Pressure-Treated Wood, Why Does It Rot?


We have all heard those stories. Someone built their fence or deck using pressure-treated wood. A few years later, low and behold, they had to replace it because it was rotten. Often it’s not the whole thing but a critical joist, beam under the deck or a rotten post.

Which raises the question, if pressure-treated wood is treated against rot, why does it rot? What causes pressure-treated wood to rot?

Pressure-treated wood rots because of excess moisture, unprotected wood surfaces or insufficient levels of treatment when in contact with high levels of fungi. Also, wood damaged by UV will increase the likelihood of rot.

Well, these are all true. Water, Fungi and UV will damage your exterior wood projects, but my guess is you are not really looking for the what’s and whys but how to prevent pressure-treated wood from rotting.

If you want to jump right to the solutions, especially for deck joists, click the link and learn 15 Ways to Prevent Deck Joist from Rotting Plus My Top Picks.Opens in a new tab. Not to steal the show but my top 3 picks.

  • Ventilation
  • Treat end cuts
  • Joists Membrane

These 3 will prevent deck joists from rotting most of the time. So much that the other 12 may not even need to be considered. But there are always exceptional circumstances where other ways will be more beneficial.

So let’s look specifically at what causes pressure-treated wood to rot and possible solutions. Again, if you are building a new deck jumping straight to my article may be a more practical answer.

Pressure-Treated Wood Rots Because of Excessive Moisture

Outdoor wood is treated to make the wood edible for fungi not sealed against moisture.

Which often surprises many DIYers. But it’s for outside. Why is it not fully protected? Good question. There are reasons but let’s just say for now that water can and will destroy treated wood.

Trapping or pooling in water will cause treated wood to rot.

Since I am a deck builder, I will focus on deck solutions and situations but can be applied to any outdoor wood project.

Trapped Water

There are many ways that water can be trapped on or around a deck.

The first and biggest is under the deck. Water runs off the decking and under the deck needs to be able to drain away and the wood to dry out.

Solutions.

Water Trapped in the deck. This one is not as obvious but is still critical. Wherever you have wood sandwiched together like the deck beam or the ledger tight against the house wall, you have the potential for wood to get trapped beside the wood, with nothing to do but rot the wood.

Solutions.

  • Wide Joists membrane on top of multiple framing members
  • Deck Flashing

The ledger is the most important one here to protect. The beam can dry with air movement, but the ledger tight against the house does not. If you don’t stop the water from getting behind the ledger it will rot your ledger and house. Not might it will. Deck flashing is super important.

I am not a big fan, but deck ledger spacers allow the water to drain and air to dry out the wood. Which in a high rain region may be worth it.

Water can also get trapped against the rim joist with side attached wood railing. This becomes obvious when you pull the railing of and every section of the covered rim is rotted.

Solutions. Don’t install wood tight to the edge of the joists. I know that sounds to simple, but it is the best solution. Just another reason for top-mounted deck railing.

Trapping Water by Paint. Yes, we paint things to protect them from the elements.But paint also creates a film over the wood, trapping moisture. If it seals the wood 100%, it wouldn’t matter, but it never does. But the thin-film prevents water from evaporating out of the wood. Increasing the time that the wood remains wet and can rot.

Solution: Stain the pressure-treated wood instead of painting. Stain protects the wood against UV damage but still allows the wood to breathe and dry.

Pooling Water

Like trapped water, wood sitting in water will increase rot. Often with a deck, it’s more water pooling on the deck or on the concrete piles.

Solutions

  • Slope Deck to Prevent Pooling on the Decking
  • Use Saddles under Deck Posts

Sloping the deck a 1/8 or ¼” over a foot will drain rain off the decking, allowing it to dry out.

Water can easily pool on concrete. With a post or beam sitting directly on the concrete, the pooled water will rot the bottom of the wood. A post saddle raises the wood out of the water, keeping it dry and preventing rot.

Pressure-Treated Wood Rotting Because of Unprotected Surfaces

Surprise, pressure-treated wood is only surface treated.

Exposed Untreated Wood After Cutting Pressure Treated Wood

All that untreated wood will rot. It has nothing to protect it. Therefore it will rot the same as untreated wood.

Two common ways that pressure-treated wood becomes unprotected is by cutting or fastening it. Both reveal untreated wood but require different solutions.

While building a deck, you should end treat every cut. It only takes a few extra minutes to brush every cut but will add years to the deck’s life.

Screws or nails damage the wood in two ways.

Face screwing decking creates small pools of water and dirt on the wood. This is why I am a fan of Camo edge screws. By installing the screws on the edge of the decking, the water and dirt do not collect. Reducing rot. It also improves the look of the deck.

Penetrating the treated wood, channelling water and rot into the joists. It doesn’t matter if you face screw or edge screw. The screws create channels for water to get into the wood.

Solution. Protect the wood and seal the screw holes with joists membrane. The joist membrane shelters the wood preventing rain from soaking into the wood. It also seals around the screws preventing water from penetrating into the wood.

Pressure-Treated Wood Rotting Because of Insufficient Treatment

Two general ways treated wood may not be sufficiently treated.

  • Contact with Dirt
  • Ultraviolet sun Damage

Wood in contact with the groundOpens in a new tab. requires a higher level of treatment than above-ground wood. Roughly twice as much chemical treatment. So, whenever you are building within 6″ (150mm) from the ground use wood treated for ground contact. The extra fungi in the soil requires a higher level of protection.

Yes, all outdoor pressure-treated wood is treated against fungi, but there is a higher concentration of fungi in the dirt than in the air. Requiring a higher level of resistance and protection.

Not as obvious, but dirt can also get on a raised deck rotting the wood. Often collecting in the gaps between the decking on the joist. The dirt and crud holds both water and fungi spores. A perfect environment for the rot to develop and start to eat away the wood.

There are 2 solutions for dirt on the deck.

First, spring and fall cleaning of the deck will extent the deck’s life. Especially pay attention to the gaps between the deck boards. Mind the gap and the chance of rot are low.

But you can also use joist membrane tape under the decking. The membrane will protect the wood from both the water and fungi, reducing rot. Yes, I really am a fan of joist membrane under the decking.

Pressure-Treated Wood Damaged by UV. Another aspect about exterior wood is it is not treated for, the sun.

“UV weathering, a process initiated primarily by the ultraviolet portion of the solar spectrum, causes surface degradation of wood.”

NC State UniversityOpens in a new tab.

Regarding rotting, the old enemy is still water, but the degenerated wood will crack and become weak. The cracks collect both water and dirt, rotting the wood.

Solution. Regular staining of the wood. Quality stains do more than just enhance the wood’s colour and appearance. Wood stains contain pigments that protect the wood against UV damage. Becoming a sacrificial part of the wood. Instead of damaging the wood fibres, the pigment absorbs the UV, increasing the life of the wood.

The stain helps protect the wood against the sun, but it also seals the wood against rain. Locking out the moisture that can rot the wood.

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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