When the wood is broken down and utilised 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.
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 colonise. 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 colonise.
A minute spore of dry rot lands on wet wood and germinates. In the laboratory this takes around 7 - 10 days, but may be longer in the wild under less than ideal conditions,
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 wood is broken down by the enzymes secreted by the growing fine filamentous hyphae the wood becomes even more porous so allowing further water to penetrate into the timber. Furthermore, the by-product of the decay process is water which can also contribute to the moisture within the wood.
It is reported that the spores can remain viable (in a 'dry' state) for up to 3 years; however, one report identifies that such conditions in the spore stage can be tolerated for up to 30 years. thus spores may land and be present but not germinating until the timbers become suitably wet (probably greater than 28% moisture content)
It should also be considered that the spores are 'omni-present', ie, they are everywhere. They are extremely small and literally float around in air currents (along with many other organisms, pollen, spores, etc); effectively they are present in all houses - and even found at elevations of 5 miles above the earth's surface.
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.
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, wood in contact with damp masonry, 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 origin of a dry rot attack is most likely to be associated with a severe source of water ingress, the most common being defective rainwater goods. Rising damp does not appear to be common as the originator of an infection although it will certainly support growth where infected wood is in contact with such dampness.
Whilst timber needs to be wet for growth to be initiated, at moisture contents of around 22% existing mycelial growth ceases and the fungus will eventually die; decay just above 22% 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 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 2 metres away from its food source have been recorded, and it is this ability to grow over and through damp inert material that can lead to significant problems of spread.
Like all wood destroying fungi dry rot flourishes in the slightly acidic conditions found in wood. But unlike the others it also flourishes under slightly alkaline conditions which explains the frequently encountered rapid growth behind and through old mortars and renders.
Growth rates of up to 4 metres per annum have been recorded; in other cases the organism may only have spread a few millimeters in the same period of time. However, Building Research Establishment give a figure of about 0.8 meters per year as a general purpose maximum growth rate (BRE Digest 299) and Coggins (1980) gives a general figure of about 1 meter per annum. 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.
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 18-20ºC without a food source for up to 10-12 months. But under the damp, humid conditions such as found in a cellar with temperatures of 7-8ºC, 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.
Control Of Dry Rot
The principles for the control and eradication of dry rot are outlined as follows:
The most vulnerable feature of the fungus is it requirement for water, and it is the control and elimination of this essential requirement that forms the fundamental measure for the control and elimination of dry rot.
- Locate and rectify the source of water causing and maintaining the rot.
- Promote and maintain rapid drying conditions.
The removal of the source of water is the first point of attack. It is therefore absolutely essential to stop further water ingress. This action alone will eventually control and eliminate the activity. Indeed, it is the fundamental measure in eradicating the organism. Included in this action is the promotion and maintaining of rapid and good drying conditions.
- A specialist contractor such as Lifecote will fully understand the factors involved Please note: where dealing with historic properties and where it is deemed necessary to keep as much of the historic timbers as practically possible then it is essential that dry techniques are used. This WILL require careful monitoring of both conditions and the state of the rot; such practices should only be conducted by specialist professionals in the conservation of historic timbers.