Wood Rot and Wood-Destroying Fungi | Pest ID Guide
Wood-Destroying Fungi: Quick Facts
Wood rot is caused by fungi that feed on wood and slowly break it down. It’s one of the most common sources of structural damage in Tennessee homes, and it has one root cause: wood that stays wet. The same moisture that feeds rot also attracts termites and other wood-destroying insects, which is why moisture control is part of protecting a home from termites.- Cause: Fungi growing in wood that stays damp
- Main types: Brown rot, white rot, and soft rot
- Common locations: Crawl space framing, sill plates, subfloors, window sills, decks, and trim near the ground
- Key signs: Soft or spongy wood, cracking, discoloration, fungal growth, buckling floors, and a musty smell
- “Dry rot”: A misleading name: all wood rot needs moisture to start
Why Moisture Control Is Part of Termite Protection
Moisture problems and termite problems usually show up together, for three reasons.- Moisture attracts termites: Subterranean termites need damp soil and wood to survive. A wet crawl space puts their food and water right next to each other.
- Rot softens wood for pests: Decaying wood is easier for termites, carpenter ants, and anobiid powderpost beetles to attack.
- Moisture wears down termite treatments: Standing water and constantly wet soil around a foundation can shorten the life of soil termite treatments.
Entomologists have estimated that wood decay fungi cause about as much damage to wood structures as termites, and because both depend on the same damp conditions, removing that moisture helps control both.
Inspectors call these “conducive conditions”: things that make a home more likely to develop termites, wood-destroying fungi, and other pest problems. Controlling them protects your termite treatment and your home. See our termite control services and wood-destroying insects guide.
Types of Wood Rot
Brown Rot
The most common and destructive type in homes. It breaks down the wood’s cellulose, leaving it brown, shrunken, and crumbly, and it cracks into cube-like chunks. It’s often called dry rot because the damaged wood looks dry and powdery, but it started with moisture.
White Rot
Breaks down both cellulose and lignin, leaving wood bleached, lighter in color, and soft, spongy, or stringy. It’s more common in hardwoods but affects softwoods too.
Soft Rot
Develops in very wet wood, often in contact with soil. It softens the surface first, and the wood may look fine until you probe it.
Surface Mold and Mildew on Framing
Before rot sets in, damp framing often grows surface mold or mildew: fuzzy or powdery white, gray, green, or black patches on joists and subfloor. It doesn’t weaken wood the way decay fungi do, but it’s an early warning that moisture is too high. See our mold and moisture guide.
How Much Moisture Causes Wood Rot?
Technicians measure moisture inside wood with a moisture meter, reported as a percentage.- Below about 18 percent: The range where wood generally stays sound
- About 18 to 22 percent: Surface fungi such as mold and mildew can start growing on framing
- Above about 22 percent: Wood decay fungi can begin breaking down the wood itself
Tennessee’s humid summers, damp soil, and heavy spring rains push crawl space framing toward those levels, especially in crawl spaces without a vapor barrier or with poor drainage. Readings are taken in several spots, because one wet area over a plumbing leak or a low spot can be much higher than the rest.
Signs of Wood Rot and Moisture Damage
In the Crawl Space
- Soft or spongy wood: A screwdriver pushes in easily
- Cracking: Especially cube-like cracking across the grain
- Discoloration: Wood that’s darker, browner, or bleached compared with sound wood
- Fungal growth: White or gray fan-shaped growth, strands, or mushroom-like growths
- Sagging insulation or a wet vapor barrier: Water pooling on top of the barrier or insulation hanging down
- Musty odor: Strongest near the crawl space access
In Your Living Space
- Cupping: Hardwood boards with edges higher than their centers, often from moisture rising from below
- Buckling: Floorboards lifting or rippling, usually from serious water under the home
- Crowning: Boards higher in the middle than at the edges
- Cracked tile: Tile cracking as the subfloor beneath it moves
- Soft or bouncy floors: A sign joists or subfloor may be weakened
- Musty smell indoors: Air from a damp crawl space rising into the home
Where Wood Rot Starts
- Crawl space sill plates, joists, and subfloors, especially in damp crawl spaces
- Subfloors under bathrooms, kitchens, and laundry areas
- Window sills and door frames where water collects
- Fascia, soffits, and trim behind clogged gutters
- Decks, porches, and posts in contact with soil
- Siding and trim close to the ground
What a Crawl Space Inspection Checks
- Moisture readings in joists, sill plates, and subfloor
- Fungal growth and wood decay
- Standing water, puddles, or signs of past flooding
- The condition of any existing vapor barrier
- Plumbing and HVAC leaks or sweating ducts and pipes
- Drainage around the foundation, including gutters and downspouts
- Termite mud tubes and other wood-destroying insect activity
- Signs of rodents, cockroaches, and other moisture-loving pests
How U.S. Pest Treats Wood Rot and Moisture
The goal is to stop the fungus, dry the wood, and keep moisture from coming back, so your termite protection keeps working.- Antifungal wood treatments: Applied to crawl space framing, joists, and subfloors to stop fungal growth. U.S. Pest has provided them for more than 30 years, on their own or alongside other moisture work.
- Vapor barriers: A heavy-duty barrier over the crawl space ground keeps soil moisture from rising into the framing.
- Dehumidifiers: Installed in the crawl space to pull humidity out of the air.
- Drainage work: Correcting where water collects or enters so it moves away from the foundation.
- Crawl space encapsulation: A full seal of the crawl space: vapor barrier on the ground and walls, sealed openings, and a dehumidifier. See our crawl space encapsulation service.
These options are often combined, and they can be part of a U.S. Pest service plan. Wood that has already lost its strength should be repaired or replaced; treatment and moisture control keep the rest of the structure from following.
How to Prevent Wood Rot
- Fix leaks promptly: Roof, plumbing, window, and appliance leaks.
- Manage rainwater: Clean gutters and extend downspouts well away from the foundation.
- Grade soil away from the house: So water drains out rather than toward the crawl space.
- Break wood-to-soil contact: Keep siding, trim, and deck posts off the ground.
- Keep crawl space vents and access clear: And have the crawl space checked at least once a year.
- Paint and seal exposed wood: Finishes slow the spread of moisture into wood.
Frequently Asked Questions
What causes wood rot?
Fungi that grow in wood that stays damp. Without moisture, wood rot can’t start or spread.
What moisture level causes wood rot?
As a general guideline, surface fungi can grow when wood moisture reaches about 18 to 22 percent, and decay fungi can begin breaking down wood above about 22 percent.
Does moisture affect termite protection?
Yes. Moisture attracts termites, softens wood for them, and can shorten the life of soil termite treatments, which is why moisture control is part of termite protection.
Why are my hardwood floors cupping?
Cupping, with board edges higher than the centers, often means moisture is rising from a damp crawl space. Have the crawl space inspected.
Can wood rot be treated?
Yes. Antifungal treatments stop fungal growth, and moisture control keeps it from returning. Wood that has lost its strength should be repaired or replaced.
