DRY ROT AND ITS CONTROL
The
true dry rot fungus,
Serpula lacrymans, is often regarded as the "cancer" of a building. Many
myths have built up concerning what this fungal decay is capable of doing,
occasionally leading to the belief that the fungus is indestructible and
that the whole of the building will have to be torn down.
However,
dry rot is vulnerable to certain environmental effects and like all wood
destroying fungi it has essential needs, and it is those needs that limit
the extent of spread and damage that this organism can inflict. Unfortunately
dry rot is a very secretive organism, favoring dark, damp stagnant conditions
to develop. This is frequently why it is able to spread extensively before
the damage is first noticed. Often times, once it is noticed, extensive
damage has occurred.
One
might expect
large volumes of nasty chemicals to be used and that they will have to put
up with the risk of any toxic effects and unpleasant odors and fumes, which
may be a part of the treatment. That is not the case here. We will describe
the borate wood preservatives and how they can be best utilized in an existing
structure to stop rot.
Borates
are a relatively new series of fungicides based on boron, a naturally
occurring mineral. These products are virtually odorless and have mammalian
toxicity (humans and pets) about the same as common household salt! Also,
these formulations are environmentally friendly and have a very significant
advantage over the traditional dry rot preservatives. They are water diffusible
and therefore use the moisture that is present in rotting wood as a carrier.
The water becomes the Trojan Horse that carries the poison pill into those
areas that are particularly susceptible to dry rot and other decay organisms.
Traditional preservative will not diffuse into wet timber therefore leaving
such wood at great risk of decaying. Epoxy systems alone will do nothing
to kill the fungus and the cancer will continue to spread. Epoxy systems
are good for restoration, but not for preservation.
The
correct use of Borates, namely IMPEL Rods, Bora-Care and Tim-Bor,
coupled with good building practice, will ensure that a building will be
at very little risk from further dry rot activity. This can be done without
putting the occupants or the environment at risk from the problems which
can arise from the use of traditional preservatives.
The wood destroying fungus, Serpula
lacrymans, is commonly known as dry rot. However, the name dry rot is a
misnomer since all wood destroying fungi requires water for germination,
growth and survival. It also needs a food source (the wood itself), a temperature
range between about 40 and 90 degrees and oxygen. One might say that if
you cut off the moisture source, you cure the problem. But can you be sure
it will never come back. Once established, the dry rot fungus can survive
in wood as dry as 20%. That is why wood is kiln dried to 19%.
WOOD AS A FOOD SOURCE:
FORMATION OF WOOD
Wood is a natural
material being the end product of a complex chemical process called photosynthesis,
which occurs in all green plants. Wood basically consists of boxes and tubes
made of sugars which are linked together to form cellulose, the basic building
material of plants. Chains of cellulose are laid down in different orientations
and bonded by another material, lignin, which adds rigidity and strength.
It is the arrangement of cellulose, which give wood its characteristic properties
and its ‘cellular’ structure.
The
wood forming the outer part of the tree
is known as the sapwood and transports water, nutrients and stores food.
This is the most vulnerable part of wood to fungal decay and attack by wood
destroying insects. The inner wood in the heartwood and forms the older
wood in the center of the tree; it does not conduct sap or store food but
it does contain some excretory products and is therefore more resistant
to decay than the sapwood. It is also more resistant to the movement of
water and preservatives in general. The heartwood of different timbers varies
in its resistance to fungal decay and it is this heartwood resistance to
decay by which timbers can be classified.
WOOD DECAY
Wood
decay is basically
the reverse of wood formation. Dry rot fungus releases enzymes that dissolve
the cellulose and hemicellulose of the wood to break it back down into its
sugar components. The sugars are respired with air to produce carbon dioxide.
Water is not metabolized and this causes the darkening in color of the wood.
A number of wood destroying fungi other
than dry rot also decay the wood in the same manner, leaving the lignin
untouched. The characteristic darkening of the wood by these fungi give
them the loose title of ‘brown rots’; dry rot is one of the brown rots.
When the wood is broken down and utilized
for food, shrinkage, loss of weight, loss of strength and cracking occur.
It is the shrinkage which causes the typical ‘cuboidal’ cracking (cracks
to form small cubes) of dry rot and the other brown rots. Indeed, it is
this shrinkage and cracking which is often the first signs of a problem.
