Rot and infestation management

Introduction

We have all seen woodworm damage by the common furniture beetle (CFB), whether in chairs and tables or boards and joists.

Do not be influenced by those who say that CFB causes little structural damage to buildings in this country. To a certain extent it does depend on what we mean by 'structure': the floorboard of a traditional floor is 'structural', as well as the rafters, purlins, roof battens, etc.

Many biocides and insecticides act to control and kill a range of organisms, so exact identification may not be as critical a factor. Try and avoid the use of Latin names.

CFB life cycle Adult female beetles lay eggs in or on suitable timber – and prefer timber with a roughened surface, or with cracks and crevices, favouring too the end grain of timber, or even a pupal chamber. Eggs hatch and larvae emerge. Larvae bore into the wood, and may continue boring for several years. Eventually they munch and burrow towards the wood surface, pupate and by metamorphosis change into a beetle, which chews its way out from the wood leaving a roundish exit or flight hole.

Emergence of the common furniture beetle is between May and September. During the time that the larvae are tunnelling away inside the timber, there would be no visible external evidence of their presence. The evidence is only visible once the beetle bites its way out to leave a hole, together with collectable and identifiable bore dust (frass). So it can be risky to pronounce in a report that woodworm in timber are not present – they could be unseen. Even after timber has been chemically treated, woodworm can still be active, but might succumb eventually to the insecticide, which will often only have protected the outer margins of the timber.

If you wished to check whether an infestation was live by filling up flight holes or covering areas of timber with paper or card, remember that you would need to wait till the end of the flight season to find this out.

Timber of less than 12% moisture content should be safe from woodworm. The optimum moisture level for growth of larvae is much higher.

Identification of insect responsible for damage

You will find useful advice for surveyors in Surveying Buildings. The standard guide to identification, Recognising wood rot and insect damage in buildings, explains in great detail how you might confirm the cause of wood damage in buildings. Basic identification advice follows below:

• Bore dust – frass: usually collects in cobwebs under attacked timber – light in colour, approximating to the colour of the freshly cut timber. Lemon-shaped pellets, gritty.
• Beetle: 3–5 mm long.
• Larvae: up to 6mm long.
• Flight holes: 1–2mm diameter. Recently cut holes would exhibit sharp edges and you would see the lighter inner sides of the hole. Eventually this light hole interior would darken from oxidisation. You can easily insert a screwdriver into damaged timber. Timber will typically be quite crumbly where badly affected.
• Damage is restricted mainly to sapwood, except where rot is present.

It is worth investing in an illuminated car map reader or a 10× magnification lens to aid your inspection of flight holes, frass, insect damage and the insects themselves.

You may find other insect holes and damage on the same area of damaged timber, e.g. the smaller holes of parasitic wasps, or in damper wood, the damage from wood-boring weevil.

Where to find woodworm damage

The BRE tell us to expect woodworm where timber is damp – but follow the example of many surveying textbooks and single out timber near loft hatches or in stair cupboards. 

There may often be evidence of woodworm and weevil in boards and joists near a WC pan, but this is not always due to WC pan leaks. Floor joists may be built into a wall and be very damp, or floorboards may be damp too – butted against damp masonry or plasters. Ground floors are becoming increasingly difficult to investigate due to the popularity of laminated flooring – so often fitted over original timber boardings. Woodworm may not be discovered until the infestation is quite advanced.

If you follow through the decision-making flowcharts contained in the BRE Good Repair Guide 13, you should be able to confidently find where woodworm damage might be. But you may also have sympathy for those who have a strong aversion to any type of chemical treatment. More and more surveyors are cautious concerning use of chemicals, and more often recommend a very focused or 'targeted' treatment, developing improvements to building detailing to make conditions less favourable for any aspiring woodworm or fungal spore. Chemical suppliers have responded by marketing safer and more user/building-friendly chemicals.

Remediation

For any woodworm or rot outbreak, decide between chemical and non-chemical remediation. Consider each case on its merits and make sure that, if you do decide to use or specify chemicals, the treatment is carried out as safely as possible and in accordance with current good practice advice.

Sustainable remediation in most cases involves careful upgrade of construction detailing in tandem with targeted chemical treatment. To ignore 1 side of this remedy equation is to court trouble. No matter how carefully treated, a joist end built into a wet wall will eventually rot. If it does, any guarantee will probably be invalidated as the property has not been properly maintained, i.e. kept free of undue dampness.

So for most scenarios consider:

• targeted chemical treatment of retained timber or use of pre-treated new timber;
• isolate all timber from damp masonry; and
• ventilate it wherever possible.

This is the remediation trilogy for rot or woodworm.

Recognising wood rot and insect damage in buildings classified CFB as a 'Damage Category A' insect – meaning that they would normally consider insecticidal treatment to be necessary. It is clear from this advice that the BRE are supportive of chemical treatment against woodworm, i.e. CFB.

The BRE states in Part 1 of GRG 13 that:

'Infestations, [i.e. of common furniture beetle] even if long standing, are usually of little structural significance and therefore require little or no replacement of timber.'

In the author's opinion this is an underestimation of the amount of damage we see in the UK year after year from this particular woodworm. Much of the softwood in British homes is fast-grown pine, with a significant proportion of vulnerable sapwood.

Figure 1: Note how the woodworm damage is restricted to the outer sapwood at board edges. This is very common. Such a floorboard would crush at its edges when levered up by a surveyor's bolster or crowbar.

