BUILDING BRIEFING:
Timber Infestations
A series of briefings on property defects and remedial techniques that property managers will commonly encounter and oversee throughout their management role.
Wood is an excellent construction material being strong and relatively durable; it has been used successfully in the construction of dwellings for hundreds of years. However, the strength and durability of timber can be adversely affected by decay and insect infestation.
As one of the principle construction components of all dwellings, it is essential that property inspectors and managers can identify conditions where the material comes under attack or decay before serious structural damage occurs. Arrested early, these conditions will not unduly affect strength and integrity but, if allowed to continue, expensive and comprehensive replacement may be necessary.
Wood may be attacked by a variety of timber pests, and these may be divided into two groups:
- fungi or rot
- wood-boring insects
In most cases, ventilation and damp are also key factors especially with rot which results from wood being exposed to damp conditions.
Rot
Fungi, or rot, is a word that strikes fear into the minds of many homeowners, and inevitably leads to a visit from the myriad of preservation and treatment companies that exist. Anecdotal reports describe owners incurring thousands of pounds of repair bills to remedy the damage inflicted by a severe attack of dry rot.
Types of fungal attack
There are two principle types of fungi that attack and break down wood, and both result from wood becoming exposed to damp conditions for prolonged periods of time. These are:
- dry rot
- wet rot
This division, although somewhat simplistic is important as it defines the type and extent of remedial treatment necessary.
Dry Rot
Although wet rot is the most commonly seen, it is dry rot that is the most serious type of fungal attack and less easy to control once established. The term dry rot is something of a misnomer, as, with any forms of rot, it requires damp conditions to create it. Dry rot is a particular form of brown rot and its name refers to its effect on wood which becomes dry and crumbly. Dry rot conditions can develop when timber and its surrounding air contain excessive moisture (a moisture content of between 17% and 22%) – conditions that can easily develop in poorly ventilated places like under the floor or behind skirting boards. Dry rot can spread extensively around a building and it is this characteristic that makes it so much more threatening than wet rot which is generally very localised. The mycelium of dry rot can spread extensively along the surface of wood and even along masonry, growing outwards around a building in search of food and favourable conditions. Dry rot can also enter a dormant stage if the damp environment disappears, returning once again when condition become more favourable.
Identification of dry rot can be more difficult as it can develop in inaccessible places and spread throughout the building, whereas wet rot does not spread in the same way. Dry rot can appear in a variety of forms; the most common being an off-white net or web – not unlike cotton wool. Grey or white strands may also be visible along the surface of wood or masonry, and the rusty-red coloured fruiting body which is either a pancake or bracket-like in shape and which releases spores so that the organism can spread and regenerate further. The spores, en-masse, appear as a rusty red-coloured dust. Wood infected by dry rot develops a distinctive cuboidal cracking pattern as it becomes dry and crumbly, and develops a dull brown colour.
Although many types of rot impart an odour, there is a distinctive mushroom-like smell that can often be detected within a building infested with dry rot – this smell can be particularly noticeable when entering a building that has been shut up, unventilated for some time.
Wet Rot
Wet rot: Poor maintenance of exerior paintwork and other exterior detailing led to severe wet rot and early failure in this door
If wood is kept continuously wet, but still exposed to the air, it is likely to develop a soft fungus – wet rot. Typically areas where wet rot may be found is in window sills, parapet gutters, and other places where dampness can accumulate.
Wet rot thrives in timber with a high moisture content – around 50-60% - particularly where the wood is repeatedly wetted.
Wet rots vary more in terms of visible identification. They can appear as a brown rot – causing the wood to become dark (often with similar cuboidal cracking to dry rot – although generally less pronounced) or as a white rot where the timber becomes bleached. Wet rot does not travel through masonry or plaster which means that any problem is generally localised to near the source of dampness.
Remedial Treatment
The first step in remedial treatment for either wet or dry rot is to determine the cause and extent of the outbreak. Depending on the type of rot and outbreak, professional inspection and treatment may be required. Localised wet rot to the edge of a window frame will, of course, be of significantly less threat and concern than, say, the identification of dry rot and cuboidal cracking in a length of skirting board. The latter situation is a typical case for employing professional help.
