Dry Rot and its Treatment in Historic Buildings

Janice Carey and Colin Grant

'Dry rot' is the decay of wood caused by the fungus Serpula lacrymans, the effects of which in buildings can be truly dramatic. Diagnosis of dry rot in a building, like the diagnosis of cancer in a patient, has the ability to strike terror into the heart of the recipient of this dire news. However, all wood-rotting fungi require both food (wood or other cellulosic material) and water, and the dry rot fungus is no exception; deprived of either, it cannot survive.

Much of the mythology surrounding dry rot is founded on the ability of its strands to penetrate through non-wood building materials, to transport water to otherwise dry areas and for the fungus to 'manufacture' its own water. In reality, the delicate hyphae are the primary colonisers and the ability to conduct water is limited and can be negated by good ventilation. The process of wood decay itself produces water but in this respect dry rot is no different from any other wood-rotting fungus and, likewise, its ability to produce moisture in this manner can be negated by ventilation. Decay will cease if the moisture content of the wood is reduced to below about 20 per cent, and many extinct outbreaks of dry rot are discovered in buildings where the fungus has died out as a result of this happening, probably following maintenance which has eliminated a water source

CONTROL STRATEGIES

Because of the total dependency of dry rot on moisture, the primary control strategy must be based on environmental considerations aiming to restore and maintain dry conditions. However, in many cases drying will take a long time, often measured in years, especially where some types of historic buildings are affected. Therefore, secondary measures will often be required to prevent further damage by the fungus before it is effectively arrested by the drying

Primary Control Measures

A detailed survey should be carried out to identify and locate sources of moisture ingress. Particular attention should be paid to roofs and rainwater systems with emphasis on gutters and downpipes, parapet roofs and roof coverings. Rain penetration can also be through renderings and flashings or around windows and doors. Rising dampness through missing, bridged or otherwise defective damp-proof courses must be rectified. Any plumbing should also be inspected for leaks.

Rapid drying should be encouraged through the provision of heating and ventilation which may also require specific building work to prevent moisture ingress and transfer, and to encourage aeration. Dehumidifiers can remove moisture from the air but their effectiveness in aiding drying of walls depends on the rate of evaporation from the wall surfaces.

Other Measures

Assessing the outbreak
It is necessary to determine how far the dry rot has spread. All woodwork in the vicinity of any outbreaks should be inspected carefully to assess the extent of decay and the current moisture content of the timber. Extensive removal of plaster is necessary only if it is suspected that timber is embedded in the walls and is at risk.

Removing affected timber
Removal of all timber affected by dry rot is destructive but necessary in principle. Retaining affected timber presents problems for the structural integrity of the building and falling debris can be a hazard to occupants and others if decay continues. Timber already below 20 per cent moisture content presents little risk of further decay but, at higher moisture contents, the level of risk depends upon the speed with which drying can be induced and the ease of monitoring the reducing moisture content. Higher risks may be acceptable where timbers are of historic value or where their removal cannot be achieved without damage to important historic fabric - for example, where they support a fine plaster ceiling. In such cases the retention of some timbers may be essential or at least highly desirable.

If the wood can be removed, it can be sterilised in a kiln. The temperature throughout the wood must be maintained at just over 40C for 15 minutes. Care is needed to prevent splitting and distortion and this method provides no protection to the wood after reinstatement.

Special building measures are necessary if timber is to be retained, including isolation from damp masonry.

Wood preservative treatments

Liquid preservatives can be applied to the surface of sound timbers left in situ to help prevent new infections developing during the drying process. However, they should not be used or regarded as an alternative to physical methods of protection.

If timber infected with dry rot has to be retained for special reasons and decay cannot be arrested in the short term by drying, preservative treatments that penetrate throughout the affected part of the timber can be used. For example:

  • application of a preservative paste
  • repeated addition of liquid preservative to sloping holes drilled into the wood or by pressure injection
  • insertion of borate rods or tablets (these are only effective if the wood is wet).

Treatment of hardwoods must include an insecticide if there is a risk of infestation by death watch beetle.

All new timber used in repairs should be pre-treated with a wood preservative. Detailed guidance on the treatment required for various timber components is given in the British Standard BS 5268: Part 5, which deals specifically with structural timber, and BS 5589 which covers a wider range of uses. Guidance is also given on the use of naturally durable timbers (see also Digest 429 published by the Building Research Establishment Ltd).