Figure 2: Even a well-built raised timber ground floor, with dpcs under its wall plates, can suffer woodworm attack. This softwood joist has woodworm damage along its underside, i.e. to its outer sapwood. Old suspended floors are often built directly off damp lime sprinkled earth. The void can often be humid – making the softwood damp enough to suit wood-boring insects, say 16–18% moisture content. Timber close to or built into very damp masonry will commonly become much damper and suffer rot and wood-boring weevil damage. Suspended floors with a concrete 'oversite' can suffer a worse fate – when water ingress creates a pond on top of the slab and timber becomes covered in moulds.

Most textbooks cite CFB as the insect causing the greatest amount of timber damage in this country. As mentioned above, the BRE appears to understate the sheer amount of damage woodworm can cause. This is not the author's experience nor that of many remedial treatment surveyors consulted, who have usually been quite categorical concerning just how often CFB causes damage to structural timber.

The Good Repair Guide 13 (Part 1 and Part 2) covers 'identifying and assessing damage', and 'treating damage'. The very title of Part 2 leads one to suspect the emphasis will be on 'treating' woodworm attack, and 'treating' means application of chemicals to kill off insects and kill or prevent damage by others that follow in the future. Part 1 covers basic information on the various wood borers; where to find the damage, information on the main culprits, etc. There is a sketch showing where you might find insect damage, and typically crosses are placed where the general house surveyor would normally look, e.g. near a loft trap, near a sanitary appliance, in the stairs cupboard. In Part 2 is a flow chart for decision making on whether or not to apply remedial treatments, i.e. chemicals.

In designing any remediation of woodworm damage we would quickly move down the left hand column of the BRE flowchart in The Good Repair Guide 13 from:

• 'Has insect damaged wood been found during inspection?' (YES) to
• 'Is damage of a type which could be active?' (YES) to
• 'Are there obvious fresh flight holes or fresh dust?’ (YES) to
• 'Is damage widespread?' (YES) to
• 'Apply remedial preservative treatment to all timbers of affected floor or roof area.'

But the BRE chart incorporates some quite complex threads of questioning too, and would veer away from chemical treatment in the following instances:

• there is evidence of previous treatment;
• infestation is inactive;
• less than 20% of timber is sapwood;
• timber affected is not structurally important;
• fresh bore dust (frass) or flight holes not present; or
• the damage is not widespread.

If the damage is not widespread there is the option of very localised and 'targeted' chemical treatment or timber replacement.

There are many surveying challenges here. It could be quite expensive to confirm a previous treatment. It would help if you knew the company that carried treatment out, as once the type of chemicals used were known, specific tests could be commissioned to verify this. If you have no knowledge on the type of treatments used, then a number of tests would need to be paid for.

Confirming whether or not an infestation is active could be difficult from a snapshot pre-purchase inspection. The definitive clue is the presence of freshly formed flight holes – so shine a torch across the woodwormed timber and look for light-coloured wood inside the hole, i.e. the colour of the freshly cut timber. Then seek out piles of freshly emitted bore dust (frass). Once timber is cut, oxidation darkens it.

You should be able to know whether damaged timber is 'structural', but if in doubt, consult a structural engineer. The difficulty is that damage may be quite serious but within the invisible thickness of a timber. There exist quite sophisticated instruments for assessing internal condition of timber, such as the 'resistograph', which drives a very narrow drill into timber, and creates a print out of the varying resistance through the member's thickness. Carpenters have in the past traditionally tested timbers using a hammer, or perhaps a wood bit driven in by hand drill – to get a feel for varying resistance as rotten wood or voids are encountered.

Establishing just how widespread a woodworm attack is could be a daunting task.

To fully check out just a timber ground floor could take hours. Many properties these days are covered by laminated boarding or glued vinyl, making inspection very difficult.

There may be a limited opportunity to actually inspect the timbers. Houses are likely to be fully accessible when they are empty between sale and purchase. This might be for a limited time only. Once carpet and furniture arrives, the floors become yet again difficult to access., so precautionary treatment is commonly carried out at this time. The preparatory work to treat, i.e. lifting every fifth floorboard, would be the same amount of access required for a full inspection – so there is a strong case to treat at this time.

Many properties have been given precautionary treatment against woodworm, probably often just because it can be so time-consuming and difficult to fully assess the scale and significance of an outbreak. This could be 'blanket treatment' when a fuller inspection might confirm a localised treatment only to be necessary.

If you do plan to specify or carry out chemical treatment of timbers as a precaution, then the decision to treat should be taken following a COSHH (Control of Substances Hazardous to Health) assessment, in which, among other things, ease of future access should be considered.

You could learn more about past treatment by desktop research and site enquiries:

• Past maintenance records could shed light on past chemical applications.
• Guarantees may be available for specialist work and may even detail the exact chemicals used and the extent of treatment.
• You may find joinery repairs that indicate remediation of past insect or rot problems.

Monitoring woodworm

Church wardens have been known to keep records of numbers of death watch beetle collected. If you have the opportunity to check a building for common furniture beetle (CFB) over a period of time, you could affix paper or card to vulnerable timbers and check for flight holes periodically. You could, as Rideout mentions, clog up flight holes with furniture polish and observe whether new holes appear. Such investigations take time, as the beetles only emerge in the summer. Monitoring might take a year.

In practice, most house surveyors will be making a one-off visit to a property, most often for a pre-purchase inspection. Such an inspection might only allow a few minutes for assessment of a woodworm outbreak – but in a limited inspection it would be often be quite possible to look closely at discovered woodworm affected areas to see if the holes appear fresh or old, and to verify the type of insect responsible for the damage.

Being the most common wood borer in UK properties, a house surveyor should have knowledge of the CFB. To pronounce an infestation active or inactive requires quite considerable experience. So many cases are passed on to specialist remedial contractors, where a chemical treatment and the accompanying guarantee might be the end result.