Conditions necessary for decay of wood
|
|
| The amount of oxygen surrounding wood and the atmospheric temperature are quite difficult to control in wood in service. Moisture content, on the other hand, can be regulated in wood which is not likely to be exposed to liquid water or to extremely high humidities following drying. If wood is maintained at 20-25% MC it will not rot. Therefore, wood used indoors for most uses need only be dried to provide long-term protection from rot. | |
|
|
| The way that fungi cause decay is through the action of enzymes secreted at the tips of decay hyphae. These enzymes attack the linkages between sugar units that comprise cellulose and hemicellulose, thereby liberating the constituent sugars and making them available as food. But the enzymes cannot reach the wood surface, nor can sugars that might be liberated travel back to the tip of the hyphae, without a film of liquid water. The water serves as a transport medium, and without it wood cannot decay. | |
|
Thus, your furniture, cabinets, interior trim, and other wood that is
kept indoors - and kept at a moisture content below the fiber saturation
point, 20-25% MC, will never decay. Therefore, wood used indoors for most
uses need only be dried to provide long-term protection from rot. It is
important to realize, however, that if wood is exposed to liquid water,
its' moisture content may quickly rise above fiber saturation even if
the wood has been previously kiln dried.
Conditions which can lead to decay of wood used indoors include:
A common source of problems is collection of liquid water against untreated wood. |
|
|
Shown here is the rim joist of a house exhibiting advanced decay. The corner of the front porch is visible at the left. Normally, the joist shown here would not require preservative
protection as it is well above ground level. In this case, however,
the porch was sloped slightly
toward, rather than away from, the house setting up perfect conditions
for decay.
(Image from: DeGroot, R.C. 1976. Your Wood Can Last For Centuries. Forest Service, United States Department of Agriculture. |
|
|
Here a planter has been installed such that the ground level is raised
above the bottom of the wood siding and the wood framing members to the
inside. To make matters worse, frequent watering is employed to help
the flowers grow. The homeowner has unwittingly set up perfect
conditions for decay.
(Image from: DeGroot, R.C. 1976. Your Wood Can Last For Centuries. Forest Service, United States Department of Agriculture. |
|
Another common situation is one in which backfilling brings the ground
level to a height above untreated framing members. Homeowners sometimes
do this on purpose when the exterior is brick or stone, thinking that
soil contact for these materials poses no problem. However, water can
readily move through the wall, creating ideal conditions for decay in
the wood members inside.
(Image from: DeGroot, R.C. 1976. Your Wood Can Last For Centuries. Forest Service, United States Department of Agriculture. |
|
|
Where wood is used outside in contact with the soil it too will regain
moisture even if previously dried. In this case, and in situations such
as that described above, other steps must be
taken to guard against fungal attack.
The strategy employed in protecting wood to be used outdoors, or in a wet environment, is to make the wood unavailable to fungi as a source of nutrients and energy. If wood can be poisoned or made undesirable as a food source, fungal attack can be averted. Methods of treatment will be discussed a bit later on. |
|
|