Masonry Treatments

Although strands can grow through and across masonry, the dry rot fungus derives no nourishment from it. The concept of killing the fungus within masonry by wide-spread irrigation with a fungicide traditionally has provided a 'comfort factor', but it has to be questioned in each case whether this procedure can be justified. First, it is usually difficult to achieve a thorough treatment and, secondly, the treatments introduce large quantities of water which then need to be removed, increasing the risk of salt efflorescence and damage to the masonry, as well as prolonging the time it takes to dry the structure.

The most important role of chemical treatments of the masonry is to prevent the fungus from obtaining access to a fresh food supply in the form of timber in adjacent areas, or replacement timbers being introduced into the area. For this purpose, localised chemical treatments of the masonry can create a useful barrier between the fungus in the wall and the wood. Examples of such treatments are:

  • surface application of fungicidal fluid (which also helps prevent fruit-body formation during the drying phase)
  • use of fungicidal renderings
  • insertion of preservative plugs or pastes
  • localised irrigation treatments.

Whilst these localised treatments play a role in the overall control strategy, they must not be regarded as a substitute for getting the building dry.

Heat sterilisation of masonry walls and timber in situ
In the past, the use of heat to sterilise walls was condemned because it was too difficult to apply effectively and provided no residual protection. In the search for chemical-free control measures, sterilisation with hot air is now increasingly being used, particularly in Denmark. However, the process needs to be carefully controlled so as to prevent damage to the building as well as to ensure that the necessary temperature has been achieved deep in the affected area.

MONITORING

The importance of monitoring the conditions in buildings cannot be over-emphasised. Dry rot develops very slowly, so early detection and curing of moisture ingress will prevent decay occurring in the longer term. Routine monitoring can be as simple as regular visual inspection to check the integrity of the building fabric against ingress of moisture, and taking measurements of moisture content of vulnerable timbers with a hand-held probe. However, sophisticated permanent monitoring systems are now increasingly used involving computer-based equipment linked to probes permanently installed in timbers or other parts of the building fabric. Specific sensors can also be installed in rainwater goods to indicate overflows

Dry rot is potentially a cause of seriously damaging decay for timber in historic buildings, but it does not have to be devastating or outrageously expensive to cure if caught in time. A careful diagnostic approach is required to identify and cure dampness, to treat in a very selective and targeted way and to re-instate with appropriately pre-treated or naturally durable replacement timber. Installation of monitoring systems to facilitate routine maintenance checks can enable massive economies compared with destructive re-build approaches and will provide greater assurance for the future.

Recommended Reading

Building Research Establishment (BRE) Information Paper
IP 19/88           House inspection for dampness

British Standards Institution
BS 4072, Wood preservation by means of water-borne copper/chrome/arsenic compositions, 1987 (revised version to be published shortly)

BS 5268 Code of practice for the structural use of timber Part 5: 1997 Preservative treatments for constructional timber

BS 5589 Code of practice for preservation of timber, 1989

BRE Digests
163       Drying out buildings
299       Dry rot: its recognition and control
304       Preventing decay in external joinery
307       Identifying damage by wood-boring insects
321       Timber for joinery
327       Insecticidal treatments against wood-boring insects
345       Wet rots: recognition and control
364       Design of timber floors to prevent decay
371       Remedial wood preservatives: use them safely
378       Wood preservatives: application methods
429       Timbers: their natural durability and resistance to preservative treatment

BRE Timberlab Papers
Dry Rot - Causes and Remedies, TP No: 44-1971 (Reprinted 1981)

Books
DH Jennings and AF Bravery (eds), Serpula Lacrymans: Fundamental Biology and Control Strategies, John Wiley & Sons Ltd, Chichester, 1991

For further information contact BRE, Garston, Watford WD2 7JR
Tel 01923 664000 Fax 01923 664010 E-mail GrantC@bre.co.uk

 

This article is reproduced from The Building Conservation Directory, 1999

Author

Dr JANICE CAREY is a Wood Durability Projects Manager and COLIN GRANT is Operations Manager in the Building Research Establishment's Centre for Timber Technology and Construction (CTTC). The CTTC is the BRE's focus for promoting and developing the effective use of timber.

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