Wet rot is decay that happens in wood that is not treated and is often exposed to water or high levels of moisture. Fungus grows on wet wood because it feeds on it and grows. Even though most decks are made with wood that has been treated with pressure, it can still get wet rot. The most vulnerable spots are where the wood was cut without being treated and where water pools or seeps into cracks.
When the wood has more than 30% moisture, the fungus spores can start to grow. If you don't stop the fungus from growing, it could spread to structural parts of the deck and cause it to collapse.
Read more: How to Build a Raised Deck
Wet Rot’s Early Signs
Catch wet rot early by inspecting for:
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A damp, musty smell
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Wood that is cracking and beginning to soften
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A board that moves (or bounces) when you walk over it
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Discolored or weak wood
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Visible fungal growth
If the deck is painted, it may be difficult to spot any signs of wet rot. The presence of rot can be determined by inserting a screwdriver into the wood and observing how easily it goes in.
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Wet Rot Causes
Wet rot can be caused by a number of different sources of moisture that can be found in buildings. Some of these sources include faulty roofs and plumbing that leaks. When it comes to wooden decks, common causes include inadequate drainage that causes the decking, railings, or posts to remain wet; leaking gutters or downspouts; moisture that is trapped in the ground; and the accumulation of dead leaves and other forms of tree litter on and around the deck.
Read more: How to Design and Build and Above-Ground Pool Deck
Wet Rot Treatment
Stopping the flow of moisture is the first step in treating wet rot, which is followed by either repairing or replacing the rotten wood. The price will be different depending on how badly the wood has rotted and how far the rot has spread throughout the wood.
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Can I Treat Wet Rot Myself?
If the damage is not significant, such as severe deterioration of structural elements, you should be able to treat and eliminate the rot on your own after you have eliminated the source of the water that is causing it. After determining and eliminating the source of the moisture, the next step is to locate all of the rotten wood. Once you start digging into it, you might be surprised at how extensive the rot is, but you shouldn't be afraid to remove the decay once you've discovered it. Solid wood is the only type of wood that can be correctly fastened and refinished.
This maintenance work may also serve as a helpful reminder to take preventative measures. When constructing a deck, one of the most effective preventative measures is to wrap all of the joists, beams, rim joists, and ledger boards with butyl tape that is water resistant. Inspecting the wood decks on a routine basis to look for rot is another recommended maintenance practice. The earlier you discover the problem, the simpler and less expensive it will be to fix it.
Read more: How to Brace a Deck from Swaying
Cost of Wet Rot Treatment
The costs of repairing damage caused by water are comparable to those caused by a fallen limb, despite the fact that water damage does not splinter the wood. When it comes to structures of any kind, whether it be a house, a shed, or a deck, the most common cause of damage is water. Decks, which previously required more effort to access in order to make repairs, are now, thankfully, more easily accessible. As a direct consequence of this, the majority of repairs are not particularly difficult and can be carried out at a cost that is acceptable. Damage caused by water can be one of your largest expenses, particularly if you need to replace a roof or repair damage brought on by a leaking pipe. The price will vary depending on the specifics of the situation.
Read more: How to Build a Freestanding Deck
Dry Rot vs Wet Rot: What’s the Difference?
Although both wet and dry rot are caused by an excessive amount of moisture in the wood, dry rot gets its name from the fact that moisture is not always detectable when it occurs. In point of fact, the threshold for dry rot to become established is lower (>20 percent) than the threshold for wet rot. Additionally, once dry rot has begun to develop, it will draw moisture from the wood as it feeds on the cellulose in the wood. It is not necessary to use any other source of moisture in order to spread it. Because of this, dry rot is generally considered to be the more serious form of fungal decay.
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Both wet and dry rot have the potential to cause significant damage, but there are some key distinctions between the two types of rot, including the following:
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When compared to dry rot, wet rot necessitates a higher moisture content in order to become established.
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The progression of wet rot will be halted if the source of moisture is taken away.
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Because it draws its moisture for growth from the wood itself, dry rot does not require a reliable source of moisture in order to spread.
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Dry rot is more likely to spread, which also means that it is more likely to require the assistance of a professional.
FAQs
What is wet rot—and how is it different from dry rot?
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Wet rot is wood decay caused by fungi that need high moisture in the timber; it stays local to damp wood. Dry rot can progress at lower moisture and sometimes spreads farther through structures. Fixing the moisture source is the key to both.
At what moisture level does wet rot start?
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Decay fungi typically do not attack wood below ~20–25% moisture content; growth is much faster once wood is ~40–80% MC. Keep timber ≤20% MC to prevent decay. (Use a pin moisture meter to check.)
I’ve found wet rot—what’s the step-by-step fix?
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a) Eliminate the moisture source (leaks, drainage, ventilation). b) Dry the area. c) Remove and replace any structurally unsound wood. d) Treat remaining sound wood and adjacent at-risk areas with a preservative.
Can I “save” slightly affected wood instead of replacing everything?
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If the wood is still sound after drying (passes a screwdriver/probe test), treat it; severely softened sections should be cut out and replaced. Borate/copper preservatives are common for salvageable areas.
What products actually work on wet-rot fungi?
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Look for registered borate preservatives (e.g., DOT/borate powders/solutions, rods) used on framing/joists; these diffuse into damp wood and are listed by the U.S. EPA for decay fungi. Always follow the product label.
How do I stop wet rot from coming back on decks?
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Control water and debris, keep framing ventilated, and protect tops of joists/beams with deck flashing (joist) tape so fastener holes and horizontal faces don’t stay wet.
Will wet rot keep spreading after the wood dries?
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No—decay activity stops when wood dries below ~20% MC. But if the damp conditions return, decay can resume, so moisture control is non-negotiable.
When should I call a specialist?
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If rot involves structural members (ledger, beams, joists), if the moisture source is unclear, or if it’s widespread, bring in a qualified contractor or a Property Care Association specialist for diagnosis and repair planning.
