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). Be sure to follow all appropriate safety procedures when working with cantilevered loads.
The maximum load for a single-degree-of-freedom (SDOF) system is 1/4 of the total span, or 25 percent of the average load on the supporting members. For a double-degree-of-freedom (DDOF) system, the limit is 1/2 of the total span, or 50 percent of the average load on the supporting members. If you're not sure whether your supports are SDOF or DDOF, check the manufacturer's documentation or consult an engineer. The default assumption for most systems is SDOF support unless otherwise indicated.
For example, if a house is 100 feet long and each floorboard weighs 10 pounds, then the maximum load that could hang from the ceiling without damaging its structure is 120 pounds. Because this is more than the average weight on a house floor, the board will sag under its own weight. To prevent this, two people are needed to nail them down at their ends.
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. For example, if a house has 12-inch-wide joists and a 10-foot overhang, the cantilever for that story is 75 inches.
The reason for this limit is that most buildings experience lateral loads that are borne by the end posts or abutting walls. If the overhang in a given situation exceeds 24 inches, shear forces will be present on these end posts or abutting walls. The design load should be sufficient to produce no more than a 1/8 inch gap between the bottom of the beam and the floor or ground surface. Any more than that and you have created a cantilevered structure that is prone to failure.
Cantilevers are usually not recommended for houses because they are difficult to build and therefore cost more. But they are used in large buildings such as factories and warehouses where the cost of building code compliance is too high.
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 of clear space below the beam for insulation and other items.
Cantilevers are commonly used as roof supports because they can carry a lot of weight without damaging the building's structure. The strength of a cantilever depends on how much weight it has to support. The heavier the load, the more likely you will see damage to the building.
The most common type of damage caused by unsupported beams is called "tie-in" damage. This happens when one end of the beam hits something hard (such as a wall) and then continues into space without any support. Even if the other end doesn't hit anything, the force of the impact can cause serious damage to the beam itself. Tie-ins can also cause walls to collapse, posing a threat to anyone under or near them.
Tie-ins can be avoided by supporting the beam with another structure. For example, if a 2x8 beam is being used as a roof support, it should be tied back to the wall at both ends. This will prevent it from pushing out into empty space and causing damage.
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 distance between the supports.
If you were to try this at home, note that if you were to use 2-by-10s for your joists instead, you could go much farther than 2 feet. A 2-by-10 is actually rated for an internal force of 2,000 pounds! That's more than enough pressure to support a small house.
The important thing is to ensure your joists are well-supported. The best option is usually a foundation or slab built under the entire length of the projected area. This way any potential stress on the joists is taken off their shoulders and distributed across the ground.
You should also carefully consider what type of anchors you will use before you start framing. For example, if you were to use common household nails to attach the joists to the supports, you would need to make sure they are not longer than 1/4 inch so they will not pierce through the wood.
Finally, make sure nobody is going to walk off with your house!
This is not based on actual construction but simply applying the rules of code enforcement. A real building code would limit this to 2 feet 10 inches.
A double-beam cantilever with 2x10s instead of 2x8s for the main beams and 2x6s for the secondary beams would have a carrying capacity of 40 pounds per square foot. This is more than enough load for most buildings.
The total weight of a building isn't always equal to its strength. For example, a house might appear to be strong enough but it's actually not because one side is shorter than the other. Or one floor is lower than the other. Or there are other factors such as unsupported spans or uneven loading that reduce the overall strength of the structure. In these cases, the codes require some sort of load distribution between beams to prevent them from being placed too close together.
In conclusion, the double-beam cantilever is a very reliable and efficient method for transferring loads from one end of a building to the other. Although it was originally designed for use with timber framing, it can also be employed using steel framing materials.