I'm not a carpenter, but I did stay at a Holiday Inn last night. Heres what I think:
Load-bearing cantilevers need careful engineering
Many builders lay out cantilevers according to a simple rule of thumb: "One out, two in." This rule means that for whatever length the joists extend past their bearing point, they should run back in at least twice as far. While technically correct, the rule applies to non-load-bearing applications only, and even then has its limits. In nonbearing applications, a joist may not cantilever more than four times its depth. Therefore, a 2x10 joist should cantilever no more than 37 in. (4 x 9-1/4 in.), regardless of its length.
Non-load-bearing cantilevers can include sun decks and even bay windows (the cantilever supports only the weight of the window; any loads above are carried on a header set into the main wall). On the other hand, a zero-clearance fireplace with a two-story wood-frame chase would impose a significant bearing load on a cantilever.
Some simple load-bearing cantilevers can be built without paying an engineer. Because loads transfer through solid-sawn joists at 45° angles, codes allow load-bearing cantilevers that extend the same distance as the joists are wide. In other words, you could set a bearing wall on the end of 2x10 floor joists that are cantilevered 9-1/4 in. without risking a correction notice.
Cantilevered joists that run perpendicular to the main floor joists may have another problem: If the connection between the two is not constructed properly, a teeter-totter effect could force the inside edges of the cantilevered joists upward, creating a hump in the floor.
To prevent this unpleasantness, the cantilevered joists should butt into a main joist that has been doubled to serve as a header. The connection between the cantilevered joists and the header should be securely constructed to prevent independent movement. As an added measure, subflooring should overlap the joint where the cantilevers and the main joist meet.