Building pathology: timber decay and infestation

Satisfaction guaranteed?

12 May 2011

Treating timber decay and infestation does not necessarily mean pumping biocides into a building, says Brian Ridout, the solution just needs a little considered thought and understanding

Most wood decays eventually if it is damp and the rapidity of this depends upon its durability, which may be inherent or artificially provided by impregnation with chemicals.

Most people would accept these statements but they push the theory of timber conservation about as far as many professionals wish to take it. A good spray with something labelled ‘preservative’ is deemed a wise precaution, particularly if it is accompanied by something called a ‘guarantee’. Unfortunately, the guarantee is probably a little hazy; it might mean that timber, which hasn’t been infested with ‘woodworm’ in 200 years, will not become infested in the next few decades; or that wood will not be decayed by fungi if kept dry, thus taking us back to where we started.

The problem with this cheerful acceptance of perceived wisdom and trade hype has been that vast amounts of biocides have been flung unnecessarily into buildings. Teams of ‘specialists’ have caused more damage to fine interiors in the pursuit of death watch beetles or dry rot than the organisms they sought to destroy.

Understanding what you see

There are questions that should be asked whenever damaged timber is found and remedial action contemplated.

  • Why is the problem there?
  • How much damage has the decay or infesting organism caused and why is that damage limited?
  • Is the decay or infestation current or historical?
  • Will a targeted biocide treatment serve any purpose?

Figure 1: Dry rot treatment where the plaster has been stripped and the brickwork has been extensively drilled and irrigated - was this the correct solution?

It is important to avoid a stereotyped response to remedial timber treatments. It is also generally assumed, for example, that dry rot will always require extensive treatment, while ‘wet rot’ (decay caused by any other fungus) is easier to control. Dry rot, however, may be dead or unable to cause much damage, while a ‘wet rot’ may be causing extensive decay. Calling the fungus dry rot may inevitably lead to a response that is out of all proportion to the problem.

All of the fungi and insects that damage the timbers in European buildings belong in the woodlands outside. None of these organisms have adapted to a life in buildings, and a poorly maintained roof is just a heap of dead wood to a fungus or furniture beetle.

Despite popular mythology, dry rot’s moisture requirements do not differ from those of other brown rotting fungi. It may grow feebly at a wood moisture content as low as 22% but it will not cause much damage below about 28% (for context, wood in a dry but unheated building would have a moisture content around 15%). Many people now realise that the key to the decay process is water; however, most forget that the water has to remain if the fungus is to stay alive. Dry rot does not produce sufficient moisture from the breakdown of wood to survive in a dry building, and the primary control method must be to remove the sources of water.

Similar limitations restrict the activities of furniture beetles. These are not likely to infest, and their population is likely to decline, if the wood moisture content stays permanently below 15%. The beetle larvae will only attack the sapwood of our commonly used construction timbers (pine and oak), and a few faces of timber full of beetle holes in an otherwise intact old roof indicate a little sapwood and not roof timbers in danger of being comprehensively infested. Sapwood damage in old timbers is generally historic and does not require treatment.

The effective treatment of death watch beetle infestation is frequently impossible because the insects are living in inaccessible timbers. A few beetles do not, however, mean that all timbers are at risk. Localised infestation will be present because the chemistry of the heartwood has been modified by fungus. The focus of an infestation may be removed when timbers are repaired and the insect population will decline when the structure dries. Far more buildings have death watch beetle holes in their timbers than support an active infestation and any treatments must be based on a correct assessment of the problem.

If the conservation of historic materials is to be maximised then biocides must be just one tool in the workbox. They must not be allowed to replace thought and knowledge. The amount of knowledge required to make an informed judgement is actually not that great.

Brian Ridout is a Senior Architectural Conservator with English Heritage

Further information