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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.
Instead of relying on chemical treatment of Dry Rot you should chemical
free treatments as advocated in ‘TAN 24- Environmental Control of Dry
Rot’ written by John Palfreyman & Gordon Low, published by Historic
Scotland.
The cost of a timber survey can be found here here and can be arranged by phoning 0800 028 1903 or click the enquiry button to the left of the screen.