History of Dry Rot
Dry rot, a timber decay fungus whose environmental
requirements are mostly not significantly different to
those of any other decay fungus, has gained an
extraordinary notoriety and mythology. Now, as the 20th
Century slides into the history books, it is a good time
to reappraise dry rot and its treatment, so that we
don't carry one century's disastrous habits into the
next. If we are to do this properly then it is vital
that we understand how our current attitude to the
fungus has developed.
In 17th century Britain most structural timbers were
fashioned from oak and a few other hardwoods. If these
timbers stay wet enough for long enough then they rot.
The surfaces generally became fibrous or lint-like and
the damage slowly spread through the timber. This decay
was believed to be due to wind and rain acting on the
surfaces and people called it 'wet rot' or 'common rot'.
Towards the end of the 18th Century, construction
practices changed for a wide variety of reasons, and
softwood timbers (from coniferous trees) came into
prominence. They were used as joists and roof timbers in
the now fashionable brick buildings, and the navy used
them in enormous quantities in their ships.
With the prominence of softwoods in an enclosed
environment came an increase in one particular type of
decay which proved to be an enigma. The damage seemed to
start from the centre of the timber, and to leave a
sound outer skin, while the wood itself broke into
little cubes. This was 'obviously' not caused by an
adverse environment acting on the surface. Those
concerned eventually concluded that it was caused by the
fermentation of 'natural juices' which became
destructive when the tree was felled, and, because the
damage did not appear to be caused by water, they called
it dry rot. Even then a few persons claimed that fungi
caused both wet rots and dry rots, but their opinions
were ridiculed or disregarded.
It is easy, now that timber decay is a firmly
established science, to understand what had happened.
Their wet or common rot is our 'white rot'. These rots
are caused by fungi which may attack all the structural
components of the wood, but tend to preferentially
destroy lignin, a substance which contributes to the
woody strength of the cell walls. They are generally
more common in hard woods than in softwoods. The 18th
century concept of dry rot describes any of the 'brown
rot' fungi. These fungi, which are common in softwoods,
are unable to utilise lignin so that it remains as a
fragile brown matrix that cracks into cubes as it dries.
Brown rots, as a group, tend to prefer drier conditions
than white rots, and damp building timbers make a good
habitat.
Brown rot fungi must always have occurred in the British
Isles, but they did achieve a massive notoriety during
the late 18th Century when naval ships sank before they
could be commissioned. The fungi had not changed, but
the use of softwoods, and the environment within which
it was placed, had. Several books were published on the
subject, and a gold medal was offered by the Royal
Society of Arts to anyone who could demonstrate a cure.
However, the cure remained elusive because the problem
was not understood, despite the appearance of a book,
which correctly identified the causes, as early as 1797.
During the 19th Century, decayed timber was generally
replaced, if it was noticed at all, and the problem was
eventually accepted as a consequence of poor building or
poor maintenance which could be avoided with care. The
term 'dry rot' still encompassed any of a wide array of
brown rots which appeared in buildings, although towards
the end of the century it tended to be used more for
those species which produced extensive visible growth.
The 20th Century brought with it a new set of problems.
The supply of wild-grown pine trees dwindled and was
replaced with plantation-grown material possessing a
greatly reduced durability. The First World War used up
vast timber resources, and the wood imported from Europe
during the inter-war years was generally poor and
frequently of grossly inferior quality. This coincided
with a rapid expansion of mediocre building using poor
quality materials. The Second World War capped these
difficulties by a nation-wide and unavoidable neglect of
maintenance, augmented by a range of actions which
included blocking ventilation to exclude gas attack, and
piling damp sandbags against walls to reduce the
consequences of explosions. Under damp, humid conditions
and a generous supply of suitable timber the fungi
proliferated, and when normality returned, timber decay
was rife.
