Why wood decays: food, moisture, oxygen and temperature

The four needs of a decay fungus

Decay fungi are living organisms. They need four things at the same time: food (the wood itself), moisture, oxygen and a suitable temperature. Remove any one of them and decay stops. In a sawmill the factor you control best is moisture, so most of this course starts there.

A sawmill handles wood at its most vulnerable moment. 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 the dry, stable board there is a window, often a few weeks in warm weather, in which discoloration can begin and decay can start.

Moisture: the limiting factor

The figures below come from a Canadian (Forintek) review by Morris and a US Wood Handbook chapter, so they are North American in origin, but the 20 % line is also used in the European use-class system.

The practical lesson: 20 % MC is the line. Above it, the clock is running, and it runs faster when the weather is warm. The European summary of EN 335 uses the same figure: for outdoor use above ground with fast drying (use class 3.1), a fungal risk exists above 20 % moisture.

Oxygen and temperature

• Oxygen. Normal air contains about 21 % oxygen. Growth of wood-rotting fungi is not reduced until oxygen falls below roughly 1-2 %. This is why wet or sprinkled log storage can work: the wood is kept too wet for oxygen to reach it. (The link between the 1-2 % threshold and wet storage is an inference in the source article.)
• Temperature. Most wood-rotting fungi thrive between 20 and 30 degrees C. Growth stops above about 46 degrees C, and the fungi may not be killed until about 60 degrees C. Each 10 degrees C drop roughly halves growth rate, and growth continues until temperatures very close to 0 degrees C.

These are figures for fungi in general. They apply to building timber and storage; fungal species and wood species vary, so treat all thresholds as approximate.

Worked example: is this board at risk?

A sawn pine board comes out of the kiln at 14 % MC and is left outdoors unwrapped for a week of rain. Its surface now reads 26 % MC.

1. Is it above 20 %? Yes, so the board is no longer in the "immune" range.
2. Is it above about 29 %? No, so fresh spore infection is not yet likely.
3. Is the temperature favourable (20-30 degrees C)? If yes, the risk rises quickly.
4. Action: get it back below 20 % (dry, ventilated storage) before a fungus establishes itself. Once established, decay can continue at 22-24 %.

Checklist: the four-needs test

When you meet a decay problem, ask in this order:

• Is the wood above 20 % MC (food is always present)?
• Is it staying wet because of design, storage or a leak?
• Is air reaching it (almost always yes)?
• Is it warm enough for growth?

If the answer to the first two is yes, fix moisture first. It is the cheapest and most reliable protection, and the one the mill controls.

Key points

• Decay needs food, moisture, oxygen and moderate temperature together; remove one and it stops.
• Untreated lumber uniformly below 20 % MC is described as immune to fungi; decay proceeds at about 22-24 % once established; spore infection probably needs about 29 %.
• Drying below about 15 % stops decay but may not kill the fungus.
• Most decay fungi grow best at 20-30 degrees C; oxygen has to fall below roughly 1-2 % before growth slows.
• Thresholds are approximate and come mainly from North American and general reviews.