CPD articles

CPD: Wood treatment and timber preservation

5 June 2017 | By Richard Walker

Richard Walker, national technical and development manager at preservation specialist Peter Cox, explores the most common dangers facing building timbers in the UK, and reviews the options for prevention and repair. 

Building timbers in the UK can be blighted by both the wood-boring beetles more commonly called woodworm and several different types of fungi which can cause timber to decay. Not only can these pests leave unsightly holes and cracking to the timbers, they can cause serious structural damage and, left untreated, can destroy the timber completely.

This means that preventative measures should be taken to ensure the conditions that allow the beetles or fungi to thrive do not exist. If they are discovered in building timbers, however, it is vital that action is taken to eliminate them immediately.

Of the many species of wood-boring beetle found in the UK, four are of particular significance:

As well as the damage caused by these beetles, timber faces additional challenges from fungi. As with the beetles, there are a number of different species, but the four most commonly encountered varieties in the UK are:

The risks of woodworm

All species of woodworm have a similar life cycle. It is the larval stage that really does the damage. A female will typically lay between 40 and 80 eggs into the cracks and end grain of timber. When laid, for example, in a joint between floorboards, the hatched larvae will bore their way into the timber. They usually follow the grain, creating a series of tunnels.  They can do this for between two and eight years – depending on the species – and will cause significant damage in the process.

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Towards the end of the larval stage, the larvae make their way near to the surface of the timber where they create a pupation chamber and metamorphosis takes place. The adult then chews its way out, creating the tell-tale holes in the surface. These emergence holes are evidence that there has been an active infestation within the timber – and confirmation the larvae will have been destroying the wood below the surface for a number of years prior to the appearance of any adult beetles.

The risks of fungal damage

While there is just one form of dry rot, Serpula lacrymans, many wet rots are commonly found in the UK. All wood rotting fungi require these essential preconditions to thrive: spores, susceptible wood, oxygen, moisture and darkness.

The only factor we can really control as building constructors, owners or managers is moisture. At the lowest moisture levels, the spores require a moisture content in the wood of around 18% so, provided the building is well maintained and kept watertight, it will be safe. However, this isn’t always the case and the consequences of an outbreak of fungal decay can be disastrous and, left untreated, will cause the total destruction of the wood.

The spores settle on timber where they germinate into hyphae. The hyphae multiply many times to form the mycelium growth, from which the strands which allow the fungus to spread develop. After the fungus has been growing for a while and is quite mature, it will develop a fruiting body or sporophore (mushroom) which then sends many new spores into the air.  For example, a single dry rot sporophore can produce up to 30 million spores.

Severe weevil infestation

Dry rot or wet rot

All rot can cause significant damage but it is important to distinguish between dry rot and wet rot. Dry rot is malignant, which means it will spread in search of fresh timber to attack, even over steelwork, masonry, concrete or through masonry. This means action taken to eliminate it has to be far more extensive.

In contrast, wet rots do not have the ability to spread through masonry, and growth is confined to the immediate area of damp and will cease when the moisture is removed. Quite often when wet rot is found, it is quite common to find wood-boring weevil infestations attacking the timber at the same time.


Although the damage inflicted on building timbers by woodworm, wet rot and dry rot can be extensive, the treatments for these different defects are very different.

Given the intrusion and expense of such measures, prevention is obviously the best approach. If this isn’t possible, then as soon as any of the tell-tale signs are spotted, it is vital to bring in professionals for a rapid diagnosis and urgent action. Whatever species of rot or woodworm is attacking a building, the damage can be extensive if it is allowed to continue – once discovered, urgent action is vital.

Taking action against woodworm

The application of an insecticide is usually sufficient to eradicate a woodworm infestation from timbers. The vast majority of treatments are carried out by applying a liquid formulation to the surfaces of the affected timbers. This is done using a brush or, more commonly, by low-pressure spraying.

It is important to balance the need for efficacy with the need to minimise any possible mammalian toxicity: any product used must be non-flammable and HSE approved. The team at Peter Cox favours micro-emulsions, water-based products that have a very good penetration capability in timber and are odourless. We use the lowest possible level of active ingredient to minimise toxicity while ensuring efficacy, so the active ingredient of Permethrin constitutes just 0.2% of the sprayed solution.

Timber damage requires expert attention if it is not to spread. Below: The deathwatch beetle is known for its distinctive tapping sound

Permethrin is a pesticide used in agriculture and is one of the safest products for controlling wood-boring beetle attack in timbers. It is also “bat friendly” and can be used in areas where bats are known to frequent, once the relevant authorities have given permission to treat the infested areas. The latest products come in pre-measured doses in dissolvable packaging, which avoids any need for contact with the concentrate and minimises packaging waste.

The permethrin solution is sprayed on to the timber surfaces, where it will penetrate to a depth of 2-5mm to create a protective envelope that is poisonous to the beetles. Permethrin is a contact poison, so if an adult beetle, larva or newly laid egg comes into contact with treated timber it will absorb some of the active ingredient and die. The insects do not need to ingest the material – on contact the insecticide attacks their central nervous system.

