What stains and rots stored wood: moisture, temperature and time

A sawmill handles wood at the moment of greatest biological vulnerability. A freshly felled log is full of water and of the sugars and starch in the sapwood that fungi and insects feed on. Between felling and a dry, stable board there is a window, often a few weeks in warm weather, in which stain can set in and decay can begin.

What decay fungi need

Decay fungi need four things together: food (the wood), moisture, oxygen and a suitable temperature. Remove any one and decay stops.

Moisture is the limiting factor. A Forintek Canada review (Morris, North America) states that decay by established wood-rotting fungi proceeds at roughly 22 to 24 percent moisture content (MC), that spore infection probably does not occur below about 29 percent, that 20 percent is frequently quoted as a maximum safe MC, and that the optimum for decay is 40 to 80 percent. The US Wood Handbook storage chapter describes untreated lumber uniformly below 20 percent MC as immune to fungal attack.

For logs, a Bavarian leaflet (LWF, 2025, Europe) gives the biological window: damaging organisms need oxygen and water, available at MC of roughly 20 to 30 percent up to about 120 percent. Above 120 percent oxygen is too scarce; below 30 percent there is no free water. The strategy is to keep logs wet or to dry them quickly below 30 percent, ideally below 20 percent.

Oxygen. Normal air contains about 21 percent. Growth of wood-rotting fungi is not reduced until oxygen falls below roughly 1 to 2 percent (Morris).

Temperature. Most wood-rotting fungi thrive between 20 and 30 degrees Celsius. Growth stops above about 46 degrees and fungi may not be killed until about 60 degrees. Each 10 degree drop roughly halves the growth rate, and growth continues until temperatures close to 0 degrees (Morris).

Worked example. If a fungus grows at relative rate 1.00 at 30 degrees, then using the halving rule: 20 degrees gives 0.50 and 10 degrees gives 1.00 x 0.5 x 0.5 = 0.25. Cool storage slows damage; it does not stop it.

Kinds of damage

• Brown rot degrades mainly cellulose, leaving a brittle, brown, cube-cracked residue. It mostly attacks softwoods and is the most economically important decay type in temperate building timber, with rapid early strength loss.
• White rot degrades lignin and cellulose, leaving bleached, stringy wood; typical of hardwoods, with comparatively slow early strength loss. "Brown in softwoods, white in hardwoods" is a tendency, not a law.
• Blue stain (sapstain) lives on the contents of sapwood cells and colonises felled logs and green timber quickly. Its main effect is disfigurement. LWF describes blue stain as a blue-black discolouration, often in summer-felled wood, that rules out visual-grade use although technical properties remain. A trade source (NELMA) reports optimal growth at about 21 to 32 degrees in wood above 22 percent MC, dormancy below about 10 degrees and death above about 54 degrees.
• Mould is surface growth on damp wood, mostly above about 20 percent MC.

Stain and mould cost grade; only rot destroys structure. Blue stain does not reduce strength, early rot does (Holzforschung Austria).

Time scales from the Wood Handbook (North America)

• Blue stain can develop in the sapwood of exposed log ends within 2 weeks after felling in warm weather.
• Discoloration may penetrate a log as fast as 1 ft (about 30 cm) per month, a maximum, not a typical rate.
• Wood at 25 to 30 percent MC solid-piled in warm weather can show stain within a week.

These come from North American conditions, so European climates and species will differ.

Worked example: how deep can stain go?

A warm-weather pile is left 3 months. At the maximum rate, penetration could reach 30 cm x 3 = 90 cm from the log end. Because 30 cm per month is a maximum, 90 cm is an upper bound, not a forecast. The practical message is that the volume lost grows with storage time, so shorter storage in the warm season is the cheapest protection.

Key points

• Decay needs food, moisture, oxygen and a moderate temperature; remove one and it stops.
• About 20 percent MC is the working line for untreated lumber (decay established at 22 to 24 percent, spores below about 29 percent unlikely to infect); logs are at risk between roughly 20 to 30 and 120 percent MC.
• Each 10 degree drop roughly halves fungal growth; cool storage slows but does not stop damage.
• Blue stain and mould cost grade, rot destroys structure; blue stain can show within 2 weeks on log ends in warm weather.