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 aspects of deck building get a lot of ink, one of the deck’s most basic components often gets dismissed: joists.
There are a couple of solid resources to reference to get deck joists right: the 2018 International Residential Code (IRC), and the American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (DCA 6), which is based on the 2012 IRC. These resources give guidance on spans and cantilevers, connecting joists to beams, treating cuts, and many other important details. Check with your local building inspector to see which best applies to your area.
Needless to say, there’s a lot more to joists than simply choosing the right lumber. But that’s still the best place to start.
Dried lumber makes building easier
Most decks are framed with pressure-treated dimensional lumber. The pressure-treating process leaves lumber wet; the moisture content may exceed 50%. As the wood dries, it shrinks and often…