information/FAQs

Fig 1 Fungal hazard zones

Fungal decay

Timber is only vulnerable to fungal attack if the four following conditions are met:

Moisture must be present; if the timber moisture content (MC) is between 0 and 20%, fungal attack will not occur. Between 20-25% MC fungal attack is retarded, and fungal attack occurs when the MC is over 25%.
Oxygen must be present; timber that is completely submerged, saturated, or more than 600 mm below ground is rarely attacked.
Temperature must be in the range of 5° to 40°C; 25° to 40°C is ideal. Fungal attack is retarded by higher or lower temperatures.
Food must be present in the form of unprotected nutrients (carbohydrates, nitrogen minerals, etc.). The timber usually provides these itself, particularly sapwood, which is normally high in sugars and carbohydrates. Nutrients in the sapwood can be protected by preservative treatment.

Removal of any one of these four conditions will prevent fungal attack although, in practice, it is usually moisture that requires the greatest consideration.

Fungi can be broadly grouped into two main types, based on whether they affect the strength properties of timber. Moulds and stains (i.e. blue stain) usually only mar appearance (blue stain can effect impact strength) whereas rot fungi (white rot, brown rot and soft rot) can significantly effect strength properties as they penetrate or break down the cell walls of wood.

Timber is best protected from fungal action by:

eliminating contact with moisture, or
where this is not possible, by using species with natural durability appropriate for the application or low durability species or sapwood preservative treated to a level appropriate to the hazard .

Figure 1 provides a general guide to the level of decay hazard likely to be encountered in Australia. It should be noted that this map reflects the requirements of both moisture (rainfall) and temperature. Local variations and the effect of micro-climates should also be considered when assessing the hazard.

Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.