Many forces in addition to gravity can affect a deck’s ability to remain stable during its lifetime as it is set under a variety of conditions. High winds can cause uplift and top heavy decks can lead to lateral shifting or racking both of which cause tremendous stress and possible failure to a deck structure. We recommend using 6x6 structural posts because they have twice the resistance to sway than 4x4’s. Tall decks are much more susceptible to these unbalancing forces. Burying your posts is another technique that will increase lateral resistance and protect your deck from racking.
Decks with an above grade pier footing type should have bracing installed to prevent the post base from acting as a pivot. Bracing comes in many forms but is always used to prevent racking and increase the frames stiffness. For instance 4x4’s can be installed at 45 degree angles parallel to the beam as Y or knee braces connecting the beam to the sides of the structural posts. Always use bolts rather than nails to insure a strong connection. The taller the structural posts the longer and more substantial the braces should be.
Unfortunately there are no hard rules regarding bracing. You will probably not be required to install bracing but it is an option worth considering because it will increase the strength of your deck. Bracing between posts using X or K bracing can be also be used to make a deck more rigid. In this method you attach 2x4’s or 2x6’x from the top of one post to the bottom of the next and vice versa resembling the understructure of a wooden railroad bridge. This technique is rarely used because it is obstructive and unattractive. One of the only situations where you would be likely to need X bracing would be if your deck had long support posts and was sited on a sloping lot.