Almost all building
materials deteriorate when they are exposed to moisture over time. Moisture
causes fungal decay and mould in timber. Also, chemicals from corroded metal
fixings can cause damage to timber.
The rot family: brown,
white and soft rots
There are three classifications of fungal decay or rot:
brown rots
white rots
soft rots
All rot is caused by fungi, spread by airborne spores. Rots
require food, moisture, oxygen and warmth. Spore germination needs the presence
of free water.
Brown rots are the most common in wet framing. They tend
to darken the appearance of the timber as they consume the cellulose only,
leaving behind the darker lignin. In its early stages the rotted timber may not
change much in appearance but will be easily penetrated with a knife. Brown
rots are considered to be more ‘malignant’ than other rots because
they decay timber more rapidly. One particularly malignant brown rot is dry
rot. It has the ability to transport moisture and is very destructive once
established.
White rots appear to bleach the timber as they consume both the
lignin and cellulose. They are less prevalent in softwoods such as
radiata pine.
Soft rots leave very little indication of their presence. The
decay takes place within the cell wall and will generally occur when the timber
is in contact with the ground.
A damp wall cavity can form an ideal environment for fungal growth. Typical
fungi are Penicillin, Aspergillus and Stachybotrys. (see also: Mould)