TIMBER DECAY.
Introduction
The decay of timber can be caused by several different factors, such as the following (Singh, 1994; Singh and White, 1997):
1. Fungal decay. Common types of fungal decay are due to dry rot, wet rot, and mold.
a. Dry rot. A fungus belonging to the same group as common mushrooms and toadstools causes
dry rot. Reproduction of the fungus is by spores, which can be produced in enormous numbers, but reproduction requires favorable timber moisture content of 20 to 30 percent. The fungus produce an enzyme called cellulase, which digests the wood cellulose, but is unable to attack the rigidifying polymer (in the cell walls), called lignin. The lignin remains as a brittle matrix that cracks into cubical pieces. The fungal strands have the ability to spread beyond the initial area of attack and these strands are known as rhizomorphs.
These strands are able to transport nutrients and water and may be up to 1/4 in. (6 mm) in diameter. If the timber moisture content falls below about 20 percent, the fungus will become dormant and eventually die within 9 months to 1 year.
In terms of the appearance of dry rot, Singh and White (1997) state:
Wood thoroughly rotted with dry-rot fungus is light in weight, dull brown in color, crumbles under the fingers, and loses its fresh resinous smell. It is also called brown rot, a term relating to the manner in which it destroys the cellulose, leaving the lignin largely unaltered, so that the wood acquires a distinctive brown color. As a result of this, the structural strength is almost entirely lost.
b. Wet rot. Wet-rot fungus develops when the timber is in a persistently wet condition with moisture content of 50 to 60 percent. Wet rot decay is responsible for up to 90 percent of wood decay in buildings.
c. Molds. Molds usually develop on the surface of the wood. They increase the porosity of the wood and allow the wood to get wet more easily and to stay wet, which increases wet-rot decay. Often the presence of surface deposits of mold will indicate that the wood has been subjected to excessive moisture.
2. Insect decay. There are many different types of insects that will attack timber. Some insects, such as beetles, are wood boring and directly consume the wood. Other insects prefer damp wood or wood that has fungal decay. Some insects, such as termites, have complex colonies of winged adults, workers, and soldiers. Termites live within the timber, often hollowing out the interior, but leaving the outer shell for protection.
3. Other factors. There are other factors that can cause timber decay. Examples include chemical decay, mechanical wear, and decomposition by physical agents, such as prolonged heating, fire, and moisture.
The decay of timber can be caused by several different factors, such as the following (Singh, 1994; Singh and White, 1997):
1. Fungal decay. Common types of fungal decay are due to dry rot, wet rot, and mold.
a. Dry rot. A fungus belonging to the same group as common mushrooms and toadstools causes
dry rot. Reproduction of the fungus is by spores, which can be produced in enormous numbers, but reproduction requires favorable timber moisture content of 20 to 30 percent. The fungus produce an enzyme called cellulase, which digests the wood cellulose, but is unable to attack the rigidifying polymer (in the cell walls), called lignin. The lignin remains as a brittle matrix that cracks into cubical pieces. The fungal strands have the ability to spread beyond the initial area of attack and these strands are known as rhizomorphs.
These strands are able to transport nutrients and water and may be up to 1/4 in. (6 mm) in diameter. If the timber moisture content falls below about 20 percent, the fungus will become dormant and eventually die within 9 months to 1 year.
In terms of the appearance of dry rot, Singh and White (1997) state:
Wood thoroughly rotted with dry-rot fungus is light in weight, dull brown in color, crumbles under the fingers, and loses its fresh resinous smell. It is also called brown rot, a term relating to the manner in which it destroys the cellulose, leaving the lignin largely unaltered, so that the wood acquires a distinctive brown color. As a result of this, the structural strength is almost entirely lost.
b. Wet rot. Wet-rot fungus develops when the timber is in a persistently wet condition with moisture content of 50 to 60 percent. Wet rot decay is responsible for up to 90 percent of wood decay in buildings.
c. Molds. Molds usually develop on the surface of the wood. They increase the porosity of the wood and allow the wood to get wet more easily and to stay wet, which increases wet-rot decay. Often the presence of surface deposits of mold will indicate that the wood has been subjected to excessive moisture.
2. Insect decay. There are many different types of insects that will attack timber. Some insects, such as beetles, are wood boring and directly consume the wood. Other insects prefer damp wood or wood that has fungal decay. Some insects, such as termites, have complex colonies of winged adults, workers, and soldiers. Termites live within the timber, often hollowing out the interior, but leaving the outer shell for protection.
3. Other factors. There are other factors that can cause timber decay. Examples include chemical decay, mechanical wear, and decomposition by physical agents, such as prolonged heating, fire, and moisture.
