Backspan: Backspan is the part of a cantilevered joist within the supporting wall.
Cantilever: A cantilever is an extension of a floor joist beyond a supporting wall or beam. The distance that one may cantilever a floor joist depends on the width of the
roof parallel to the cantilevered joists, the floor joist depth and spacing, and the design snow load. Most readers should not design floor joist cantilevers.
Cantilevered Floor Joists
1. You may cantilever a floor joist not more than (≤) the depth of the joist. Example: you may cantilever a 2×10 floor joist not more than (≤) 10 inches. You may support one and two story light frame walls and a roof on these joist depth cantilevers.
2. You may cantilever solid lumber floor joists supporting one single story light frame wall and one roof according to the following table. Column headings refer to ground snow load and the width of the roof supported by the cantilever.
3. Use #2 Douglas fir, southern pine, hem-fir, or spruce-pine-fi r for cantilevered floor joists. Use a backspan ratio of at least (≥) 1 inch of cantilever to 3 inches of backspan. Install a full depth rim joist on the ends of the cantilevered joists and full depth blocking at the cantilever support wall.
4. You may cantilever solid lumber floor joists, supporting an exterior balcony according to the following table. Column heading refers to ground snow load in pounds per square foot (psf) and building width in feet.
5. You may interpolate to calculate values for building widths and snow loads not contained in both tables.
6. Refer to the IRC for more cantilever distances.
Floor Joists Under Load-Bearing Walls
1. Install additional floor joists under load-bearing walls that run parallel with the floor joists. Ensure that the number of joists is sufficient to support the load imposed by the wall and the loads supported by the wall. Parallel floor joists run the same direction as the wall being supported. Example: install at least (≥) 2 floor joists under a wall supporting a roof and at least (≥) 3 floor joists under a wall supporting a story above and a roof.
2. You may separate the additional joists under a load-bearing wall, if necessary, to fit pipes, vents, or ducts. Install solid 2 inch thick blocking at least (≥) every 4 feet along the full depth of the separated joists.
3. Place load-bearing walls that run perpendicular to the floor joists not more than (≤) one joist depth from the supporting wall or beam below unless the joists are sized to carry the load. Perpendicular joists run at a 90 degree angle to the wall being supported.
Example: place a loadbearing wall that runs perpendicular to 2×10 floor joists not more than (≤) 10 inches from the supporting wall or beam below.
Header, Girder, and Floor Joist Bearing on Supports
1. Place at least (≥) 1 ½ inches of a floor joist, header, girder, or beam on the supporting wood or metal wall.
2. Place at least (≥) 3 inches of a floor joist, header, girder, or beam directly on the supporting masonry or concrete wall or column. The bearing surface should be at least (≥) 4 inches thick.
3. Place at least (≥) 1 ½ inches of a floor joist, header, girder, or beam on a wood sill plate bearing on a supporting masonry or concrete wall or column. Use a sill plate that is at least (≥) nominal thickness and provides least (≥) 48 square inches of bearing surface.
4. You may support floor joists on at least (≥) a 1×4 ledger if the ledger is attached to each stud and if the joist is attached to a stud.
5. You may support floor joists, headers, girders, and beams using approved joist hangers.
Floor Joist Lap at Supports
1. Lap floor joists from opposite sides that meet over a bearing support at least (≥) 3 inches at the support and nail the joists at the lap using at least (≥) three 10d nails. You may substitute a wood or metal splice of equal or greater strength for the nailed lap.
Floor Joist Attachment to Beams
1. Use an approved joist hanger or at least (≥) a 2×2 wood ledger to support floor joists that connect to a beam or girder.
Floor Joist Blocking
1. Install full depth solid blocking at least (≥) 2 inches thick at both ends of floor joists, or attach the joists to a header, band or rim joist, or attach the joists to an adjoining stud. This helps reduce joist twisting.
2. Install blocking at all intermediate load-bearing supports in seismic design areas.
Floor Joist Bridging
1. Install bridging not more than (≤) every 8 feet of floor joist length on floor joists deeper than (>) 2×12.
2. You may use solid full depth bridging, wood or metal diagonal bridging, or other means to provide required floor joist bridging. Some code officials require bridging on all floor joists regardless of what the IRC requires.