A penetration of 2-5mm is more than adequate, but the results won’t be instant.  Any larvae near the immediate surface will die, as will emerging adult beetles as they eat their way out of the wood. Larvae below the surface will continue to cause damage but the infestation will be eradicated once all adults have emerged at the end of their life cycle.

Gels and pastes, which offer a deeper penetration into the timber, are also available.

After the timbers have been treated, it is possible that you will see new holes appearing as a result of “post-treatment emergence”.  However, the escaping beetles will not survive long enough to do any further damage.

Taking action against wet rot

Sometimes it is not possible to easily distinguish between species of wet rot. In practice, this is not really necessary since the same remedial measures are required to treat all of them.

The primary action is to prevent the cause of the damp problem. Second, the decayed timber needs to be cut back to “sound” timber and any decayed sections repaired. Finally the affected and surrounding timbers should be treated with a fungicidal spray.

As with the insecticides used for treating woodworm, it is prudent to use products that offer the following advantages: they are micro-emulsion formulations, come as a spray application, have low odour, low mammalian toxicity, rapid re-entry to the property after treatment – often one hour after the treatment has dried, are non-flammable and HSE approved. In addition, any treatment regime must include the removal and replacement of decayed timbers.

Taking action against dry rot

In the case of dry rot, an extra step is required. This involves the fungicidal treatment of the masonry. This is because dry rot mycelium growth will spread behind wall plaster and through the masonry itself.

To treat the mycelium, affected masonry and wall plaster are removed, then an irrigation process is used whereby holes are drilled to half the depth of the wall, at staggered centres around the perimeter of the affected masonry. Into these holes a masonry biocide is pumped to refusal and then a surface application is applied to all the exposed masonry. The treatment has the effect of “sterilising” the masonry to prevent further dry rot mycelium growth.

Roof trusses which have been attacked by deathwatch beetle (above) and beam end bearing the marks of deathwatch beetle

It is vital that the masonry is allowed to dry, otherwise the mycelium growth in the wall will continue to spread and will reinfect any new timbers in the vicinity.

Masonry irrigation is not necessary for wet rots as their mycelium growth does not spread through masonry.

Replacing timbers

Wherever there has been a woodworm infestation or a dry rot or wet rot outbreak, decayed or structurally weakened timbers must be removed and replaced. This can be the most disruptive part of any treatment process.

The most vulnerable sections of timber are usually those that bear into the wall structure because it is at those points where the timbers typically tend to absorb moisture.

In such a situation, often it is only the ends that need to be replaced. The traditional method is to splice in new sections using steel plates or bolts and timber connectors fixed through the timber. While this technique is effective, the steel plates can be quite unsightly if the timbers are to remain exposed.

Plus, there are significant health and safety risks around lifting the steel plates into position when these are large sections – to repair the end of a traditional large roof truss, for instance.

One alternative is to use epoxy resins for carrying out structural repairs to damaged timbers. This is an economical method, since repairs can often be carried out relatively quickly without having to cut out large areas of the existing timber and with minimal disturbance to the surrounding structure.

Working with epoxy resin also presents less health and safety risk than manoeuvring heavy steel plates into position.

Peter Cox has been working with epoxy resin for many years now. Over this time, our team has found a number of applications to which epoxy resin is well suited:

“An outbreak of fungal decay can be disastrous and, left untreated, will cause total destruction of the wood.”

These types of epoxy resin repairs are both structurally and aesthetically beneficial.  They are particularly well suited to repair work being undertaken in historic buildings, since this type of repair enables the maximum proportion of the original timber to be conserved, while the repairs are often “secret” and can be hidden from view.

Early methods of using resin involved pouring solid resin into a shuttered box with reinforcement bars. The shuttering can either be removed afterwards or alternatively can be incorporated into the repair so that it looks as if timber has been used. 

A more economical alternative is to make the bulk of the replacement in timber and only use resin to effect the join. This is achieved by creating slots in the new timber to be spliced to the “host” timber. This allows the reinforcement bars to be accommodated. Once the new splice has been propped into position, resin is poured into the slots. The props can be removed after 48 hours. This very efficient regime can result in an economical repair that is both structurally sound and aesthetically sympathetic to the original construction.

Of course, as with any other beam end repair, the reinforcement will usually need to be designed or verified by a structural engineer.


As is so often the case, prevention is better, more economical and easier than cure. Since wet and dry rots all need moisture to thrive, and most woodworm prefers it, the easiest way to prevent an infestation is to ensure good construction standards and building maintenance practices are observed. Repairs to roofing, rainwater goods, defective pointing and cracked render coats and plumbing defects, cleaning of airbricks, and the removal of soil and paths above damp-proof courses should be undertaken immediately.

Developers and property managers who ensure good standards of construction and maintenance should not have much to fear from these destructive organisms.

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