 inches, so that 64 inches of the joist remains inside the building. The problem is that many framers don't realize that this rule applies only to nonbearing cantilevers. When a bearing wall comes down on the end of the joists, the cantilever is limited to the depth of the joist. In the case of a 2x10, for example, you can cantilever the joist 9 inches. A lot of the cantilevers I see are overloaded because the framers don't recognize the load that is being applied to the end of the joists. The 2x10s shown in Figure 2 overhang the supporting wall by almost 2-1/2 feet and are tied back into a double joist about 5 feet away, running perpendicular to the cantilever.

Figure 2. The floor framing in the photo, which was cantilevered to allow for a zero-clearance fireplace, follows the 1-out/2-in rule of thumb for nonbearing cantilevers. The weight of all the framing and finish materials, however, plus the fireplace and chimney, adds up to well over 1,500 pounds. The illustration shows how the cantilever should have been framed. The solution, however, depends on the species of lumber used. The author recommends that an engineer design any loadbearing cantilevers. The framer thought this was okay because the cantilevered joists were not supporting any roof loads, just a zero-clearance fireplace. What you can't see in the photo, however, is that the wood chase enclosing the fireplace and chimney extends upwards for two stories, continues along the gable end of the 10/12-pitch roof, and extends several feet beyond the peak. In addition to the weight of the fireplace itself, you have to add the weight of some 30 feet or more of insulated metal chimney, plus the studs and sheathing used to construct the chase. On top of that, the chase is clad with hardboard siding, a fairly heavy material. Altogether, these materials add up to well over 1,500 pounds. That kind of load will cause the cantilever to fail both in bending and in shear. The way this cantilever should have been framed is shown in the illustration. First, the joist holding the ends of the cantilevered joists inside the building should have been doubled to carry the extra load. To resist the tendency of the cantilevered joists to lift up where they meet this beam, the subfloor should span the joint by at least 6 or 8 inches, enough to get 2 or 3 nails on both sides of the beam. A 12-inch overlap would be even better. Next, the outer cantilevered joists, as well as the rim joist, should be doubled, and the cantilevered 2x10s should be placed closer together. In most cases, the next smaller on-center spacing would probably work - switching from 16 inches on-center to 12 inches, for example. Finally, the cantilevered joists need to be blocked where they cross the supporting wall. This solid blocking resists the tendency of the joists to rotate under load. This solution is not a rule of thumb, however. It would have worked in this case, provided the framing material was southern pine or Douglas fir. My recommendation is to have an engineer specify the framing for any loadbearing cantilevers.