It is generally considered that wood with a moisture content of 18% or less will not be attacked by fungi. In fact, it is probably a prerequisite that wood must be wetted to above 30% in order to allow the initial germination of fungal spores.
Once established, fungi can lay dormant in dry conditions and re-emerge when the moisture content increases once again. Fungal spores are everywhere, so I sometimes wonder whether they have begun to germinate in wood that I see on building sites left unprotected from the rain, thus storing up a problem for the future.
Some panels are, of course, used in wet conditions, such as marine plywood. Such products are either made with wood species that have a high natural resistance to fungi, or are impregnated with chemicals to prevent attack.
The mechanical properties of wood are not really affected by bacterial decay, so if it is re-dried it is very likely to regain its mechanical strength. Bacteria may, however, increase the permeability of wood, because they tend to attack the cellulose-rich zones, such as the pits which interconnect cells.
The three basic types of fungal attack are: brown, white and soft rot. Brown rots attack the cellulosic components of wood, leaving behind a lignin skeleton. This leaves the wood looking characteristically brown in colour, with frequent splits along and across the grain, creating a brick-like texture that is very fragile to the touch.
As their name implies, white rots tend to bleach wood, especially in the advanced stages of attack, whereas soft rots tend to create pockets of soft tissue. Soft rots tend to be found in very high-humidity situations, such as cooling towers and submerged wood.
You may ask about ‘dry rot’ found in many old buildings. This is, in fact a brown rot fungus. It derives its name from the fact that it can cross dry areas and actively wet dry wood by transporting water to it. This is why it is such a menace.
The fungal decay of wood can be arrested by drying it. Precautions need to be taken to ensure that any infestations of dry rot are removed as it may re-infect the dried wood area.
Wood based panels tend to be a little more resistant to fungi than solid wood. So a particleboard made of spruce is more resistant than solid blocks of spruce (see fungal attack in spruce in photo below). This is because the panel has a higher density – and it contains synthetic adhesives.
Correct design, good site practice and subsequent maintenance can ensure that panels are good, safe products to use in buildings.