These cracks are typically across the grain.
THE INITIATION OF FUNGAL DECAY
The
essential requirements
for any fungal decay to take place are both food and water, especially the
latter, at a sufficient level. Fungal decay is generally initiated in several
stages.
First the water penetrates the wood
and this allows bacteria and micro fungi to colonize. Typically these are
the mold and mildew organisms. These break down part of the cell structure
but do not cause weakening of the wood. Instead, the wood becomes more porous
which allows it to become even wetter. Provided that the wood is now sufficiently
wet and remains wet and that other conditions are suitable, the wood rotting
fungi such as dry rot can colonize.
DRY ROT-SERPULA LACRYMANS
COLONIZATION
A minute spore of dry rot lands on wet
wood and germinates. The first growth that emerges from the spore is known
as the germ tube. This grows and divides to produce fine filaments, hyphae,
which invade the timber and secrete enzymes to break down the wood.
As the enzymes break down the wood it becomes even more porous so allowing
further water to penetrate into the wood. Furthermore, the byproduct of
the decay process is water, which can also contribute to the moisture within
the wood. In other words, once you have rot, it begins to generate its own
water as a byproduct of metabolism and the process is like an unstoppable
chain reaction.
VEGETATIVE GROWTH
The fine filaments of fungal growth,
the hyphae, develop into a larger mass, the mycelium, which grows into
and across the damp wood. Under humid conditions the mycelium is white
and cotton-like. In a very humid and stagnant environment droplets of
water will form on the mycelium like teardrops. The fungus removing excess
water from the wood probably causes these droplets.
Under less humid conditions the mycelium
forms a silky gray colored skin which is often tinged with yellow and
lilac patches. This form of the mycelium can be peeled rather like the
skin on the cap of a mushroom.
Strands: Within the mycelium special
thick walled hyphae develop-these are known as strands. They are resistant
to desiccation and assume their real importance when the fungus spreads
over and into ‘inert’ materials such as mortar and brick.
In these situations they conduct water
and nutrients to the growing hyphal tips so allowing the fungus to continue
to spread over non-nutrient substrates. It is this ability to travel away
from the food source, over and through inert materials allowing the fungus
to reach more wood. This is what makes dry rot so potentially destructive.
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FRUITING BODY (SPOROPHORE)
When growth is usually advanced, a fruiting body-sporophore may develop.
This can occur as the result of two different mycelia meeting. The onset
of stress conditions such as drying out the wood or environment as well.
Light is also thought to be the cause of fruiting body formation in some
situations.
The fruiting body takes the form of
a fleshy pancake or a bracket, the surface of which is covered with wide
pores of corrugations. The surface is orange/ochre colored. The corrugations
form the spore bearing surface. The spores themselves are very small,
ovoid in shape and orange in color. They develop on a structure known
as the basidium, four spores to a basidium. When fully developed a small
droplet of fluid forms at the junction between the spore and the fine
stalk on which it developed. The pressure exerted by the droplet of fluid
trying to form a true sphere is sufficient to eject the spore an inch
or more away from the fruiting body, dispersing into the air.
Large numbers of spores frequently
collect around the fruiting body under still conditions and form the red
‘dust’ often visible where there is a significant attack of dry rot.
GROWTH AND SURVIVAL
It is essential to understand that water is absolutely fundamental
to the growth and survival of not only dry rot, but all wood destroying
fungi; wood decay cannot occur, exist or survive without it!!
Spore germination: To initiate growth
from a spore the wood must be physically wet. In other words it must be
subject to a source of water ingress, e.g. leaking gutters, wet masonry,
condensation on cool wall voids etc. In practical terms the wood must
have a moisture content in excess of 28-30%. Spores will not germinate
on dry surfaces or surfaces which are not suitably wet. In other words,
unless the wood is wet, dry rot cannot become initiated.
Growth: While wood needs to be wet
for growth to be initiated, a moisture content of around 20% existing
mycelial growth ceases and the fungus will eventually die. Decay in wood
below 20% moisture content is likely to be very minimal. However, for
practical purposes when dealing with fungal decay as a whole, moisture
contents of 20-22% should be taken as the threshold figure and assume
moisture contents in excess of this level will put the timber at risk.
The fungus flourishes under humid,
stagnant conditions; hence growth tends to be secretive and hidden and
is therefore often extensive before it becomes evident. Unlike other wood
destroying fungi, dry rot can grow significantly on and through damp masonry.
