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Rot, stain, mould and insects: telling damage apart
Not all fungal damage is equal
Distinguishing the types of damage tells you how urgent a problem is and what it costs.
| Damage | What it does | Effect |
|---|---|---|
| Brown rot | Degrades mainly cellulose and hemicelluloses; brittle, brown, cube-cracked residue | Rapid early strength loss; mostly attacks softwoods; the most economically important decay in temperate building timber |
| White rot | Degrades lignin and cellulose; bleached, stringy wood | Typical of hardwoods; strength loss comparatively slow at first |
| Soft rot | Attacks wood in permanently moist conditions such as soil or cooling towers | Relevant where wood stays wet |
| Blue stain (sapstain) | Fungi live on sugars in the sapwood and colonise fresh logs and green timber quickly | Mainly disfigurement; costs grade |
| Mould | Surface growth of spores on damp wood | Often confined to the surface; costs grade; some moulds can cause respiratory problems in sensitive people |
"Brown rot in softwoods, white rot in hardwoods" is a tendency, not a law. Stain and mould cost grade; only rot destroys structure.
A common yard test (general practice): rub the surface. Mould smears or brushes off; blue stain is in the wood and does not.
Stain timing: why the mill must be fast
Figures from the US Wood Handbook storage chapter (North American conditions):
- Blue stain can develop in the sapwood of exposed log ends within 2 weeks after felling in warm weather.
- Discoloration can penetrate into the log at up to about 30 cm (1 ft) per month. This is a maximum, not a typical rate.
- Wood at 25-30 % MC, solid-piled in warm weather, can show stain within a week.
- Stain fungi grow best at about 21-32 degrees C in wood above 22 % MC, are dormant below about 10 degrees C and, per a trade source, die above about 54 degrees C.
Insects in general terms
Insects have a different timeline from fungi. The Wood Handbook chapter says insects are inactive at or below about 10 degrees C and that activity roughly doubles with each 5-6 degrees C rise, reaching a maximum near 27 degrees C. Ambrosia beetles are usually a minor hazard in lumber stored away from forests or sawmills. Powder-post beetles are particularly damaging to large-pored hardwoods such as oak and ash. European durability sheets classify timber against two beetles as simply susceptible (S) or durable (D).
Worked example: a warm-season storage plan
You saw pine boards in July. Antistain dip treatments have limited safe times before the board must dry: 1 day at 27 degrees C or above, 2 days at about 21 degrees C, 1 week at about 15 degrees C and 1 month at about 10 degrees C.
- Daily temperature is 27 degrees C: after a dip the boards must be drying within 1 day.
- Stickered boards under a roof will not reach 20 % MC in a day, so the dip alone is not a plan. You also need prompt stickering and shelter.
Checklist for the yard
- Move logs first-in, first-out; keep storage short in the warm season.
- Use end coating, wet or sprinkled storage, or other listed options when logs must wait.
- Sticker green boards promptly with dry stickers; do not close-pile wood above 20 % MC.
- Keep sawn timber off the ground and protect it from rain.
- After drying, wrap or shelter: kiln-dried wood stored outdoors in wet weather regains moisture, and stain-free packaging can stain later from condensation.
- Never treat sapwood as durable.
Key points
- Brown rot is the most damaging in temperate construction timber; blue stain and mould mainly cost grade, but only rot destroys structure.
- Mould can usually be brushed off; blue stain is inside the wood.
- In warm weather expect stain within days to about two weeks on exposed sapwood; at 25-30 % MC solid-piled it can appear in a week.
- Antistain dips buy only limited time, from 1 day (warm) to 1 month (cool).
- Insect activity rises with temperature and is negligible at or below about 10 degrees C.
- Fast drying, good stacking and shelter are the mill's main protection.