Dry Rot: What’s in a name?
by Dr Matthew Green
Two main forms of timber decay were recognized in Bowden's day, and these were common or wet rot and a relatively new phenomenon they called dry rot. We still use the same terms but our meaning is rather different, and it is important that we understand this difference.
In the eighteenth and early nineteenth centuries wet or common rot was seen as a form of decay which progressed inwards from the surface of the timber and was caused by the actions of wind, heat and water. The damage was thought to be chemical or mechanical. The resulting modified timber was considered particularly suitable as a substrate for fungus.
The connection between the dry rot and water was not made for a long time because the decay was seen as progressing from the inside of the timber outwards. The concept of water being absorbed and held within timber resulted in fungal decay was not grasped until later.
It is clear that what the earliest investigators termed ‘dry rot’ was in fact brown rot, and common or wet rot was white rot. The application of two different names is understandable when we consider that the two forms of decay do not look alike, and progress in a different fashion. The term was used because the damage was thought to be caused by internal ‘fermentations’ rather than water. It is also important to note that the term 'dry rot' was used for all brown rots, and would therefore have encompassed a wide variety of fungi which we would now consider to be wet rots (e.g. cellar rot).
Photograph 2: Water is a requirement for fungal decay, including dry rot.
The fact that fungi caused the decay rather than just living on it was not firmly established until work by the German botanist Robert Hartig was published in 1878, although the suggestion had been made as early as 1803. Considerable research into decay fungi ensued, not all of which was relevant to practical building situations. The conclusions reached, however, stay with us and have been woven into the mythology of dry rot.
Gradually throughout the nineteenth century the term became restricted to fungi which produced substantial mycelium, and even during the first half of the twentieth century there was a tendency to include the mine fungus Antrodia (Fibroporia) vaillantii as a dry rot. Eventually, however, the name referred to only one fungus Serpula lacrymans, and this became known as the 'true dry rot'. All other decay fungi (brown and white rots) were lumped together as wet rots. The ‘true’ prefix was gradually dropped towards the end of the twentieth century, although some literature still cites it as such.
The term ‘dry rot’ has come a long way from describing the fermentations of sap in eighteenth century ships’ timbers. Work undertaken by the Forest Products Research Laboratory for the production of a British Standard in 1963 perceived the difference between wet rots and (‘true’) dry rot was that the strands of dry rot had the ability to grow through walls and over inert surfaces. In contrast, the strands of wet rots (those that produced them) did not. Dry rot could therefore spread through the building where conditions allowed, whilst wet rot decay remained localised.
This is not the case: many species of wet rot will grow over or through
walls. However, the separation of S. lacrymans
from other building decay fungi and from other brown rot fungi has resulted
in its inadvertent and quite unfounded status as a unique organism capable
of quite the most biologically impossible feats. Coupled with a commercial
treatment industry that is only too happy to pander to this unabashed
scare-mongering in order to sell fungicide it has come to be feared by
homeowners up and down the country.
[Continued...]