Wood is a durable material which is resistant to most attack provided it remains dry. Through prolonged wetting it can lead to a risk of decay by wood rotting fungi and insect attack.
There are two main classes of wood rotting fungi, the brown rots and the white rots. Brown rots cause the wood to become dark in colour and crack along and across the grain. White rots cause the wood to become lighter in colour and fibrous in texture.
Dry Rot or Serpula Lacrymans is a major building decay fungus often causing extensive damage. The dry rot fungus has the ability to spread on wood and other cellulose containing material, and is also able to grow through brick and plaster. Dry rot will only affect timber that is damp, typically affecting timber with a moisture content of 20 - 35%. In suitable damp conditions dry rot growth can extend a distance of several metres from its food source.
This type of rot is extremely destructive, however it is sensitive and can only survive in a limited range of moisture and temperature conditions. If you remove these conditions the rot will stop growing, become dormant and die after a few years. If these favourable conditions return during the dormancy stage it can begin to grow again. The primary aims of remedial works are to establish dry and well ventilated conditions.
The rotten wood shrinks and splits into cuboidal pieces, the wood becoming lighter in weight, dull brown in colour. Fungal strands are grey or white 2 - 3mm in thickness, in damp darker places white silky 'cotton wool' growths can be found.
The most common Wet Rot is Cellar Fungus or Coniophora Puteana which develops in saturated or very damp timber in buildings. It is normally found in bathrooms, roofs, cellars and window frames where persistant leaks or condensation occurs. The moisture content of timber for decay by this fungus is around 45 - 50%.
The attacked timber becomes dark brown in colour or black with yellow-brown streaks in patches. The fungus itself can produce dark brown branching strands on the surface of the wood, decay taking place within the wood. The strands unlike dry rot do not spread any distance from the timber. The fungus attacks wood that is very wet and will die as soon as the timber dries out.
Other forms of wet rot fungus include the mine fungus (Fibroporia Vaillantii) and oak rot (Donkioporia Expansa).
Measures to control an outbreak of wet rot are much less than dry rot attack as it is only limited to areas of damp timber. Removing the source of moisture and drying the timber can be all that is required, however assessing the structural integrity of all timbers attacked by fungal decay should always be carried out.
A number of insects, mainly beetles are able to use wood as a food source and some of them can cause serious damage to building timbers. Damage caused by wood boring insects does not always indicate a need for remedial treatment so correct identification is essential before any consideration can be given to whether treatment is needed.
Five main species are responsible for most of the damage caused in the UK, these are the Common Furniture Beetle, Death Watch Beetle, Powder Post Beetles House Longhorn Beetle and the Wood Weevil.
Common Furniture Beetle (Anobium Punctatum)
This beetle is the most frequent cause of damage. It attacks the sapwood of softwoods and some hardwoods. It can be found in all constructional timbers such as loft access, under stairs, ceiling beams and timbers in contact with solid walls.
Death Watch Beetle (Xestobium Rufovillosum)
The attacks are mainly confined to damp or decayed hardwoods, particularly Oak. It is mostly found in the structural timbers of older buildings such as wall plates, lintels, joist end and other built in timbers wher it can spread. The beetle is much larger than the Furniture Beetle and therfore causes damage more quickly.
Powderpost Beetle (Lyctus Brunneus)
This beetle attacks only the sapwood of recently seasoned hardwoods such as Oak and Elm which have a high starch content. They very rarely attack softwoods. They are commonly found in furniture, block strip flooring and oak timbers, the attack usually originates in the stockyard or storeroom and so by the time the attack has been discovered, the timber has a reduced starch content preventing further attack.
House Longhorn Beetle (Hylotrupes Bajulus)
This beetle attacks softwoods and is most commonly found in roof timbers. The beetle is currently only found in South West England, with a few isolated cases reported out of this area. Any outbreaks found must be reported to the Building Research Establishment.
Wood Boring Weevils (Pentarthrum Huttoni)
The weevil attacks both softwoods and hardwoods in damp conditions and is usually secondary to fungal decay. They are commonly found in poorly ventilated floors, cellars and wood in contact with solid damp floors and walls.
Insect infestations can be put into three categories;
Insecticidal treatment usually needed
A number of insects are a primary cause of serious damage and these require an insecticidal treatment. Some can cause structural damage (eg Deathwatch Beetle / House Longhorn Beetle) and with these a structural survey may be necessary.
Treatment necessary to control Wet Rot
A number of insects can only feed on damp wood rotted by fungi. As they cannot attack dry wood, further infestation by these insects is prevented by controlling the wood rot.
No treatment needed
A number of insects attack partially dry timber which may be incorporated into buildings. These insects though have usually been killed off during drying and therefore no treatment is necessary.