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Dry Rot and Mycelium

Dry rot, also known as Serpula Lacrymans and formally known as Merulius Lacrymans (from the Latin for tears owing to the unique tear shape of the fungus at microscopic level) is a special type of wood destroying fungus and it has peculiar features which make it the most serious type of timber decay in buildings. It can cause decay of timber at a lower moisture content than the wet rot fungi and it has the ability to grow through damp masonry, brickwork and behind plaster. Thus it can spread to a greater extent and its eradication is more expensive than that of wet rot fungi.

Timber that is decayed by dry rot is brown in colour and tends to split into large cubes up to 50mm in size. These splits can occur along and across the grain. This brown rot attacks and decays the cellulose component of the cell walls of the timber. In addition, there is often masses of cotton wool like, white to grey mycelium. Mycelium is the vegetative part of a fungus and consists of a mass of thin, thread like branches known as hyphae. Hyphae is Latin for web and indeed these structures do resemble spiders webs. Mycelium can also be yellow or lilac in colour and appears on the surface of timbers. The presence of mycelium is very characteristic and aids in the identification of dry rot, as does an unusual mushroom like odour.

The mycelium strands are up to 5mm in diameter and become characteristically brittle when dry. They may spread from infested timber into damp masonry and support further outbreaks in other parts of the building. When conditions are suitable, the fungus forms a flat plate. The fruiting body has a characteristic rusty red centre with a white outer margin. The central portion produces masses of rust red spores which often form a red dust over surrounding areas.

Dry rot can be considered as a form of timber decay that will develop under conditions of poor ventilation, high humidity and condensation rather than extensive leaks. Blocked air bricks or a faulty damp course are common causes. Whilst it can destroy timber at moisture content levels of 20%, the optimum moisture level content for the growth of the fungus is 30-40%. The optimum temperature for dry rot to grow is 20c and the fungus can be killed by exposure to 40c. It is more susceptible to heat than many other species of timber decay fungi. Dry rot is unusual as it is found attacking timber only in buildings, mines and boats and is never found causing decay of outdoor timber or on trees.

Wet Rot

Other types of wood destroying fungi found on timber in buildings is usually grouped together under the term wet rot. All wet rot fungi require higher moisture contents than dry rot and many will achieve optimum growth and decay rates at between 45 and 60% moisture content.

Some wet rot initiate their attack in the standing tree, in the freshly felled log, or during seasoning. If such decayed timber is used in a building in a situation where it becomes wet, this type of decay may continue in the infected piece and can spread to adjacent timbers.

Coniophora puteana is a cellular fungus of the Boletaceae family of fungi which are characterized by developing their spores in small pores or holes under their caps, rather than the usual gills of species such as button or shitake mushroom. It is the commonest cause of wet rot in timber in buildings. It usually causes a brown rot in softwoods. The decayed wood splits along the grain and any cross grain cracking is not as deep as that of dry rot. This decay is commonly confined to the wood interior, leaving a veneer of apparently sound timber. This type of fungus produces very little surface mycelium but thin, brown, branch like strands are often formed on the surface of affected timber. The fruiting body – or the fungus ready to release its spores, is rarely found on timber inside buildings.

Coniophora may also attack buildings where there has been a serious leak, perhaps as a result of burst pipes, failed plumbing runs or faulty gutters and downpipes. It can also cause decay in exterior painted joinery and other timbers.

Another fungus commonly found causing wet rot of softwoods in buildings if Fibroporia spp – usually known as Poria. There are a number of related species of this fungus that can cause decay in timber.

How to Spot

Not sure what the signs of dry or wet rot are? Find out more information.


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