TIMBER DECAY & ITS TREATMENT by
Brian Ridout
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 was 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 1950's 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. 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 and Historic Scotland.'.
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