DRyrot 130B


 Report the presence of wood rot is the #1 issue we find during our inspections. Most homes that are 10 years or older will have some form of “rot”. Its just a simple fact in the Pacific NW that anywhere that water can sit, wood rot will occur.

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.

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%.

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.

dry_ro1Borates 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.


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 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 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_ro2A 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.

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.


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.


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.

Dealing With Decay

The first and most important thing to do once decay is discovered is to figure out where the water is coming from. Check for the obvious – roof and plumbing leaks, and missing or punctured flashing. Look for stains and drip tracks caused by ice dams. Are eaves wide enough to prevent water from cascading down sidewall’s? Are gutters poorly maintained or missing? Do finish grades slope towards or away from the foundation? Are foundation cracks admitting water? Is untreated wood in direct contact with concrete, masonry, or soil? Check to see if crawl spaces have soil covers, and if venting and/or insulation is present, adequate, and properly placed. The same goes for attics. Peeling and blistering paint often signal inadequate interior ventilation, or a missing vapor retarder. Water stains on framing and sheathing inside walls suggest condensation. Remember that to make the remedy permanent, you’ve got to cure the disease -water infiltration, not just treat the symptoms- mildew, mold, and decay.

Once the source of water has been eliminated, remove as much decayed wood as is practical and economical.
This is especially important with girders, columns, and other critical members whose load-carrying ability may have been compromised. There’s no known way of accurately determining the remaining strength of decayed wood left in place. Cut back rotted members to sound wood, keeping in mind that difficult-to-detect incipient decay can extend well beyond visibly rotted areas. When a partially decayed structural member can’t be replaced, reinforce it with a “sister” anchored to sound wood. Decayed wood absorbs and holds water more readily than sound wood, so let rotted areas of members not removed dry out before making repairs and closing in. Otherwise, you’re just adding fuel to the slow fire.

In damp crawl spaces or other places where water is likely to appear, replace decayed members with preservative treated wood. The major model building code agencies -BOCAI, ICBO, and SBCCI- require that treated wood be used for sills and sleepers on concrete or masonry in ground contact, for joists within 18 in. of the ground, for girders within 12 in. of the ground, and for columns embedded in the ground supporting permanent structures.

In-place treatment with borates
Dormant fungi can be reactivated when dry, infected wood is re wetted. Consider treating infected, but otherwise serviceable wood left in place with a water-borne borax-based preservative that will not only kill active fungi, but guard against future infection as well. An effective homemade version of Bora-Care is “HERE”

Borates have low toxicity to humans and are even approved for interior use in food processing plants. They don’t affect wood’s strength, color, or finish ability, don’t corrode fasteners, and don’t outgas vapors. Widely used in treating new timbers for log homes, they’re the preservative of choice for remedial treatment of wood in service. Because of the decay hazard posed whenever wood bears on concrete or masonry, solid borate rods are often inserted into holes bored near contact areas. Should wood ever get wet, the rods dissolve and ward off infection. Infected wood can be treated with
Boric Acid an extremely effective cure for a multitude of problems including control of wood rot in homes and boats and it is natures insecticide for control of fleas,roaches,termites,ants,spiders and many other household pests. Before any repairs or replacement of damaged wood is started, I recommend a through treatment of damaged areas with Boric Acid to eliminate future problems and stop the spreading of the fungi.

Epoxy repair of decayed wood
Sometimes replacing rotted wood isn’t an option. In conserving historic buildings, for example, the goal is to preserve as much of the original “architectural fabric” as possible. Stabilizing deteriorated wood with epoxy is often the only choice. Epoxies consist of resin and hardener that are mixed just before use. Liquids for injection and spatula-applied pastes are available. After curing, epoxy-stabilized wood can be shaped with regular woodworking tools and painted. Epoxies are useful for consolidating rotted wood, restoring lost portions of molding’s and carvings, and for strengthening weakened structural members. In the last case, they’re used to bond concealed metal reinforcement inside holes or channels cut into hidden faces. Epoxies aren’t preservatives and won’t stop existing decay or prevent future infection. They can be tricky to use; follow the manufacturer’s mixing, application, and safety instructions to the letter.