Under special conditions, very limited growth might occur over and through
dry materials. Distances in excess of 20 feet away from its food source
have been recorded. This ability to grow over and through inert material
can lead to significant problems of spread. Like all wood destroying fungi,
dry rot flourishes in the slightly acidic conditions found in wood. Unlike
the others it also flourishes under slightly alkaline conditions, which
explains the frequently encountered rapid growth behind and through old
mortars and concrete. Growth rates of up to 12 feet per year have been
recorded. In other cases the organism may only have spread a few inches
in the same period of time. Because there are large variations in growth
rates, the age of an outbreak cannot be positively determined. The problem
is further complicated since it is not always possible to tell if an outbreak
is the result of a single outbreak or the coalescing of numerous outbreaks.
Without a source of food (wood) growth
will quickly cease and the fungus eventually die. But research has shown
that in the laboratory the food reserves in the mycelium may allow up
to 20% growth before spread ceases. This might have important implications
in control measures since it could theoretically allow the infection to
pass to immediately adjacent non-infected wood even though the original
food source had been removed but leaving the mycelium on, say, damp brickwork.
For these reasons, the Borates are by far the best means to control the
rotting fungus since they will poison the wood as a food source, regardless
of the moisture content, temperature or oxygen supply.
Survival: The spores are reported
to remain viable for up to 3 years. They could therefore lay dormant until
such times when conditions become suitable for their germination, that
is, when any exposed wood surface on which they have landed becomes wet.
The mycelium can remain viable in damp masonry at around 50 degrees without
a food source for up to 10-12 months. But under the damp, humid conditions
such as found in a crawl space with temperatures of above 40 degrees,
the mycelium may remain viable for up to 9-10 years! If untreated wood
is put in contact with damp infected masonry there is always the potential
for the new wood to become infected.
CONCLUSIONS
In considering the requirements for
the growth and survival of dry rot and methods and practices for it’s
control, the emphasis is on attacking the essential requirements for growth
and survival.
Where chemical control is used as
a support measure or to reduce the risk of decay to damp wood it is essential
that the whole area at risk be treated, i.e. deep within the timber. This
is unlikely to be achieved with ‘conventional’ preservatives. Indeed,
Holland and Orsler (1992) reported in a paper to The International Research
Group on Wood Preservation (Preservatives and methods of treatment) that
" treatment against wood destroying fungi by this means (conventional
preservative pastes) may be insufficiently effective for more severe risk
situations in the longer term."
Unlike conventional preservative pastes
the boron based materials are designed to work under high risk situations.
By drilling holes into the affected wood members and injecting Bora-Care
prior to inserting IMPEL Rods, you have the added advantage in that the
borates will distribute more rapidly. Then, spray Bora-Care on the outside
of the wood to ensure greater potential protection and lowering more rapidly
the risk of rot. Furthermore, the IMPEL Rods also leave a ‘reservoir’
of solid borate for long term diffusion and effectiveness.
The prevention of dry rot is rarely
addressed in this country unless the wood is in ground contact or exposed
directly to rain etc. There are buildings less than 5 years old that are
literally falling apart because water is getting trapped into the stud
cavities and rotting. There are a couple of theories as to why this is
happening, but regardless of the cause, rotten wood is being replaced
with more untreated wood. The same thing will happen again unless measures
are taken to treat the replacement wood with borates.
Pretreatment of entire houses can
be done with Tim-Bor Industrial. It is a powder you mix with water and
spay apply to all wood including studs, crawlspaces, subfloor, rafters
and exterior sheathing.
Remedial Treatment, or treating an
existing problem is best done with Bora-Care. It is a liquid concentrate
that is mixed with water and sprayed on the affected wood and all of the
wood surrounding that area. A three-foot minimum perimeter treatment is
recommended.
IMPEL Rods can be used whenever the
wood is larger than 2" in thickness or when the area has a moisture content
in excess of 30% below the surface or in areas subject to repeated wetting.
They are inserted into drilled holes where they dissolve and diffuse on
the inside of the wood. They are often used in combination with Tim-bor
and Bora-Care.
We will be delighted to
serve you. Thanks to my friends at Safeguard Chemicals in the UK for the
basis of this paper. We are fighting the same battles. Jim Renfroe
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