Dry rot



Dry rot is wood decay caused by certain species of fungi that digest parts of the wood which give the wood strength and stiffness. It was previously used to describe any decay of cured wood in ships and buildings by a fungus which resulted in a darkly colored deteriorated and cracked condition.
Dry rot is the term given to brown rot decay caused by certain fungi that deteriorate timber in buildings and other wooden construction without an apparent source of moisture. The term is a misnomer because all wood decaying fungi need a minimum amount of moisture before decay begins. 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 an extremely serious infestation that is hard to eradicate, requiring drastic remedies to correct. Significant decay can cause instability and cause the structure to collapse.
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 (2830%). 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 (London) Times on Tuesday 12 March 1793 carried an advertisement that informs the reader that the British Colour Company, No. 32, Walbrook, London continues to use, manufacture and sell paints prepared with the Oil of Coal, which is of a very penetrating nature, and hardens wood in an uncommon degree protecting it from weather, dry rot and ice.
Dry rot can be very difficult to remediate unless all of the decayed wood and spores are removed. In some cases after this is done, decayed areas can be treated with special epoxy formulations that fill-in the channels of the damaged wood, killing the rot and improving structural integrity. However, this type of treatment can actually promote decay in wood in exterior service unless the epoxy application is designed to shed water. Epoxy or other polymers will trap moisture behind the patch, causing more decay in the surrounding previously uninfected zones unless the repaired zone is protected from recurring water events. Commercial ethylene glycol (commonly sold as antifreeze) and many other toxic diffusible compounds can diffuse into the wood to kill the fungus, but they also can diffuse out of wood that is repeated wetted. Diffusion of these toxic compounds out of the wood, and into surrounding soils and plants is not appropriate from a toxicity or environmental standpoint, and these types of treatment are not recommended for wood in-service. Certain copper compounds, such as copper naphthenate, are available as a brushable solution and are frequently used when dry-rot damage is repaired by splicing in new wood; after removal of bulk rotten wood the remaining original surface is saturated with such a compound (typically green in color) before application of the new wood.