Get Your Deck Joists Right
Prescriptive guides provide textbook solutions.
Synopsis: In this article, Mike Guertin takes an in-depth look at prescriptive deck-building codes when it comes to one of the deck’s most basic components: joists. He describes how joist spans and cantilevers are determined in both the IRC and Residential Wood Deck Construction Guide (DCA), and includes a joist-span table from the 2018 IRC to help explain how to calculate cantilevers and determine simple spans. He also includes a series of installation tips for creating robust joist connections, weatherizing, and more.
Old decks are time capsules. It’s interesting to ponder, “What were they thinking?” whenever I replace or upgrade them. I see built-up beams with unsupported joints, ledgers attached with seemingly any fastener that was in the truck, and joists of every size without regard for span.
Thankfully, prescriptive deck-building codes have largely squashed most bad practices. But while deck footings, ledger and lateral connections, and various other…