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Home > Knowledge Base > Aritcles and Technical Info > Dry Rot and It's Control
Dry Rot and It's 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:

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

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.
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 by-product 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.

FRUITING BODY (SPOROPHORE)


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.
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.

The Cure

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.
Pre-treatment of entire houses can be done with Tim-bor. It is a powder you mix with water and spay apply to all wood including studs, crawl spaces, subfloor, rafters and exterior sheathing. It provides a termite and rot resistant envelope treatment that can last for 30 years or more. 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 solution of Tim-bor can also be used if the wood has a moisture content of 20% or greater.
After the wood has been treated with borates, the cancer will have been cured and it will not return. However, you still have damaged wood. ROTFIX is a two part penetrating liquid epoxy that soaks in and hardens all soft and punky wood. Often times small holes are drilled into and adjacent to the rotten area and filled with the liquid epoxy. This will consolidate the loose and spongy wood to a very hard mass.
To fill in any missing areas us SCULPWOOD which is a two part epoxy wood filler. It has the consistency of play-dough. Each part is kneaded thoroughly, then pushed into the void. SCULPWOOD is sculptable and moldable. It cures in about 4 hours at room temperature and becomes strong and hard. It can be sanded, machined, primed and painted. Cured SCULPWOOD has very similar characteristics to wood. It will hold a nail or screw and can be shaped just like wood. It can also be textured before it cures with a little practice.
One of the biggest needs for this system is wood window sills. Windows are being ripped out and replaced right and left because of a little rot pocket on the sill. While the contractors and window dealers love it, the homeowner is spending much more money that is necessary. A window sill can be repaired for under $500 while a replacement usually cost $1500 and up.
Other areas are beam ends that protrude beyond the roof line. Timbers and beams in crawl spaces. Wood under kitchen and bathrooms usually has some degree of rot. It is not always necessary to use the epoxies. When it will be inside a wall void or covered up, then the wood can be treated with Tim-bor and IMPEL Rods. If there is weakening of the wood, it may be a good idea to sister on another piece to carry the load.