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Dry rot treatment


Dry rot treatment refers to techniques used to eliminate dry rot fungus and alleviate the damage done by the fungus to human-built wooden structures.

Dry rot (Serpula lacrymans) is considered difficult to remove, requiring drastic action. Remedial timber treatment and damp proofing companies typically recommend stripping out of building fabric beyond the visible extent of the infestation and the use of fungicide. More holistic approaches attempt to eradicate dry rot by controlling the local environment to reduce timber moisture levels and increase ventilation in order to promote drying.

The first priority when treating dry rot is to find and remove the dampness within the building that caused the outbreak, and to promote drying out by taking measures, such as increasing ventilation. Treatment approaches differ after these steps are taken.

S. lacrymans is a form of brown rot, a group of fungi which digest the cellulose and hemicellulose in timber. This particular species poses the greatest threat to buildings since it can spread through non-nutrient providing materials (e.g., masonry and plaster) for several meters until it finds more timber to attack.

Dry rot is spread by spores which are present in most buildings. The minimum moisture content of timber for spore germination is 28–30% (lower than other rots), and the relative humidity must be in excess of 95%. Spores are resistant to desiccation and may still be viable for germination when they are several years old.

If conditions are suitable, the spore will germinate producing microscopic fungal threads called hyphae.

Table 1. Environmental conditions for mycelial growth following germination

The average moisture content of modern softwood timbers in dry buildings is generally in the range of 12–15%, and heating systems may reduce this to a far lower level. There is, therefore, no prospect of a dry rot infestation developing in a building that has been properly designed, constructed, and maintained.

Once the environment starts to dry out, the rot will become dormant and eventually die. The length of time that the fungus can remain dormant under dry conditions seems to depend on the temperature, with approximate times of nine years at 7.5 °C and one year at 22 °C being quoted.

Most fungi will only thrive in acid conditions, but dry rot will remain active in alkaline conditions. This gives it the ability to grow through damp mortar, masonry, and plaster, infecting other areas of the building. Thick, conducting strands known as rhizomorphs, are produced to cross inert surfaces and penetrate masonry.

A fruiting body (sporophore) may develop naturally or in response to unfavourable conditions of humidity, temperature or exhaustion of nutrients. Often the stress that provokes this is the exposure of the infestation. The fruiting body will produce millions of rust brown spores.


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