According to the new span tables and IRC provisions, cantilevers can extend up to one-fourth the backspan of the joist. This means that joists, such as southern pine 2x10s at 16 inches on-center, spanning 12 feet are allowed to cantilever up to an additional 3 feet (see illustration, below).
If the code span tables allow a 2×8 to span 12 feet, then the cantilever can be 3 feet.
Likewise, how far can deck boards overhang? Allow for Sufficient Overhang of Decking A one to two inch overhang beyond the outer fascia board is sufficient. This means the installed deck boards should have at least 1.75 to 2.75” overhang before the fascia is installed.
Looking at the table below, we see that 2×6 floor joists can be cantilevered up to 2 feet and 6 inches (2′-6″) when the joists are spaced 16″ apart.
Many building departments limit joist cantilevers to 24″ maximum. You must use at least five framing nails to attach the rim joist to the ends of your joists. The American Wood Council states that cantilevers are limited to 1/4 the span of the joists.