Timber Decay

Dr Jagjit Singh

 
Sporophores of the dry rot fungus  Serpula lacrymans affecting floor joists  

Building materials are decayed by the effects of adverse environmental conditions and the extent of damage depends on both the materials and the conditions. Among the most vulnerable materials are timber, paint, textiles and paper. Timber remains one of the most useful in a world of diminishing resources and is a major component in most historic buildings. It has many positive structural and aesthetic properties as well as being an energy-efficient and renewable resource. However, timber provides specialised ecological niches and many organisms have evolved to use it as a food. The most common and destructive to timber are dry rot, wet rot, common furniture beetle, and death watch beetle.

Orthodox remedial treatments often entail the loss of irreplaceable decorative finishes, floors and ceilings. Furthermore, treatment of the infestations with insecticidal fungicidal chemicals is not only expensive, inconvenient, hazardous to the operatives and occupants but also environmentally unacceptable and usually unnecessary. Environmental control and preventative maintenance provide an alternative, less destructive solution, and remain the most widely used methods for preventing biological decay.

BIOLOGICAL DECAY MECHANISMS

Biodeterioration of materials was defined by Hueck in 1968 as 'any undesirable change in the properties of material of economic importance brought about by the activities of living organisms'. A wide range of materials are subject to microbiological deterioration, which are caused by a broad spectrum of micro-organisms.

Beetles responsible for the decay of timber principally include woodworm (Anobium punctatum), death watch beetle (Xestobium rufovillosum), powder post beetle (Lyctus spp), and house longhorn beetle (Hylotrapes bajulus). It is their larvae which cause most damage as they bore through the wood, feeding off it and causing damage to the structure and strength of the timber.

Decay fungi are capable of enzymatically degrading complex cellulosic materials, such as wood, into simple digestible products. The decay of wood cells by these fungi results in the loss of weight and strength of the wood. There are two main types of wood-rotting fungi found in buildings; wet rot and dry rot (see Table 2).

The principal environmental factors favouring the biodeterioration of building materials are temperature, humidity and a lack of ventilation. Moisture may be contributed by penetrating or rising damp; condensation; building defects and disasters such as leaks; and from construction moisture introduced in mortar, concrete and plaster for example.

ENVIRONMENTAL CONTROL - THE GREENER APPROACH

Environmental control relies on controlling the cause of the problem by controlling the environment. It is designed to ensure the future health of the building and its occupants by avoiding the unnecessary use of potentially hazardous and environmentally damaging chemical pesticides where possible and their consequential legal and management complications. Eradication of dry rot spores or insect pests in an historic building and its contents is in practice, impossible. The volumes of chemicals necessary and the toxicity required would be damaging both for the buildings and all its occupants. Where chemical treatment cannot be avoided materials and techniques should be used which have minimum adverse environmental effect.

By reducing the need to expose and cut out infected material, environmental control also reduces damage to the fabric and the finishes of a building. Where an historic building is concerned, this is particularly important, and the specification should ensure maximum conservation of existing materials to maintain the historic integrity of the fabric, as well as to avoid unnecessary expenditure. Its success depends on a thorough investigation of cause and effect. Through a methodical approach such as this, it is possible to decrease the cost of remedial timber works significantly or in some cases eliminate it altogether.

First the building should be thoroughly inspected using non-destructive techniques to locate and identify all the significant decay organisms within it. In cases of actual or suspected problems of woodrot or wood boring insects in buildings, investigation should be by an independent specialist consultant, architect or surveyor to establish the cause and extent of the damp and timber decay, including the potential risk to the health of occupants before specification or remedial work. Correct identification of the fungi and insect material is important as not all fungi are equally destructive. Some rots are present in timber when it is cut or are acquired in storage. Fungal material may also be dead or dormant, the product of conditions now past.

Having identified the nature of decay, the environmental conditions which are required to support it should be considered (see Table 1). Only then will it be possible to devise a scheme to deal with the problem.

The aim of remedial building works is to control the timber decay, to prevent further decay and to correct any significant building defects resulting in conditions of high moisture content or poor ventilation of timber. In particular, it is important to reduce sub-surface moisture content of all timber to below 16-18 per cent. Timber should be isolated from damp masonry by air space or damp proof membrane, and free air movement should be allowed around timber in walls, roofs and suspended floors. All other sources of water should also be eliminated, such as overflowing gutters, leaking plumbing, condensation and rising or penetrating damp. Humidity in voids should not exceed an average relative humidity of 65 per cent. In addition, all active fungal material should be removed together with all rotten wood, and the structural strength of the remaining timber and fabric construction should be assessed to determine whether reinforcement or renewal is required. In the case of insect infestation, measures should also be introduced to avoid recontamination. Dirt, dust and builders' rubbish provide a haven for insects and fungi. Voids and cavities should be cleared and the areas cleaned with a vacuum cleaner to remove dust. A programme of building maintenance and monitoring may then be instigated to prevent any future problems.

