7B Ch7 Cheat Sheet: Wood-Damaging Fungi

🎯 Top 5 Traps

1

Wood at 19 percent moisture or less is SAFE; decay requires about 30 percent. Air-dried wood (19 percent or less) and kiln-dried wood (15 percent or less) are safe from decay. The 30 percent figure is the fiber saturation point — that's what decay fungi need.

2

"Dry rot" is a misnomer. Wood must have moisture to decay. The term refers to brown-rot wood that has since dried. A few brown-rot fungi have water-conducting strands (rhizomorphs) that pull water from damp soil into otherwise-dry wood — these are the "dry rot fungi" or "water-conducting fungi."

3

Eliminate moisture FIRST. Fungicides alone do not stop decay. The first step in correcting a fungus condition is to find and eliminate the moisture source — not to spray. The exam will list "apply fungicide" or "insert borate rods" as plausible-looking distractors. They aren't first.

4

Stain fungi do NOT significantly reduce wood strength. Sapstaining (often blue) and molds are mostly cosmetic. But their presence INDICATES conditions favorable for decay fungi — pull a moisture meter and check.

5

Vapor barrier: cover MOST of the soil — leave a small portion UNCOVERED. Total coverage is the wrong answer. The uncovered portion lets the subarea breathe and gives standing water a place to escape. Inspect 1 to 3 weeks after install for adjustments.

🔢 Numbers You Must Know

Number

What It Represents

19 percent

Air-dried wood moisture content — at or below this, wood is safe from decay

15 percent

Kiln-dried wood moisture content — even safer threshold

30 percent

Fiber saturation point — wood moisture content required for decay fungi to grow

12 percent

Anobiid larvae will not survive in wood below this moisture content

50 to 90 degrees F

Favorable temperature range for fungal growth

70 to 85 degrees F

Optimum temperature range for fungal growth

below 35 / above 100 degrees F

Wood is basically safe from decay — temperature too low or too high for fungi

5 to 10 percent

Loss of weight at which the pick test can detect early wood decay

3 to 4 mm

Soft-rot soft-layer depth from the wood surface inward

up to 100 lb / day / 1,000 sq ft

Water produced by a poorly ventilated crawl space

1 sq ft per 25 lin ft

Crawl-space ventilation opening per linear foot of wall (greater if screened or louvered)

1 sq ft per 150 to 300 sq ft

Attic vent ratio per square foot of attic floor space (locate at ridge AND eaves)

4-mil to 6-mil

Polyethylene sheeting thickness for vapor barriers (or use roofing paper)

1 to 3 weeks

Vapor-barrier inspection delay after installation, to allow adjustments as wood recovers

🔀 Easily Confused

Pair / Group

Distinguishing Feature

Brown rot vs White rot vs Soft rot

Brown: cellulose only — dark brown, cross-grain cracking, crushable to brown powder. Most important softwood-decay above ground. White: BOTH cellulose and lignin — bleached look, normal shrinkage, gradually spongy, may show zone lines. Mostly hardwoods. Soft: green/saturated wood — surface darkens, soft layer 3 to 4 mm deep.

Decay fungi vs Stain fungi

Decay: change physical and chemical properties of wood — significantly reduce strength. Stain: discolor wood (often blue) — feed on wood at very slow rate, do NOT significantly reduce strength. But stain fungi presence INDICATES conditions favorable for decay.

Sapstaining vs Mold vs Chemical stains

Sapstain (e.g., blue stain): fungal, deep within wood, CANNOT be brushed/planed off. Mold (Penicillium, Fusarium): fungal, surface growth, CAN usually be brushed/washed off. Chemical stains: NOT caused by fungi — chemical changes during seasoning; prevented by rapid air drying or low kiln temps.

Anobiid beetles vs Fungus beetles

Anobiid: actually FEEDS on wood. Larvae need moisture content of at least 12 percent to survive. Fungus beetles (Cisidae, Cryptophagidae, Lathridiidae, etc.): feed on FUNGUS GROWTH, not wood — not wood-damaging pests, but indicators of moisture problems.

