The remedial treatment of timber decay with chemical preservatives is mostly a twentieth century invasion. In previous times rotten wood was replaced, and infestations by wood boring beetles were ignored until serious. The major reasons why the situation changes were a substantial loss in the durability of softwood building timbers, and two world wars.
The remedial industry as we recognize it today, that is a man, a van, and a spray lance, came into prominence in the 1950s as the country sought to repair the destruction wrought to its buildings by wartime damage and neglect. As the industry expanded the guarantee, a marketing ploy was invented. These documents were carefully worded so that they were of little actual value, but they became an end to themselves, and nobody cared what timbers had been treated against provided that a treatment guarantee had been issued. The general perception was that if a timber decay organism ate wood, and a roof was made of wood, then the decay organism could destroy the roof. This was invariably far from the truth, but nobody noticed, and chemicals are still sprayed on to timber that could never be attacked.
The roof will not be destroyed because wood is not the uniform material it appears to be. The woody stem has two main functions. Water conduction from the roots to the leaves, the first function, takes place in the outer, or sapwood zone. This active function requires living cells, which in turn require nutrients. When the tree is felled the cells die, but the nutrients remain, and the sapwood in buildings is always susceptible to decay organisms.
As the trunk of the growing tree expands the inner sapwood cells die, chemicals are deposited in the dying cells to protect the tree against decay, and the heartwood, thus formed provides the second function, a strong core for the trunk.
The heartwood of our traditional building timbers, European Redwood (Scotch Pine) and oak have a good resistance to decay. The damage that can be caused by beetles in a roof will therefore depend on the amount of vulnerable sapwood in the construction timbers. Sapwood has increased in modern softwoods because the plantation grown trees are felled when they are still young, but have reached a commercially acceptable trunk diameter. This provides logs that are considerably narrower than the diameter of eighteenth or nineteenth century wild grown logs. The volume of sapwood in a tree trunk tends to stay constant throughout its length. This means that the thickness of a sapwood band will depend on the width of the trunk, and the thinner the log the thicker the sapwood.
The practical implication from this is that the spray treatment of a Victorian roof for example, as protection against furniture beetle (woodworm) is likely to be a completely unjustifiable use of chemicals. A few beetle holes in the sapwood edge of old rafters usually mean that an infestation is extinct, and that most of the timber is immune from attack. It will never mean that the beetles could destroy the roof, and it certainly does not justify spraying every timber in the house with insecticide.
If precautionary treatments against furniture beetles are unnecessary in old buildings, then will they protect against fungus? The answer is no. Some fungi can attack the heartwood the heartwood of durable timbers, but only if here is a considerable amount of water present for a prolonged period. Decay will only occur where there is a neglected fault, and damage is likely to commence in surfaces that were not treated by the spray lance. Once the fungus is growing then it will be within the timber, and totally unaffected by spray treatments which will not penetrate more than a few millimetres into the surface. These remarks also apply to dry rot.
There is probably more rubbish talked about dry rot than any other decay organism, and this misinformation is frequently accompanied by unnecessary treatment. The fungus is a pest of softwood, and usually does not cause much damage to oak, although it may grow over it. The term 'dry' is a confusing legacy from the eighteenth century, and it has nothing to do with moisture requirements. Dry rot requires plenty of water, and will not flourish at timber moisture content below about 25%. Timber in a dry building will usually have a moisture content below about 16%. The fungus does not transport water to wet up dry timber, and it will die if all sources of water are removed. Whilst vigorously growing dry rot in a wet environment can be immensely destructive there are many situations where it does not cause much damage, and can be killed by drying alone.
More information on timber decay and treatment can be obtained from
'Timber Decay in Buildings. The conservation approach to treatment' by Brian Ridout (ISBN 0-419-18820-7). Awarded the Best Technical Publication of 1997/2000 by Association of Preservation Technology.
'Timber Decay in Buildings is the first book to tackle all the issues relating to timber decay. It presents the facts and explores timber decay problems through case studies. These are illustrated with clear self-explanatory photographs for the reader to use as a diagnostic aid and discusses various subjects such as timber as a living material, decay organisms, the effect of moisture content on timber as well as an integrated knowledge on decay organisms with holistic preservation methods and the appropriate use of targeted chemical treatments.. The methods outlined here are intended to reduce unnecessary damage frequently caused to buildings by uninformed timber treatments, and form the basis of the timber conservation methods advocated by English Heritage, Historic Scotland and The Society for Protection of Ancient Buildings (SPAB).'.
The cost of a timber survey can be found here and can be arranged by phoning 0800 028 1903 or click the enquiry button to the left of the screen.