Timber Preservation in Historical Buildings


Listed and period buildings will likely be constructed from a combination of both hard and soft woods, or in some instances, completely of hardwoods. The timbers used in period properties were often reclaimed timber which had been used in previous construction, timber taken from redundant horse drawn vehicles or commonly timbers reclaimed from ship building, particularly near coastal areas. Inevitably, these timbers will be of unusual shapes and may have completely unrelated joints, shakes and crevices. It is also inevitable that these timbers will have seen infected by some form of wood boring insect during its lifetime.


Fungal Attack to Timber: Wet & Dry Rots

fungal attack

An expert is required for correct initial identification.  Outbreaks of wet rot are usually quite localised and may only require the removal of the moisture source to result in the removal of the fungus, followed by timber repairs if necessary.

Dry rot (Serpula lacrymans) however should be treated with extreme caution and specialists will be required to eradicate the outbreak, for which you should always obtain a long term underwritten guarantee from a PCA registered company. The source of the initial problem must be identified and rectified.Dry rot can spread rapidly causing major structural defects.

Wet rot fungus is commonly mistaken for Dry rot, which is unique by the large scale cubed cracking and timber crumbling to dust when rubbed between the fingers, rather than becoming fibrous (a symptom of Wet rot). A significant proportion of both Wet and Dry rot found in listed buildings is due to condensation. If a building is left empty over the winter months, localised condensation can produce the ideal environment for the germination of rot to timber.


Woodworm Attack to Timber

The insect ‘flight holes’ present in structural timbers are commonly mistaken for active infestation by Deathwatch Beetle (Xestobium rufovillosum) or Common Furniture Beetle (Anobium punctatum), when in fact, the timber can be solid and the infestation completely inactive. Insect bore dust from legitimate previous infestation can remain in the timber for many years after introduction into the property and may subsequently be dislodged by redesign of the interior of the property, resulting in falling bore dust, which is commonly mistaken for new active infestation.

The evidence of infestation to these old timbers, should, as a rule, be viewed as normal. It would be our opinion that blanket treatment of hardwood timbers within any building should not be carried out as a matter of course. Apart from softwood found in roof spaces, which are readily accessible, the use of modern timber treatment products should be limited to actual areas of proven active infestation. This can usually be limited to softwood, or hardwoods where they abut softwood, or where the hardwood contains an abnormal amount of moisture (thus rot), and in particular, where timber is embedded within masonry without the provision of an adequate damp proof membrane (DPM). Thatched roofs may be treated, but you should ensure that, if possible, non-flammable, water based materials are used.

Some species of wood boring beetle do not actually require any treatment. The Wood Boring Weevil (Euophryum confine) is one such case, and this is the only adult beetle to actually cause damage, whereas in other species it is the larva that causes the problem. The simple removal of the moisture source and drying of the timber will result in removal of the infestation without the need to resort to chemical treatment. Bark borer beetle (Ernobius mollis) is also frequently mistaken for an active infestation requiring chemical treatment, when in fact it will usually be historic and inactive, and if active, only requires the removal of the bark (its food source) to eliminate the infestation.


How do we detect for an active infestation?

Professional diagnosis is essential. ‘Woodworm’ is a generic term covering many different types of wood borer, and correct identification is paramount. This can be done from a captured adult, or by viewing the ‘frass’ (bore dust) through a microscope. The size of flight holes, type of timber, moisture content and the time of year are all important factors, see below.


The treatment of an active infestation

adult deathwatch beetle

When an active infestation by wood boring beetle is found it is prudent to isolate the area of the outbreak and treat accordingly with regular monitoring, possibly on an annual basis, during the active season. This varies slightly according to each species, however as a general rule the season runs from March to early July for Deathwatch, and to August or September for Common Furniture beetle.

In England, the Deathwatch Beetle is one of the major enemies of historic buildings, and its favourite food is oak. It is extremely difficult to eradicate. Generally the infestation will be in timber with some decay or moisture content. As an infestation can only be detected by the presence of insect ‘flight holes’, the beetle will usually be well established before being detected.

Modern insecticidal timber treatment is reliant on breaking the cycle of infestation, from larva to adult beetle, therefore eradication may take a similar period and emergence of adult insects must be anticipated for several years prior to complete eradication, known as the initial “fly-out” period - although they do not actually fly. The cycle of Deathwatch from larva to adult insect can be as little as 5 years, but 10 years is considered normal (however, a 20 year cycle has been known, but much depends on the type of timber and its moisture content). For Common furniture beetle the ‘fly-out’ period is currently around 3 years, but 5 years has been recorded. Deathwatch beetle is well established in the southern part of the UK, but is common throughout the country (and Europe), however it is rare in Scotland and has only been found twice in Ireland, and this was most probably in imported construction timber.

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