Pick test vs Moisture meter

Pick test: subjective; an icepick or sharp tool pried up — decayed wood breaks abruptly over the tool, sound wood breaks away from it. Detects 5 to 10 percent weight loss. Moisture meter: electrical-resistance reading between two needles. Affected by species, grain, chemicals, weather, temperature.

Vapor barrier: full vs partial coverage

Wrong: cover the entire soil surface. Right: cover most of the soil but leave a small portion uncovered for breathing and water escape. Inspect 1 to 3 weeks after install.

Pressure-treated vs Surface-applied preservatives

Pressure-treated: preferred for wood that cannot be kept dry — creosote, zinc chloride, pentachlorophenol, copper naphthenate. Wear gloves and long sleeves; never burn scraps. Surface-applied / borate: easier and lower hazard; borates effective against fungi AND wood-destroying insects, water-soluble (will leach if wood gets wet).

🍄 Decay Fungi Quick Reference

Type

Chemistry

Appearance & Hosts

Brown rot

Breaks down primarily CELLULOSE; brown lignin residue remains

Dark brown wood, excessive shrinkage, cross-grain cracking; dry wood crushes easily to brown powder. Wood greatly weakened BEFORE decay is visible. Most important cause of softwood decay aboveground in U.S. "Dry rot" misnomer.

White rot

Breaks down BOTH lignin and cellulose

Bleached/whiter look. Normal shrinkage; does not collapse or crack across grain. Loses strength gradually until spongy to the touch. Sometimes shows thin dark zone lines around decayed areas. Usually hardwoods. White pocket rot: 1/8 to 1/2 inch spindle-shaped pockets — fungus dies when wood seasons, no control needed.

Soft rot

Surface chemistry — gradual softening from outside in

Attacks GREEN (high-moisture) wood. Surface darkens; superficial soft layer up to 3 to 4 mm deep. Resembles brown rot but is shallow and surface-based. Example: Chaetomium globosum.

🌬️ Moisture Control Quick Reference

Element

Specification

Notes

Crawl space vents

1 sq ft of opening per 25 lin ft of wall

Locate for cross-ventilation. Where screening, wire mesh, or louvers are used, total opening must be GREATER than 1 sq ft per 25 ft. Closeable in winter.

Attic vents

1 sq ft per 150 to 300 sq ft of attic floor

Locate near ridge AND at eaves to induce airflow. Inlet openings under cornice/roof overhang in all cases. Screen and protect from weather.

Vapor barrier material

4-mil to 6-mil polyethylene sheeting OR roofing paper

Cover MOST of soil surface — leave a small portion uncovered to allow breathing and water escape.

Vapor barrier inspection

1 to 3 weeks after installation

Allows wood to slowly recover from excess moisture; adjust barrier as needed.

Daily indoor water load

About 1 lb per day from cooking/cleaning/laundry; up to 100 lb / day / 1,000 sq ft from poor crawl space

Larger and more airtight homes have INCREASED condensation problems — humidifiers and tight envelopes magnify it.

💡 Memory Hooks

Moisture thresholds: "19 safe, 30 decay." Air-dried wood at 19 percent or less is safe; decay fungi need 30 percent. Anobiid larvae need at least 12 percent.

Three rots — "BCWBSS": B rown breaks C ellulose. W hite wins B oth. S oft S urfaces soaked wood. The third one names its target wood (green/saturated).

"Dry rot": "Dry rot is a wet word." Wood must have moisture to rot. The "dry" in dry rot means the wood dried AFTER decay — not that it was dry when fungi attacked.

Correction order: "Fix the leak first." Find and eliminate the moisture source before any chemical treatment. Without this, fungicides won't stop decay.

Vapor barrier: "Most, not all." Don't cover the entire soil — leave a small portion uncovered for breathing and water escape.