Dry Rot: What’s in a name?
by Dr Matthew Green
Dry rot is a destructive fungus whose history and name has left a legacy of confusion. Many still consider any brown, cuboidally cracked decay to be dry rot, and the inference from the name that dry rot needs less water than wet rots has had disastrous consequences for a number of buildings and the finances of their owners.
Dry rot is essentially a very efficient brown rot decay fungus with quite specific environmental requirements, but it has a reputation as a house-destroying superorganism. Much of the exaggeration and confusion concerning dry rot stems from the rather poor use of common names used to describe building fungi.
There are four terms in which building decay fungi are often described:
• Dry rot. In the UK this is a common name applied to one species of fungus only: Serpula lacrymans. It is sometimes applied to Meruliporia incrassata in North America.
• Wet rot. This is a catch-all term for every species of fungus that is not Serpula lacrymans.
• Brown rot. This term describes the mode of action of the fungus. In this respect, the term has some scientific relevance. Brown rots decay timber leaving the wood brown.
• White rot. In contrast, white
rot fungi leave decayed wood white.
Dry rot (S. lacrymans) is a brown rot. Wet
rots may use either mode of decay.
Photograph 1: The corner of an elm roof of a rotunda showing brown rot decay in the end grain of the plate with white rot damage at the base of the principal rafter and further along the plate. The contractor’s use of a 20mm auger to test the condition of the timbers was a little overzealous- this damage is historic.
It is unfortunate the use of the terms dry and wet rot are so ingrained in the literature of building decay fungi as they are poor and misleading descriptors of these organisms. A brief overview of the history of building decay fungi in the UK reveals how we arrived at this unfortunate situation.
The origin of Serpula lacrymans in the British Isles remains obscure. The fungus does not occur outside of buildings over most of its range across the temperate regions of the world with the exception of a few colonies from the Himalayas, East Asia and the Czech Republic. Recent research has shown that two sub-groups of of the fungus exist, a wild type predominantly found in North America at high altitudes, and a more aggressive variety possibly of Asian origin.
It seems logical to suppose that dry rot reached these shores in infected timbers from Europe and the lack of genetic variation in the variety seems to support this. We know that the fungus attacked softwood on ships, and we also know that fungus had frequently to be scraped off cargoes of softwood when they were landed. Many cargoes of timber from the New World and from Europe had been largely destroyed in the ships' holds before they reached port.
In 1759 shipbuilders along the Thames were asked to give an opinion on
the comparative durability of English and French ships. They concluded
that the English 'ship of war' should long outlast the French. By the
beginning of the nineteenth century, however, the situation had reversed.
Matters came to a head in 1810 when the second HMS Queen Charlotte was
launched at Deptford. Close examination revealed that all her upper-works
('ends of the most of the beams, carlings, and ledges, the joinings of
the planks etc.,') were infected with 'the dry rot'. The situation was
investigated by A. Bowden of the Navy Office who published his conclusions
in 1815 under the title 'A Treatise on the Dry-Rot'.
[Continued...]