Remedial treatment for biodeterioration

Chemical, biological or environmental control

Chemical control

A great variety of toxic chemicals are available on the market for use as wood preservatives. The ideal wood preservative should possess these characteristics:

  • high toxicity towards wood-destroying organisms;
  • permanency in treated wood (low volatility and a high resistance to leaching);
  • ability to penetrate deeply into the wood;
  • non-corrosive to metals and non-injurious to the wood itself; and
  • reasonably safe to handle and without injurious effects on operatives and occupants.

Wood preservatives are regulated in the UK by the Control of Pesticides Regulations 1986.

Conventional treatment of dry rot

The conventional approach for remedial treatment of infection by dry rot fungus would be:

  1. Establish the size and significance of the attack. In particular, if structural timbers are affected, carry out or arrange for a full survey to determine whether structural repairs are necessary. If they are, take appropriate steps to secure structural integrity.
  2. Locate and eliminate sources of moisture.
  3. Promote rapid drying of the structure.
  4. Remove all rotted wood, cutting away approximately 450mm beyond the last indications of the fungus.
  5. Prevent further spread of the fungus within brickwork and plaster by using preservatives.
  6. Use preservative-treated replacement timbers.
  7. Treat remaining sound timbers that are at risk with preservative (minimum 2 full brush coats).
  8. Introduce support measures (for example, ventilation pathways between sound timber and wet brickwork or, where ventilation is not possible, provide a barrier such as a damp-proof membrane or joist hangers between timber and wet brickwork).
  9. Do not retain dry rot infected timber without seeking expert advice. There is always some risk in retaining infected wood that can be minimised by preservative treatment and subsequent re-inspection.

Biological control

Microbial interactions and biological control methods are an alternative to chemical control methods, which cause extensive environmental degradation, pose potential hazards to wildlife and are of grave concern to public health authorities.

The prevention of biodeterioration may be achieved by biological control methods. This involves placing a microorganism into a material that does not affect the properties, but which successfully prevents invasion by species capable of damage, i.e. the use of one biological agent to suppress another.

Successful examples of biological control include:

  • control of wood-decay fungi, such as the use of Trichoderma species against various wood-rotting fungi (e.g. Polyporus adustus, Polystictus hirsutus, Polystictus versicolor and Stereum purpureum) and Scytalidium lignicola against Lentinus lepideus;
  • pure cultures of Bacillus subtilis have been shown to inhibit the growth of various wood-staining fungi such as Alternaria tenuis, Trichoderma reesei and Aureobasidium pullulans in vitro; and
  • immunising commensal fungi can be used both as preventive and curative treatments in reducing the incidence of Lentinus lepideus.

Environmental control

For prevention of any form of biodeterioration, there are 3 factors to take into account: the material, the environment and the organism. The removal or alteration of any one of these can prevent the growth of decay organisms.

The control of the environment of a susceptible material, instead of the application of biocides, is the oldest and still the most widely used method of preventing biological deterioration. Traditionally, the control of physical conditions has been by far the most important method of preventing biodeterioration. For example, in the use of timber in construction the object has been to prevent timber moisture content from rising to levels at which wood rotting fungi can thrive.

The basic principle in the control of fungal growth is to render the micro-environment in or around the material in buildings as hostile as possible to the settlement, germination and spread of micro-organisms. This can be achieved by:

  • preventing or limiting biological agents' growth and proliferation by means of toxic chemicals; or
  • ensuring that the material to be protected is kept, or keeps itself, in such a physical condition that growth of biological agents is severely limited or prevented entirely.

The second approach is discussed in more detail as, traditionally, the control of physical conditions has been the most important method of preventing biodeterioration. The application of the general principles of the control of physical conditions and the reactions of microorganisms to these conditions often results in the most effective and economical prevention of deterioration.

The 'greener' approach

Environmental control relies on controlling the cause of the problem by controlling the environment:

  • locate and eliminate sources of moisture;
  • promote rapid drying;
  • determine the full extent of the outbreak;
  • remove decayed timber;
  • determine structural strength of timber and fabric construction; and
  • institute good building practice (such as ventilation, damp-proof membrane, isolation).

Environmental control is complex and requires a multidisciplinary team of scientists, engineers, surveyors and those with computing skills.