Get Your Deck Joists Right
Prescriptive guides provide textbook solutions.
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 changes shape. If this movement happens before the lumber is used, you’ll have fewer problems down the road. Lumber that dries in place is more likely to move, leaving the decking looking wavy.
One way to reduce wood movement is to use wood that is kiln-dried after treatment (KDAT) or air-dried after treatment (ADAT). Drying after treatment brings the moisture content below 19%, stabilizing the wood somewhat so it is less likely to warp or shrink beyond what you’d expect from seasonal…