Dealing with Woodworm in Period Properties

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.

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.