Our story of the identification of dry rot commences
again in the inter-war years. W P K Findlay of the
Forest Products Laboratory observed that one
strand-forming brown rot fungus (Serpula lacrymans) grew
remarkably well under the prevailing conditions and this
he considered to be the true dry rot fungus. All others
were relegated to the wet rots, thus obscuring the
origins of the terms. From this point in our story the
term 'dry rot' refers to Serpula lacrymans alone.
It is important to emphasise that the name 'dry' rot is
historical and does not indicate a reduced requirement
for water except that brown rots, as a wide group, tend
to require less water than white rots. Cellar rot for
example (also a brown rot but now included by Findlay
within the wet rots) has very similar moisture
requirements to dry rot. Dry rot like any other decay
fungus needs large quantities of water for a prolonged
period of time before it can destroy timber, and is,
itself, destroyed if the source of water is removed and
the structure dries. If dry rot is dry, then it is dead,
but it doesn't disappear, and often it will still be
enthusiastically treated with fungicides even though
treatment is entirely unnecessary.
Dry rot is ideally suited to timber in buildings because
of a dietary requirement for calcium. Mortar and plaster
are rich in calcium, and the fungus will readily attack
adjacent damp timbers. The fungus produces strands
(known as 'mycelium') as it develops, which spread
through walls and over inert surfaces, but this
characteristic is not unique to dry rot. Many other
fungi produce strands and these may also be found within
walls. This is important, because any structure that is
perceived to be unique in nature tends to attract
adverse speculation. The function of the strands is to
conduct the products of timber decay around the fungus
so that it can grow and spread. Contrary to popular
belief, they do not wet-up dry timber to make it
suitable for attack.
The presence of fungus strands deep within a wall
suggested that some form of whole-wall treatment is
required if the fungus was to be killed. During the
inter-war years the surface application of heat with a
blow-lamp became popular, and when this was shown to
have no effect the blow-lamp was up-graded to the
oxy-acetylene torch. Unfortunately the surface of a
moderately thick brick wall has to be brought to near
vitrification before a temperature lethal to the fungus
is achieved at its core. The treatment was found to be
more dangerous than the fungus. Then in the early 1950s
J Bayliss-Butler, professor of Botany in Dublin,
perceived the advantages of the newly invented masonry
bit and 'wall irrigation' with fungicides was born.
The 1939–45 war had not only caused a massive spread of
the fungus, it had also allowed a substantial growth of
the chemical industries. Pesticides were cheap,
plentiful, and considered to be safe. Fungicides could
be pumped into holes drilled into the walls in order to
kill the fungus, and a remedial industry began to
develop in order to provide this and other decay
treatments as a service. The 'full dry rot treatment'
had come of age, including the removal of all infected
timber within one metre in each direction of any visible
growth, as well as the irrigation of walls. The result
was massive destruction, which far surpassed anything of
which the fungus was capable. In the 1960s J G Savoury
of the Forest Products Laboratory suggested that the
treatment seemed to work best where it was not required.
He showed that walls were rarely homogeneous, and that
saturation with the fungicide was rarely achieved. The
industry took no notice, and wall irrigation is still an
integral part of most dry rot treatments.
The purpose of this historical sketch has been to show
that dry rot may differ from some other decay fungi in
its secondary dietary requirements, but it does not do
so in its requirements for water. Confusion resulted
because the origins of the term 'dry' became obscured
and misinterpreted. Once this is accepted then it is
clearly of primary importance to remove the source of
water and to dry the building. This is not achieved by
pumping the walls full of water-based chemicals, making
them far wetter than they were to start with.
The fungus may continue to grow a little after water
penetration is halted because the walls will take a time
to dry, but it will not rampage into dry areas of the
building. If there is an obvious previous repair and the
walls are now dry then the fungus will probably be dead.
Exposure work may need to be extensive if the damage is
extensive, but more frequently the fungus is contained
around a readily identifiable fault and only the minimum
of treatment (if any) is required.
Where dry rot is found, the best advice we can offer is
to seek at least three specifications and quotations
from remedial companies, and if these all seem excessive
or unconvincingly justified then seek independent
advice.
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