Dealing with Woodworm in Period Properties

Should this be something to worry about and what can you do if you suspect an infestation? Alan Tierney gives the lowdown
by Alan Tierney
4th July 2019

Pretty much every period property will have experienced a woodworm attack at some point in its life.

It can be a cause of much worry, but this is rarely as serious as you might fear, and in fact very frequently prompts unnecessary, expensive and potentially harmful remedial work and treatment.

Unfortunately, this is regularly recommended by building surveyors who should, but often don’t, know better.

What is woodworm?

Woodworm is actually a wood boring insect: the Common Furniture beetle.

The damage an infestation causes is largely down to the larvae, which live in timber and use it
as a food source, boring extensive, very narrow tunnels. When the larvae have reached a sufficient size, they change into beetles, mate and lay eggs within the gallery of tunnels they’ve created.

They then chew their way out to go off in search of a new piece to colonise, leaving a distinctive flight hole (usually just a couple of millimetres in diameter) on the surface of the timber.

What they need to thrive

Wood boring beetles’ natural environment is the forest floor, where they feed on dead and decaying trees. They’re not adapted for life in buildings and can only survive indoors when conditions are particularly favourable for them.

Woodworm resistant areas of log

The cut end of a log, showing the definition between the living sapwood (usually lighter) and the attack-resistant heartwood

Like most living organisms, woodworm need a number of essential factors to live. These are food, oxygen, water and a suitable host location. Timber in buildings provides that host location and there is plenty of oxygen about, but sustenance and water are more limited in a healthy house.

The larvae’s food source is the starch in the timber (the growing tree’s own food store). This starch is only present in the outer part of the tree, known as the sapwood.The structural timber  frames historically found in buildings were primarily taken from the central core, known as the heartwood, which contains no starch – so the larvae cannot feed on it.

So most structural timber is immune from woodworm.

That said, the process of turning a round log into a square beam often leaves sapwood at the corners. Hence it is common to see the edges eaten off timbers in an old house – but this has no effect on its strength.

Timber that is severely compromised by woodworm attack is usually modern wood (which is often poorer quality, containing a higher proportion of sapwood) that has been incorporated into the building at a later date. This might include floorboards or rafters if it’s been re-roofed.

Very old houses will inevitably have experienced periods of damp, so there will always be signs of past infestation. In a historic timber frame building you will invariably find that all of the sapwood has been eaten away at some time.

Is it active?

Remedial treatment is very often specified in response to evidence of long extinct woodworm. It’s vital to recognise whether an infestation is active or historic.

There are clear signs that will tell you whether or not it’s a live case.

Bore dust under a major infestation in modern rafters
The common adult furniture beetle can reach up to 7mm in size - and will chew its way out of the timber leaving the characteristic flight hole.

As the larvae eat through the wood, they only consume the starch and excrete the rest as fine dust. In an active infestation, the bore dust is visible. The dust will trickle from the holes and be the colour of freshly cut timber. The more there is, the more extensive the infestation.

Fresh flight holes have sharp edges and light-hued interiors. As they age the edges disappear and the interior becomes darker.

The adult beetles emerge between May and August. During this period dead beetles can be found on floors and windowsills.

You can confirm a suspected infestation during this period by pasting tissue paper to the surface of the timber. Active beetles will punch holes in the sheets.

Treatment options

First, what not to do.

Chemical spray treatments are widely specified to deal with woodworm, but in a high proportion of cases the colony is long dead. Even if there is an active infestation, the effectiveness of sprays is very doubtful – the chemical cannot soak into the timber to any significant depth, so it’s effectively a surface treatment only.

In tests on historic timbers we have found that, even after prolonged soaking, penetration is less than 1mm. Because the larvae live deep inside the wood and the adults lay their eggs in the galleries before emerging, they cannot be reached until the adults surface.

Hence spraying will have no impact on the infestation – most of the emergent beetles die anyway, without the need for any poisons.

The chemicals used for these sprays are inevitably a form of insecticide. Recent tightening of restrictions on chemical use has further reduced their potential effectiveness. Moreover, the fact remains that this kind of treatment involves spraying poisonous substances around inside your home.

I’d suggest this is hardly a sensible way to treat the space in which you and your family live.

The key to control of woodworm lies in understanding what they need to thrive, specifically their need for water. If there is an active infestation then moisture levels in the building must be abnormally high – in other words, it is damp.

Resolving the causes of this issue will eliminate the woodworm. It really is that straightforward. If the building is dry the colony will die out.

Damp will inevitably lead to infestation and other forms of timber decay.

Dealing with damage

Because woodworm cannot attack heartwood it will rarely cause any structural damage.

Only modern timber, incorporating a high proportion of sapwood, is potentially vulnerable – and if it’s been compromised, limited repair or reinforcement is usually all that’s necessary. There is no need to replace timber that has been attacked but not structurally affected.

There is no risk of damage somehow spreading to the unaffected wood. Remember, dry timber cannot be attacked; damp timber will be attacked unless it is dried out.

Most issues will be cosmetic and relate to floorboards, skirting, joinery and similar elements. The extent to which these items require replacement is essentially a personal choice depending on visibility, degree of impact etc.

Historic woodworm damage to old oak timbers and the like is generally best regarded as part of the patina of age and embraced as a feature of heritage buildings.

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