 

TABLE 1: CAUSES OF DECAY

Type

 Agent

Environmental Factors

Biological

fungi (dry rot, wet rot, moulds and others) bacteria; actinomycetes; lichens, mosses and algae wood-boring insect larvae (woodworm, death watch beetle and others) carpet beetle, moths, book lice and silverfish termites

moisture and humidity
air movement
temperature
light
dust
food source

Chemical

acids, alkalis and solvents

pollution
remedial treatment

Physical

mechanical abrasion, general handling and others, decomposition by physical agents such as prolonged heating, fire and moisture

normal use, visitor wear
accidental damage
sunlight, heating, fire, damp

Radiation ultraviolet light exposure to sunlight
 
TABLE 2: FUNGI WHICH INFEST BUILDING MATERIALS

 

Moisture Conditions

Temperature Requirements

DRY ROT

Minimum moisture content in timbers of about 20 per cent
Optimum growth occurs at 30-40 per cent
Spore germination requires wood moisture content of 30 per cent

The optimum temperature for dry rot growth in buildings is about 23C, the maximum temperature for continued growth is about 25C and the fungus is rapidly killed above 40C

WET ROTS Wet rot fungi usually occur in persistently damp conditions needing an optimum moisture content of 50-60 per cent Wood-rotting fungi differ in their optimum temperature but for most the range is between 20-30C
 
TYPES OF FUNGI
Dry rot:
Serpula lacrymans
Wet rots: Cellar rot fungus (Coniophora puteana); Poria fungi, (eg Amyloporia Xantha; Fibroporia vaillantii and Poria placenta); Phellinus continguus; Donkioporia expansa; Oyster fungus (Pleurotus ostreatus); Asterostroma spp; Paxillus panuoides; Lentinus lepideus; Dacrymyces stillatus; Ptychogaster rubescens
Soft rot: Chaetomium globosum
Moulds: Cladosporium spp; Penicillium spp; Aspergillus spp; Trichoderma spp; Alternaria spp; Aureobasidium spp
Slime moulds: Myxomycetes
Plaster fungi: Coprinus spp; Peziza spp; Pyronema spp
Stain fungi: Cladosporium spp; Aureobasidium spp

MONITORING SYSTEMS

Remote monitoring systems can be very useful in increasing the efficiency and decreasing the cost of maintenance programmes. They can be especially useful for checking the moisture content of inaccessible timbers in roof spaces, behind decorative finishes and in walls.

Sensors can be placed at all critical points after the investigation or after remedial building works. Areas can then be closed up and finishes reapplied; for example sensors may be placed in lintels, joist ends, valley gutter soles or in damp walls to monitor drying. It is important to use enough sensors and to place them with an understanding of the moisture distribution processes, because conditions can vary even in a small area. It is these local variations in conditions that produce the environmental niches which decay organisms exploit.

If more than 30 sensors are deployed, taking the readings can become onerous and this may result in human error or negligence. In these situations automatic monitoring systems become desirable and a number of specialised systems have been developed. With larger systems the wiring of sensors can also become a problem. For systems requiring 100 or more sensors, a computerised unit is used, working via a single four-core mains cable connecting up any number of nodes, each supporting four sensors. This system can be programmed to record and log data at regular intervals with alarm limits for each sensor. The data is then transmitted to a remote computer via a modem connected to a telephone line. Data from the system can then be analysed using CAD and programs for statistical interpretation.

TIMBER MOISTURE MONITORING SENSORS

The water content of building materials can be determined through a range of direct and indirect methods. Direct methods involve removal of a sample of the material to be tested which is weighed and then dried to determine its water content. This has the disadvantage of being destructive and it cannot be used for remote monitoring.

Indirect methods are based on measurement of characteristics related to the moisture level in the testing material. These involve thermal conductance, electrical capacitance and resistivity. Measurement of a surrogate material in equilibrium with the first material is another method. The use of electrical resistance moisture meters provides a quick and relatively accurate method of determining the moisture content of wood if a knowledge of their limitations is taken into account. Moisture meters measure the changes in resistance, due to changes in moisture content, between two electrodes placed in the timber. Increasing moisture content results in a reduction in electrical resistance.

Miniature sensors are fabricated from hygroscopic material which has been calibrated to match the moisture content changes in timber. They are encased in a protective shell. The sensor is then inserted into a previously drilled hole to the required depth and the hole sealed. In most instances the sensor cable seals the hole to the outside. The sensor will fairly rapidly come into equilibrium with the atmosphere within the hole. Due to their small size the sensors can be inserted into the centre or ends of large dimension timbers allowing the best chance of detecting defects early.

MASONRY MOISTURE MONITORING SYSTEMS

Systems for use with masonry can be based on the direct measurement of the material's moisture content or by the use of a surrogate material which changes in moisture content in a similar way to the host material. This material may be of any hygroscopic type which, providing it has been calibrated correctly, can be used as the basis of a remote sensing system.

The sensors are placed in the material to be tested at the required depth, or in an array and the hole sealed to the external atmosphere. Sensors will come to equilibrium with the relative humidity within the cavity or drilled hole and hence with the surrounding material. Single sensors can be placed at varying depths but must be sealed within the area to be measured. A series of sensors individually sealed within the drilled hole can provide a profile of readings across the material. These can either be wired up and resistance measured remotely or can be removed, weighed and oven dried to calculate their water contents. Changes in the water content of masonry can be rapid when wetted so that it could provide an early warning of building defects leading to water penetration. However, drying down can take many weeks or years.

 

 

The Building Conservation Directory, 1996 (where it appeared as 'Environmental Monitoring and Control')

Author

JAGJIT SINGH BSc MSc PhD CBiol MIBiol AIWSc FIRTS is currently Associate Director with Oscar Faber Consulting Engineers (St Albans) and is the head of Oscar Faber Heritage Conservation. He has over 15 years' experience in both academic research and industry. He specialises in the study of the biodeterioration of building materials and associated environmental health problems.

Further information

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