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
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
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
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.