Cantilevers can extend up to one-fourth the backspan of the joist, according to the revised span tables and IRC requirements. This means that joists, such as southern pine 2x10s spaced 16 inches on center and spanning 12 feet, can cantilever up to 3 feet (see illustration, below). Beams cannot be used instead; they have no way to transfer load to other beams. Instead, hollow metal columns are used, which can cause wind problems for some home designs.
The maximum allowable depth for a cantilevered section is two times its width, measured from face of support to face of support. For example, this would mean that a cantilever could not reach deeper than three feet into a wall cavity, since the distance between the faces of the wood framing members is usually less than two feet. A deep foundation is required to support a cantilever extension.
A common mistake when calculating the load capacity of a cantilever is to use the total weight of the structure, instead of just the weight on the extending portion. The entire weight of the house must be taken into account when determining how much load it can sustain without failing. If the house is made out of thin material like cardboard or paper, then it will not be able to withstand its own weight and will collapse under its own pressure.
It is important to remember that the revised span table does not apply to window wells or other similar projections.
According to the American Wood Council, cantilevers are limited to one-quarter the span of the joists. Joist Dimensions (o.c.) The maximum permissible overhang cannot be more than a quarter of the actual main span. If the overhang exceeds this limit, you will need to either install blocking or add more joists.
The accepted practice for wood framing is to use 2x4s as studs and to attach sheathed 1×4 floorboards to them. The 2x4s serve as both roof and wall studs. They are available in lengths up to 36 inches, so they can be used in place of 2-by-4 pine boards if necessary. A pair of 2x4s spaced 16 inches on center is considered sufficient support for a single sheet of plywood.
The term "cantilever" refers to a method of providing additional load-bearing capacity by extending a beam beyond the walls of a structure. In order to prevent damage from occurring due to excessive strain on the beam, beams are often supported at some distance from the walls they carry. This allows room for expansion and contraction without causing failure. Cantilevered structures are commonly found in buildings that have wooden frames, since this is the most effective way to provide extra support while still allowing for some degree of movement.
A decent rule of thumb is that a cantilever should not be more than four times the height of the joists. 2 x 8 = 7.25 in. High x 4 in. Long = 29 in. If a 2x8 can span 12 feet according to the code span tables, the cantilever can be 3 feet. This means there are five feet between the top of the beam and the bottom of the next-highest board.
This calculation assumes uniform beams. For a beam with an open web design, the maximum allowable load for a single support is one-half the total weight over any one square foot. So, if a six-foot-long 2x8 has a nominal load of 50 pounds per foot, the open web version could support 25 pounds. The closed web version cannot be supported on its own but would need another support below it.
The actual load that a beam will bear depends on how it is constructed. A beam that is well designed and built to code will support much greater loads than this calculation suggests. Beams used as supports for other structures should be able to support at least their own weight plus any overload caused by wind or earthquakes.
The code also requires that all beams within 15 inches of the outer wall of a building have supporting posts located no more than three feet apart. These posts must be made of wood or metal and must be attached to the beam with two bolts at least 1-1/4 inches in diameter.
So the question is, "How far can we cantilever the joists' far ends?" According to the chart below, 2x6 floor joists can be cantilevered up to 2 feet and 6 inches (2'-6") when placed 16 inches apart. However, there are limitations based on the load you will be supporting and the spacing between the supports. If you go any further than 2 feet, you need to consider adding more support beams or columns.
The key here is that you don't want to put any weight on the last beam or column. In other words, make sure that you leave enough space for people to walk under the structure without hitting it!
Also note that if the distance between two supports is less than the length of a single-joist frame, then you should install additional supports. For example, if you were to install 2x6 floor joists 24 inches apart, you would need to add some kind of support at each end because the distance between the centers of two adjacent joists is less than the length of a single joist. In this case, you could use 2x4 supports at each end to keep the structure from collapsing.
Finally, make sure that you check with your local building department about how close you can place supports while still being allowed by code. Some areas require that you at least 20 inches between supports, while others allow much closer spacing.