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How to Avoid, Prevent & Repair Deck Joist Rot

The homeowner watched with dismay as her old under-deck ceiling was removed. As each section of ceiling was pulled away, large splinters of wood also came down. Her drainage system had failed to protect the joists, which were rotten and would need to be replaced.

Why Do Deck Joists Rot?

The simple answer is any wood that regularly gets wet and can’t dry out will rot. Wood joists stay wet when a deck drainage system is installed below the joists.

Under-deck drainage systems that are mounted below the joists cause damage in three ways:

  • First, they allow the wood to get wet over and over again as rain drips through the deck boards, over the joists and into the drainage system below. The water hits the pan and runs out, but not before drenching the joists.
  • Second, this type of system traps in the moisture, because with a drainage system and ceiling below the joists and decking above and on the sides, there’s no air coming in to dry out the wood. The only ventilation is through the quarter-inch or less spaces between the top boards.
  • Third, in the hot months of summer, the temperature further cooks this trapped, wet wood and accelerates the rotting. Over time, this is the perfect formula for wood rot: moisture + heat – ventilation = destruction.

Some homeowners will paint the exposed joists (on three sides), which can cause further damage. As the wood on the top gets wet, there’s no way for the painted wood to release the moisture.

How to Prevent Joist Rot

Before installing an under-deck drainage system and ceiling, do your homework. The best deck drainage systems are installed above the joists. This will provide 100% protection of all the joists and beams from moisture penetration. This type of system also helps to hold the screws in place longer.

If you’re building the deck yourself, choose an over-the-joist system that’s easy to install. Use this handy chart for comparison. If you’re hiring a builder, ask if the drainage system being installed is above the joists.

To add extra protection, especially in high humidity areas, add ventilation to your under-deck ceiling with strip vents. These vents release hot air and help mitigate temperature extremes.

As a builder or DIYer, you want to make the most of each project. If you can keep the water away, you’ll protect from rot and gain many more years of use.

How to Repair Joist Rot

If you’re handy, you can likely repair or replace rotten joists yourself. However, whether it’s a small or a large repair, you will likely need to remove decking to accurately assess the damage.

For small repairs, you may be able to use wood putty and a reinforcing joist. With this method, you’ll chisel out the rotten wood and reinforce what’s left and securely attach new wood.

However, if joists need to be replaced, you’ll have a bigger commitment of time and skill.  If you’re unsure about the safety of your deck, be sure to consult a decking pro.