What is dry rot, and why is it called dry when my wood is wet?
Short answer
Dry rot is a fungus, not a dry condition. Brown rot fungi digest the cellulose in wood and leave it dark, brittle, and cracked into cubes, which looks dry after the fact. The fungus itself needs the wood to stay wet for weeks at a time, typically at a moisture content above roughly 20 percent, plus oxygen and mild temperatures. Starve it of water and it stops. That is why every good rot repair fixes the moisture source, not just the wood.
A fungus with a misleading name
The term dry rot comes from what the damage looks like at the end: wood that is shriveled, powdery, and splits into blocks. Builders a few generations ago assumed the wood had dried out and failed. It is the opposite. The wood was wet long enough for fungal spores, which are everywhere in the air, to germinate and grow into the fibers.
The fungi behind most structural rot are brown rot species. They eat cellulose and leave the brown lignin behind, which is why decayed wood darkens and shrinks. White rot fungi eat both and leave pale, stringy wood; you see it more on hardwoods and on wood lying on soil.
What rot needs to grow
Four things, and removing any one of them stops it. Food is the wood itself. Oxygen is always present above ground. Temperature in coastal San Diego is in the comfortable range for fungi most of the year. The one you can control is water.
Decay fungi generally need wood moisture content above about 20 percent to get started and do best when it is considerably wetter than that. Lumber in a dry Carlsbad house typically sits well below that. Wood only crosses the line when something keeps it wet: a leak, a sprinkler, soil contact, a joint that traps water, or end grain left open to the weather.
- Food: the wood
- Oxygen: always present
- Warmth: most of the year here
- Water: the only one you can take away
Why the damage is usually hidden
Fungus grows where the water is, and water collects in places you do not see. The bottom of a post inside a concrete footing. The end of a joist tucked behind a fascia. The back of a piece of trim pressed against stucco. The outside face may look fine while the inside is gone, and paint can hold a surprisingly good skin over mush.
This is also why rot often looks like it appeared suddenly. It did not. It has typically been working for several seasons before a railing wobbles or a heel goes through a deck board.
How this changes the repair
Since the fungus is the symptom and water is the cause, Levi scopes a rot repair in two halves. First, where is the water coming from, and how do we stop it: flashing, a post standoff, a sprinkler adjustment, a sloped sill, a sealed end cut. Second, how far has the decay traveled, and where is sound wood again.
Decay fungi can run a few inches past the visibly soft area along the grain, so we cut back into clean, bright wood before splicing or sistering. Replacement pieces are chosen for the exposure: pressure-treated lumber for anything near grade or concrete, kiln-dried and back-primed stock for painted trim, and naturally durable species where the look matters.
Can rotted wood be dried out and saved?
Drying stops the fungus, but it does not give strength back. Wood that has lost its cellulose is permanently weakened, even when it is bone dry and looks stable. Small, non-structural areas can be consolidated with epoxy; anything carrying load, including joists, beams, posts, stair stringers, and railing posts, should be cut back or replaced. We cover that decision in a separate answer on repair versus replace.
Related questions
Is dry rot contagious to the rest of the house?
Spores are already everywhere, so the fungus does not need to spread from an infected piece. What spreads is the moisture condition. Fix the water and the surrounding dry wood is not at risk.
Will bleach kill dry rot?
Surface bleach can kill fungus on the face of the wood, but it does not penetrate far and it does nothing about the lost strength or the water source. Borate-based wood preservatives are more useful on sound wood near a repair. Neither replaces removing decayed material.
Does pressure-treated wood rot?
It resists decay far better than untreated lumber, but cut ends and bored holes expose untreated wood, and treated lumber in constant contact with wet soil can still fail over time. Field cuts should be treated and the wood kept out of standing water.