Timber Decay

There are many types of timber decay but in buildings two are significant:

Generally all decay, “wet” or “dry”, is caused by water. We can advise whether the decay is wet or, current and active or old.

The nature and characteristics of fungal decay

A General Description

Fungi are a major group of organisms that are responsible for the breakdown of organic material. They are specialised plants, able to live either on living hosts or on dead organic material. Wood is an organic material. Wood-destroying fungi such as wet rot and dry rot can only live on wood or wood-based products. Both dry rot and wet rot are thus plant-like organisms which derive nutriment from the breakdown of wood, rather than from carbon dioxide and water using sunlight as an energy source, a process known as photosynthesis which is the food production process used by most other plants. It is important to note that other building materials – brick, stone, mortar etc, and cannot serve as a food source.

Fungal decay starts from “spores”. These can be considered the equivalent of plant seeds although they are considerably smaller, being approximately 1/100th of a millimetre long. These small spores can float in the air in very much the same way a jellyfish can be suspended in the sea. In the main, they are disseminated by air currents, but also can be carried by water, animals or man. They occur in such vast numbers that, when conditions arise suitable for fungal germination on timber, the presence of a wood-destroying spore is virtually inevitable.

When conditions are right for germination the spore produces a small tube which develops into a thin thread called a “hypha”. This tube has the capacity to break down the wood by enzymic action. There are two broad categories on enzymes in wood-destroying fungi. Those which destroy the carbohydrate or cellulose part of the wood only, and those which destroy not only the cellulose but also the binder between the fibres – an amorphous material called lignin. Both dry rot and wet rot produce enzymes which attacks the cellulose. Only certain types of wet rot, white rots, attack both cellulose and lignin.

When many “hyphae” develop in the same location, the mass is called “mycelium”. It is at this stage of development that the fungus first is readily visible to the naked eye.

If suitable conditions continue, with time the fungus will mature and develop a fruiting body called a “sporophore”. This is the equivalent of the flower of a normal plant. A sporophore will produce and disseminate spores to continue the life-cycle. Sporophore formation takes place not only as part of the natural life-cycle but can also be induced at times of high stress, for example the drying out or partial chemical treatment of a dry rot attack.

The sporophore produces spores in very large numbers. These numbers are so large that it is true to say that if conditions conducive to decay occur in timber, it is inevitable that a wood-rotting spore will be present to germinate. Thus, if attacks of dry rot are present in a property and sporophores are active, all timbers which are suitably wet and unventilated are almost certain to develop a new outbreak.

The intrusion of hyphae and the breakdown of wood cells reduces both the weight and the strength of the wood. Other effects of the decay of the wood cellulose include loss of colour, splitting and cracking along and across the grain, the presence of surface mycelium and other fungal growths.

Because they are plants, in addition to a source of infection, a food source, and a suitable temperature, wood-destroying fungi need oxygen and the correct amount of water. It is only on or within wet timber in a building that all these conditions can be found.

The requirements for fungal decay

There are five requirements:-

  1. a source of infection (either a spore or some mycelium)
  2. a food source
  3. water
  4. oxygen
  5. a suitable temperature

In relation to a building I shall consider each one in turn.

Requirement 1: a source of infection (either a spore or mycelium)

In any building of this nature this first requirement is fulfilled by the natural wood-destroying spore level in the atmosphere or on the timbers of a building.

Requirement 2: a food source ie wood or a wood-based material

This requirement is satisfied by the presence of floor and joinery timbers. Timbers which are buried in or adjacent to masonry or brickwork, such as wallplates, beams, bressummers, joist ends or hearth trimmers are potentially at risk. There is a plentiful food source located within the fabric of most older buildings such that there is no constraint on the fungus attacking it.

Requirement 3: water

Wood, unless oven dried above 100° Celsius, always contains water. In timber water is measured on a weight basis and is expressed as a “percentage”. For example, if 10kgs of dry timber contains 1kg of water this is called a moisture content of 10% (1/10 x 100) weight by weight (w/w). In a centrally heated building wood may contain between 8 and 12% w/w moisture content and the newly felled log from a tree could be as high as several hundred percent moisture content. If the wood is too wet then a lack of oxygen inhibits growth of the fungus. If it is too dry, ie less than 20% moisture content, there is insufficient water to sustain growth. It is generally agreed that, to initiate growth, the moisture content has to be in excess of about 25%. For fungal decay to continue to attack wood, the generally accepted minimum water level is 20% w/w – one fifth of its dry weight. Thus, given that water is required in sufficient quantities to sustain growth, it is clear that the converse is true, ie if there is an attack of dry rot present, then it can be stated that, sufficient water present.

