Thousands of decks built in the late 2000s and early 2010s have framing that is quietly breaking down. Bad deck joists usually show insect damage, splits near the ends or edges, soft or rotted wood at key connection points, and black mold or white fungus on the lumber.
Each of these signs means the framing may be weakening. Below, we explain what to look for and what to do if you find it.
Warning Signs of Bad Deck Joists
Homeowners can check for most of these problems on their own. Start your inspection with insects and splits.
Insect Damage from Termites, Carpenter Ants, and Carpenter Bees
Wood-boring insects are one of the first things to look for:
- Termites are the obvious one to check for.
- Carpenter ants leave large tunnels running through the boards.
- Carpenter bees bore into the wood and can damage the joists over time.
Splits and Loose or Missing Knots
Check every joist for splits and for knots that are loose or missing. Pay close attention to where they are. Two zones matter most:
- The outer thirds of the joist’s span (the sections closest to each end)
- The top and bottom thirds of the joist’s height
Splits or knot problems in these zones can reduce the joist’s load capacity.
Pro Tip: Check where the joists touch the underside of the decking first. That’s where moisture gets trapped, and it usually rots out the quickest.
Where Rot Shows Up First
Rot shows up as soft wood at the spots where joists connect to other parts of the deck. Check these areas closely.
Joist Connections at the Ledger Board and Rim Joist
The ledger board attaches your deck to the house, and the rim joist runs along the deck’s outer edge. Look for softness and rot where the joists connect to both.
Where Joists Meet the Drop Beam and Decking
Check the joists where they sit on top of the drop beam. Then look at the top edge of each joist, where it comes in contact with the decking. Moisture gets trapped between the joist and the deck boards, and that area typically rots out the quickest.
Black Mold and White Fungus in Pressure-Treated Lumber
Black mold and a white fungus on your framing can point to a known problem with pressure-treated lumber from a specific era.
How the Switch Away from CCA Started the Problem
For years, the standard pressure-treated lumber was CCA, short for chromated copper arsenate. The “A” stands for arsenic. Contractors who cut CCA lumber breathed in that dust regularly, and arsenic is a known carcinogen. Through the 2000s, the industry moved away from CCA in residential lumber.
Lumber companies tried several new formulas to find a safer option that still worked. It took them a few years to get it right.
Why Lumber from 2007 to 2015 is a Concern
During that transition, from about 2007 to 2011, the chemical makeup of pressure-treated lumber was still changing. Accounts differ on what went wrong. Some say the boards did not contain enough of the chemical, and others say the formula was wrong. Either way, boards from this era may not resist rot as well as they should.
We have found this defect in lumber as late as 2015. It is very common. We believe there have been tens, if not hundreds, of thousands of cases across the United States. Black mold often appears first, followed by a white fungus. Both typically mean the board was not fully treated or the chemical makeup was off.
Pro Tip: If your deck was built between about 2007 and 2015, check the framing closely for black mold or white fungus.
Need expert help with your deck framing? Contact DW Decks for a free consultation.
What to Do If You Find Fungus on Your Joists
Spotting black mold or white fungus is a sign to act quickly. You have two options.
Replacing the Deck Joist Framing
If you see the fungus anywhere on your joists, it is a good idea to replace all of the framing.
Treating it with a Bleach and Water Solution
You can treat the affected areas with a 50/50 bleach and water solution. Keep in mind that bleach treats the surface and does not repair rot inside the wood. You will need to repeat the treatment regularly to keep the fungus from spreading and compromising the entire deck frame. Work in a well-ventilated area when you apply it.
Key Takeaway: If you find black mold or white fungus on your joists, plan to replace the framing. Bleach treatment is a temporary measure that needs to be repeated regularly.
Schedule an Inspection with DW Decks
Catching these problems early gives you more options and keeps your deck safe to use.
The Signs to Check
Look for insect tunnels, splits near the ends or edges, soft wood at the ledger, rim joist, beam, and decking contact points, and black mold or white fungus on the lumber.
Get an Expert Opinion
If you spotted any of these signs, our team can take a closer look. Contact DW Decks today to schedule an inspection and get an expert opinion on the condition of your deck joists.