Dry Rot - Discussion

Discussion

Dry rot is the term given to brown rot decay caused by certain fungi that deteriorate timber in buildings and other wooden construction seemingly without an apparent source of moisture. The decayed wood takes on a dark or browner crumbly appearance, with cubical like cracking or checking, that becomes brittle and can eventually crush the wood into powder. Chemically, wood attacked by dry rot fungi is decayed by the same process as other brown rot fungi. An outbreak of dry rot within a building can be perceived to be an extremely serious infestation that is hard to eradicate, requiring drastic remedies to correct. Eventually the decay can cause instability and cause the structure to collapse.

The term dry rot, or true dry rot, refers to the decay of timbers from only certain species of fungi that are thought to provide their own source of moisture and nutrients to cause decay in otherwise relatively dry timber. However, there has been no published experimental evidence to support the phenomena. In modern texts, the term 'dry rot' is used in reference to damage inflicted by either: Serpula lacrymans (formerly Merulius lacrymans) predominantly in the United Kingdom and northern Europe; and/or Meruliporia incrassata (which has a number of synonyms, including Poria incrassata and Serpula incrassata) in North America. Both species of fungi cause brown rot decay, preferentially removing cellulose and hemicellulose from the timber leaving a brittle matrix of modified lignin.

The term dry rot is somewhat misleading, as both species of fungi Serpula lacrymans and Meruliporia incrassata require an elevated moisture content to initiate an attack on timber (28–30%). Once established, the fungi can remain active in timber with a moisture content of more than 20%. At relative humidities below 86 percent, growth of Serpula lacrymans is inhibited, but it can stay dormant at relative humidities down to 76 percent. These relative humidities correspond to equilibrium moisture contents of wood of 19 and 15 percent, respectively.

Schilling & Jellison note the potential efficiency of these 'dry rot' fungi in growing away from direct moisture sources, although there is no reference for how efficient a brown rot fungus has to be at translocating water in order to be classed as 'dry rot'. Some have suggested the importance of these fungi providing their own source of nutrients as being more significant than providing an adequate source of moisture. Schilling suggests efficient nutrient translocation and utilization, notably nitrogen and iron, may be more distinctive in these species than water translocation. Water translocated in this fashion carries nutrients to the extremities of the organism; not, as is sometimes inferred, to simply render dry timber wet enough to attack. Coggins goes into more detail about water movement in Serpula lacrymans.

The perpetual saturation of wood with water also inhibits dry rot, as does perpetual dryness.

Dry rot would appear to be a paradoxical term seemingly indicating decay of a substance by a fungus without the presence of water. However, its historical usage dates back to the distinction between decay of cured wood in construction, i.e. dry wood, versus decay of wood in living or newly felled trees, i.e. wet wood.