How Long Do Treated Deck Joists Last?

While the framing in your home is protected by layers of siding, brick, caulking, and paint, your timber deck framing stands vigilant and exposed to rain, snow, sun, termites, and all of nature’s elements. That’s why it’s good to have an understanding of the deck joists and bearers that support your deck and how long they last.

Often, homeowners want to reuse old framing to install new deck boards. Before making this decision, consider the following factors:

• Cracks or large splinters, which are often signs of stress
• The top of the joist looks wider than the rest of the joist, which may indicate that water has penetrated the boards from screw or fastener holes 
• Soft spots — there should never be soft spots in your deck’s framing
• Signs of decay at the ends of boards
• Warping
• Support posts are buried in the ground or in concrete, which can trap moisture in the post.

If you see rot or decay, you will need to replace the affected joists, bearers, or posts. Additionally, if you are installing PVC or composite decking, you may need a new frame. Most composite deck manufacturers recommend spacing joists every 305 mm on centre. Your old deck frame likely has joists spaced 406 mm or even 610 mm.

New vs. Treated Deck Joist

While all deck joists should be pressure-treated timber, PT timber now is not the same as 30 years ago. For example, because of changes in preservatives over time, today’s timber decks are built with timber posts above ground (not making ground contact to reduce moisture content) and attached to concrete footers. This change in building practice helps to reduce timber rot.

In the past, pressure-treated timber was protected by chromate copper arsenate (CCA), which prevented rot, but due to its arsenic, it ended use in 2003. Now, the timber is treated with a preservative called alkaline copper quaternary (ACQ). ACQ, a water-based timber preservative, contains a soluble copper(II) complex and quaternary ammonium alkyl- or aryl-substituted compounds.

Facts about Treated Deck Joist

Because ACQ-treated timber contains much higher concentrations of copper, a barrier is needed between the joists and metal fasteners. When the zinc in galvanized fasteners touches ACQ, corrosion occurs. Joist flashing tape protects your timber with a thin membrane to guard against corrosive preservatives and holds your metal fasteners and screws in place.

One other consideration for deck joists is the type of timber you choose. Should you go with softwood or hardwood? Is it better to use Douglas fir, sapwood, redwood, or pine? Some timber is more naturally rot-resistant. Hint: tropical hardwoods such as ipe and old-growth teak are among the most resistant. Exceptionally resistant domestic varieties include black locust, red mulberry, Pacific yew, and Osage orange.

How to Make Treated Deck Joists Last Longer

There are a few important ways to make your deck joists last longer:

• Using pressure-treated lumber  
• Covering the joists with a moisture-resistant stain or sealer before installation
• Using metal joist hangers and leaving at least 3.2 ml between the end of the lateral joists and sides of the bearing joists to allow for airflow 
• Installing decking boards with hidden fasteners that reduce contact with the joists and ensure consistent 6.3 ml spacing between decking boards  
• Clean the deck often to prevent debris from trapping moisture onto the boards
• Use a butyl-based flashing tape to cover the tops of the joists during a new deck build

With Trex Protect’s butyl-based joist, bearer & rim tape, you can protect your deck’s joists, rim joists, bearers, the ledger board, and stringers with a thin, waterproof membrane. Each roll of Trex Protect comes with a 25-year warranty due to its durability. 

Deck flashing tape:

• Helps to reduce future maintenance  
• Gives your timber deck a longer lifespan — as a sealant protects your top decking boards, flashing tape protects what’s beneath and doesn’t have to be reapplied each year like stain or seal
• Protects your deck’s structural integrity 
• Holds deck screws and fasteners tighter and stronger

Help your deck joists last longer by protecting them from the start with Trex Protect.