It will be important to find and eliminate the damp conditions which are providing the ideal living environment for the rot. Once the dampness has been eradicated, and timber has been dried out, the rot or fungus will die. Measures must be taken to dry out the structure as soon as possible. Typically, this might involve:
- adding extra sub-floor ventilation,
- repointing a wall, or repairing a faulty damp-proof course (DPC)
- repairing a leaking pipe or drain
- redesigning an exterior detail so water does not collect
- repainting exterior timber that has been left exposed to damp and elements
It is important to emphasise that removal of the damp conditions is the key treatment in dealing with fungal decay. Chemical treatments might assist on a localised level, but new fungal outbreaks can occur as spores spread to other damp areas, or areas inaccessible to the chemical treatment. Timber removal and replacement will be necessary where the rot has caused breakdown in the strength and integrity of the wood. The remedial treatment of moderate and severe fungal infestations is an area for professional advice from a building surveyor - ideally independent of any treatment firm.
Insect Infestations
With increasing awareness of the environmental and ecological implications, it has become less acceptable to undertake repeated spraying with various cocktails of harmful chemicals at the first sign of any boreholes in the woodwork. A particular property could be end up being treated repeatedly and unnecessarily by subsequent owners (perhaps even as a condition of mortgage approval).
Modern practice is to first determine whether an infestation is active before undertaking any remedial work. A survey can be conducted, and samples collected from suspect timber to determine whether treatment is necessary.
Identification
Like rot, insect attack has been taking place on the timber of buildings for centuries, and generally makes itself visible by the exit holes (or 'flight holes') left by the adult insect as they emerge from the timber as part of their life cycle. The infestation can occur naturally, or even be already present in the timber when originally incorporated into the building. Insects that feed on timber tend, like fungal attacks, to thrive in damper conditions and will usually die out as the timber dries. The insects that tend to cause problems for timber used in buildings are often referred to as 'woodworm' but they are not worms but species of beetle. Like wood-rotting fungi, they are a threat because they use wood as a food source and eventually eat away enough of the material that the structural strength is compromised, and can eventually lead to failure. The life cycle starts with the adult that lays eggs on or in the wood. Larvae then hatch and tunnel into the wood until they develop into pupa which then hatch and emerge from the wood – through an exit hole.
One of the most common types of timber insect found in this country is the common furniture beetle, which tends to attack wood in moist or humid situations such as a damp underfloor area, or in a roofspace. Their presence (or previous presence) can be identified by the round emergence holes which are about 1-2 mm in diameter.
A common telltale sign of an active infestation is the bore dust that is excavated by the boring larvae and can sometimes be seen on flat surfaces directly underneath the boreholes in the infested timber. This is not, however, a totally watertight test as some wood boring insects do not produce bore dust (known as frass).
If the flight holes have sharp edges and clean fresh wood can be seen, then recent or current activity can be suspected
As with rot, eradication of a dampness problem can often cure the problem. Low moisture content in timber can kill the larvae causing them to desiccate (dry out) and die. The introduction of central heating has had a major effect over the last 50 or so years in reducing the prevalence of damp and thus woodworm. Chemical treatments can be applied to destroy the parasites, and prevent future infestation, although this is not always fully effective because of the limited penetration of such treatments into the timber. There are concerns about the health and safety of occupants, pets and other environmental factors of using such toxic chemicals. Bats, for example are a protected species under the Wildlife and Countryside Act 1981 and are vulnerable to the toxic chemicals used in such treatments.
Summary
Like all building problems, regular inspections with the benefit of experience and a degree of technical knowledge of buildings can help property managers to identify problem areas before they become disaster areas.
References:
Understanding Housing Defects by Duncan Marshall, Derek Worthing & Roger Heath, 2nd edition. Published by Estates Gazette Books.
Building Research Establishment (BRE). The BRE publish a series of detailed factsheets on common building defects and repair techniques.
BRE: 01923 664000