Joists perform similar functions to beams, but are smaller and are used to support flooring directly. They transfer loads from flooring or decking to beams or walls. Joists are usually spaced 16 or 24 inches on center in decks, porches and balconies. Since there are typically several joists in the structure, the failure of any one joist is less important than the failure of a beam.
Most porches and decks have wood joists. There are new products for use in exterior construction. Many of these are recycled plastic, or mixtures of plastic with wood.
Joists may be supported on top of beams, ledgerboards or other framing members. Joists may also be supported by metal brackets called joist hangers. A properly installed joist hanger may be a more secure connection than a joist resting on a beam. Joists resting on beams are typically toe-nailed to the beam. While gravity provides a good downward anchoring force, there is less protection against uplift than with a joist hanger. Also, the end of the joist is better restrained from twisting or other lateral movement with a joist hanger.
These joist hangers and their fasteners may or may not be corrosion resistant. Joist hangers should be checked for rust. Joist hangers also come in different sizes. A joist hanger intended for a 2 x 4 will not be effective on a 2 x 10.
It is very common to find that the joist hangers were not nailed properly. With very few exceptions, all of the holes in joist hangers are intended to have nails. Skimpy nailing can lead to failure of the joist hanger and movement of the joist.
The end bearing for joists should be at least 1 1 .2 inches, even on joist hangers.
Many decks are built with poor lateral stability and will rack (move slightly side-to- side) easily. A deck does not have to move very much before connections are weakened.