Wood naturally absorbs and/or gives water to the atmosphere around it. In a building structural timbers will equilibrate at about 14-16% w/w. A particularly dry item of furniture may be as low as 8% w/w. At high relative humidities (R.H.) wood will equilibrate at above 20% w/w and thus may be subject to decay. Since under natural conditions timbers are quite close to this level, in order to ensure moisture contents of less than 20%, it is necessary for the timbers in a building to be both well ventilated and isolated from direct contact with a source of water, since under natural conditions timbers are quite close to this level.

It can readily be understood that there will exist in a building interstices, such as the face of a buried timber abutting brickwork, which if they become wet have their own microclimate within the building of which they are part. This is why it is in the nature of dry rot to be hidden away out of view.

Requirement 4: enough oxygen

This is naturally present.

Requirement 5: a suitable temperature

This varies. A graph of growth rate versus temperature is shown below.

Fungal<br /><br /><br /><br />
	decay growth rate vs. temperature

In essence, the minimum temperature for observable growth to take place is 3°C, the maximum is 26-36°,and most growth occurs at 22° (the optimum). Care should be taken not to assume that below and above these extremes the fungus is killed. This is not the case. Although the fungus is heat sensitive it can withstand a larger range of temperatures for short periods and subsequently resume growth.

These above data are the results of laboratory experiments and should not be regarded necessarily as being typical of an attack in a building. It is generally considered to be true that dry rot spreads more slowly when it is colder than when it is warmer. Experimental data from Liverpool University from an attack of dry rot in a cold cellar gave rise to the suspicion that this general assumption may not always be true. In general it is not unreasonable to assume that, within the range 3-22°C, the higher the temperature the faster the growth.

C. Methods of control with particular reference to dry rot

[NB Wet rots do not infest adjoining masonry therefroe the section below on `non-timber substrates’ is only pertinent to dry rot, but the following section on `timber’ is pertinent to both wet and dry rot.]

The fungus infests not only timber but also the adjoining plaster, brick and mortar. Separate measures need to be taken for timber and non-timber substrates.

1. Timber

In general, the defective timber is removed and replaced. Often remedial contractors reserve any sound timber up to one metre from the last visible sign of attack. This is not necessary if other measures are taken. Given that sufficient structural strength is present in the timber, provided that it is dried and kept dry, no further action is necessary.

In the past it has been customary to apply a fungicide to the adjacent sound timber. Whether or not this is acceptable depends on the circumstances of the timber and the building. Since October 1989, it has been necessary to carry out a prior written “COSHH” assesment (Control of Substances Hazardous to Health). A fungicide can be water-borne, organic solvent-borne, or in the form of a preservative paste. Depending on the process, a certain degree of protection, particulary during the drying out phase, is conferred on the timber. If, however, conditions conducive to decay are present and continue to be present or recur, this chemical protection often is of dubious or limited efficacy. The key design factor of protection is isolation of the timber (or replacement timber) from the wet masonry or any other water source and ventilation.

2. Non-timber substrates

In many cases this is brick and lime mortar together with both old and new plaster. It is not generally possible or necessary to remove all the infested material. The method of control firstly is to stop the source of water and secondly to take effective measures to reduce the water content to normal levels. This can take many years by natural ventilation and depends on thickness, porosity, initial wetting and site conditions. An accepted rule of thumb is that under ideal conditions an allowance of one month per one inch of thickness of masonry is reasonable. It is becoming increasingly accepted that dehumidification can be used in conjunction with dry heating. (The burning of hydrocarbon fuels, propane gas, paraffin etc, produces large quantities of water and is not acceptable.)

Chemical treatment of masonry by drilling of holes and surface treatment has been much in vogue in the last 20-30 years, much as “burning off” by the application of blow lamps was prior to that. “Burning off”, as normally carried out on site, has been proved to have no significant effect, and there is little research published to show that chemical irrigation has a more significant effect. Nevertheless it is carried out by the majority of treatment companies, probably for commercial reasons. My own view is that hole drilling in the majority of cases is unnecessary but that the surface treatment of masonry should have some inhibitory effect. It is not an unreasonable secondary control method, but it is by no means essential. The real control is drying out, and chemical treatment is only likely to be of some use during the drying out phase. Other “treatment” methods involve fungicidal plasters, plugs and renders. Digest 299 from the Building Research Establishment covers chemical treatment quite soundly.