For years, 4x4 posts had been used to support decks with little thought to how much load the posts were carrying or how tall the deck was. That changed in 2015, when Section R507.8, a prescriptive method for sizing deck posts, was included in the 2015 International Residential Code. Prescriptive design is a cookbook for construction that eliminates the need for a design professional. However, like a cookbook compared with a chef, it’s limited in what it offers. The design tables for deck-post height included in both the 2015 and 2018 editions of the IRC were indeed limiting, as Table R507.8 was a one-size-fits-all prescription, with the maximum heights of 4x4 to 6x6 posts based on the greatest possible area of deck that could be generated with IRC joist-span and deck-beam design tables. This resulted in every deck having an 8-foot height limit when supported by 4x4 posts, based on loads coming from a two-ply 2x12 beam with 2x12 joists 12 inches on-center and cantilevering past the beam more than 4 feet. It was a good start to deck code, but desperately needed to be expanded and made more flexible. The 2021 edition of the IRC does just that. Three different categories of wood species with similar specific gravities are included, allowing southern pine posts to extend taller than redwood or cedar posts, for example. The previous tables were based on only a 40-lb. live load, but the new one includes three snow loads: 50, 60, and 70 lb. In regions where a 60-lb. live load is required by the local code, the 70-lb. snow load column is an equivalent.
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