All you need to know about dry rot!

The mycelium of Serpula lacrymans develops extensively on the surface of infected timber and in still, humid conditions produces a mass of cotton wool-like growth with bright yellowish patches. Water droplets produced on the surface of the mycelium has given the fungus its name ‘lacrymans’ (latin for ‘tears’). Lilac tinges are more common, especially in less humid situations where the surface mycelium is reduced to a thin silken grey skin. Mycelium spreads over the timber surface by the continued growth and branching of the delicate hyphal threads with time.

Thicker strands develop within the mycelium and these supply water and nutrients to the growing front as the fungus becomes established. The strands assume their real significance when the fungus spreads from infected timber onto the surface of adjacent masonry walls. The tiny hyphal threads penetrate the mortar joints and plaster layers. Therefore, large areas of damp wall can become infected. The fungus cannot derive any nourishment from the wall materials (although it is thought that calcium salts in such materials contribute to the success of the fungus in such situations). The strands, which have thick walls and are resistant to moisture loss, are able to continue to supply water and food to the growing front for considerable periods of time. It should be remembered, however, that the most careful examination of a building found to be suffering from dry rot must be made as the extent of dampness may not be clear and multiple sources of water may be present. The full extent of spread of the fungus must be determined before remedial treatment can be undertaken confidently.

General Treatment

When controlling dry rot it is essential that the necessary steps to eliminate the source(s) of moisture causing the decay are carried out as part of the overall specification of repairs. This should be carried out by a specialist contractor.

Particular emphasis should also be given to efficient ventilation, particularly of subfloors and roof voids.

It is important to establish the full extent of the outbreak by testing timbers in the vicinity of an outbreak by prodding with a bradawl or sharp pointed tool such as a screw-driver. Guidance as to the possible extent and direction of spread of fungus within walls can sometimes be obtained by observations aided by the use of an electronic moisture meter.

The extent of growth of dry rot mycelium should be determined in order to ascertain the extent of risk of infection of adjacent timbers. This usually requires opening up the affected area by removal of joinery, stripping of plaster, removal of any timber fixings and lifting of floors.

In those buildings in which it is known from previous experience that no woodwork is embedded in walls, it may not be necessary to strip large areas of such plaster even though it may be thought to overlie fungus strands. It may then suffice to remove plaster for some 300mm adjacent to woodwork at risk, to confirm that no fungus has reached it. Alternatively, the spread of fungus can be determined by removal of plaster samples at intervals.

Special consideration must be given to areas of solid flooring which are in contact with dry rot attack. Whilst it is usually economic to cut away the full extent of even lightly affected building softwoods, there are special cases, for example durable timbers (both hard and soft woods) in which the removal of lightly affected members would be disproportionately costly or would destroy historically important features.

In such cases clients will be advised of the possibilities of alternative in-situ treatments. Alternatives to complete removal may be particularly appropriate when the affected timbers are still structurally adequate and will readily dry out after being isolated from damp walls e.g. at first floor level and when effective ventilation can be arranged.

The suggested safety margin may be inadequate in the case, for example, of a wall plate or alternatively it can be excessive in the case of a floorboard where it should normally be necessary only to cut away to the next joist. The extent of the exposure work, strip out, chemical treatments and timber replacement will always be subject to variation and will be dictated by the prevailing site conditions.

It is important to isolate existing sound timbers from dampness. Timber in direct contact with damp and infected walls should be isolated by means of physical isolation. Timber beams and joist ends with bearing ends embedded in the walls should be removed and independently re-supported.

Additionally, removal of soil to lower the level of a damp or saturated oversite may be advisable.

Wood preservatives/biocides should not be relied upon to provide long term protection against dry rot in conditions where timbers are persistently wet. Unless otherwise recommended in the survey report and agreed by the client, treat exposed wall surfaces identified as showing evidence of mycelium by one of, or a combination of, the following methods:-

  • Surface application of a masonry biocide.
  • Localised treatment of specific areas by insertion of approved fungicidal plugs, pastes or gels into holes drilled in the masonry.
  • Localised irrigation by the formation of a ‘toxic box’ surrounding the outbreak.
  • Irrigation with a fungicidal solution via holes drilled in the wall.


Although good design and construction offer the best protection against decay, these can be nullified by bad maintenance.

There are, however, some parts – even in a modern house – in which any timber used should be pre-treated and isolated from damp surfaces with a damp-proof membrane.

Wood boarding laid over concrete solid floors is often nailed to imbedded battens set in the concrete. It is essential that these battens are pre-treated. Any timber boarding laid over a solid floor should be isolated with an appropriate membrane extending to protect the boards end-grain from contact with damp surfaces below damp-proof course level.

The ends of joists and other timbers should be isolated from contact with the brickwork of the outside wall. The bearing ends of the joists should be supported on isolated hangers or protected with a membrane.

Where there is any possibility that dry conditions cannot be maintained, it is essential that all timber should be pre-treated.

In exposed places where rain is likely to be driven by wind between brickwork and window or door-frames then waterproof sealing is essential.

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