Dry rot, a timber decay fungus whose environmental requirements are mostly not significantly different to those of any other decay fungus, has gained an extraordinary notoriety and mythology. Now, as we begin the 21st Century, it is a good time to reappraise dry rot and its treatment, so that we don't carry one century's disastrous habits into the next. If we are to do this properly then it is vital that we understand how our current attitude to the fungus has developed.
In 17th century Britain most structural timbers were fashioned from oak and a few other hardwoods. If these timbers stay wet enough for long enough then they rot. The surfaces generally became fibrous or lint-like and the damage slowly spread through the timber. This decay was believed to be due to wind and rain acting on the surfaces and people called it 'wet rot' or 'common rot'. Towards the end of the 18th Century, construction practices changed for a wide variety of reasons, and softwood timbers (from coniferous trees) came into prominence. They were used as joists and roof timbers in the now fashionable brick buildings, and the navy used them in enormous quantities in their ships.
With the prominence of softwoods in an enclosed environment came an increase in one particular type of decay which proved to be an enigma. The damage seemed to start from the center of the timber, and to leave a sound outer skin, while the wood itself broke into little cubes. This was 'obviously' not caused by an adverse environment acting on the surface. Those concerned eventually concluded that it was caused by the fermentation of 'natural juices' which became destructive when the tree was felled, and, because the damage did not appear to be caused by water, they called it dry rot. Even then a few persons claimed that fungi caused both wet rots and dry rots, but their opinions were ridiculed or disregarded.
It is easy, now that timber decay is a firmly established science, to understand what had happened. Their wet or common rot is our 'white rot'. These rots are caused by fungi which may attack all the structural components of the wood, but tend to preferentially destroy lignin, a substance which contributes to the woody strength of the cell walls. They are generally more common in hard woods than in softwoods. The 18th century concept of dry rot describes any of the 'brown rot' fungi. These fungi, which are common in softwoods, are unable to utilise lignin so that it remains as a fragile brown matrix that cracks into cubes as it dries. Brown rots, as a group, tend to prefer drier conditions than white rots, and damp building timbers make a good habitat.
Brown rot fungi must always have occurred in the British Isles, but they did achieve a massive notoriety during the late 18th Century when naval ships sank before they could be commissioned. The fungi had not changed, but the use of softwoods, and the environment within which it was placed, had. Several books were published on the subject, and a gold medal was offered by the Royal Society of Arts to anyone who could demonstrate a cure. However, the cure remained elusive because the problem was not understood, despite the appearance of a book, which correctly identified the causes, as early as 1797.
During the 19th Century, decayed timber was generally replaced, if it was noticed at all, and the problem was eventually accepted as a consequence of poor building or poor maintenance which could be avoided with care. The term 'dry rot' still encompassed any of a wide array of brown rots which appeared in buildings, although towards the end of the century it tended to be used more for those species which produced extensive visible growth.
The 20th Century brought with it a new set of problems. The supply of wild-grown pine trees dwindled and was replaced with plantation-grown material possessing a greatly reduced durability. The First World War used up vast timber resources, and the wood imported from Europe during the inter-war years was generally poor and frequently of grossly inferior quality. This coincided with a rapid expansion of mediocre building using poor quality materials. The Second World War capped these difficulties by a nation-wide and unavoidable neglect of maintenance, augmented by a range of actions which included blocking ventilation to exclude gas attack, and piling damp sandbags against walls to reduce the consequences of explosions. Under damp, humid conditions and a generous supply of suitable timber the fungi proliferated, and when normality returned, timber decay was rife.
Our story of the identification of dry rot commences again in the inter-war years. W P K Findlay of the Forest Products Laboratory observed that one strand-forming brown rot fungus (Serpula lacrymans) grew remarkably well under the prevailing conditions and this he considered to be the true dry rot fungus. All others were relegated to the wet rots, thus obscuring the origins of the terms. From this point in our story the term 'dry rot' refers to Serpula lacrymans alone.
It is important to emphasise that the name 'dry' rot is historical and does not indicate a reduced requirement for water except that brown rots, as a wide group, tend to require less water than white rots. Cellar rot for example (also a brown rot but now included by Findlay within the wet rots) has very similar moisture requirements to dry rot. Dry rot like any other decay fungus needs large quantities of water for a prolonged period of time before it can destroy timber, and is, itself, destroyed if the source of water is removed and the structure dries. If dry rot is dry, then it is dead, but it doesn't disappear, and often it will still be enthusiastically treated with fungicides even though treatment is entirely unnecessary.
Dry rot is ideally suited to timber in buildings because of a dietary requirement for calcium. Mortar and plaster are rich in calcium, and the fungus will readily attack adjacent damp timbers. The fungus produces strands (known as 'mycelium') as it develops, which spread through walls and over inert surfaces, but this characteristic is not unique to dry rot. Many other fungi produce strands and these may also be found within walls. This is important, because any structure that is perceived to be unique in nature tends to attract adverse speculation. The function of the strands is to conduct the products of timber decay around the fungus so that it can grow and spread. Contrary to popular belief, they do not wet-up dry timber to make it suitable for attack.
The presence of fungus strands deep within a wall suggested that some form of whole-wall treatment is required if the fungus was to be killed. During the inter-war years the surface application of heat with a blow-lamp became popular, and when this was shown to have no effect the blow-lamp was up-graded to the oxy-acetylene torch. Unfortunately the surface of a moderately thick brick wall has to be brought to near vitrification before a temperature lethal to the fungus is achieved at its core. The treatment was found to be more dangerous than the fungus. Then in the early 1950s J Bayliss-Butler, professor of Botany in Dublin, perceived the advantages of the newly invented masonry bit and 'wall irrigation' with fungicides was born.
The 193945 war had not only caused a massive spread of the fungus, it had also allowed a substantial growth of the chemical industries. Pesticides were cheap, plentiful, and considered to be safe. Fungicides could be pumped into holes drilled into the walls in order to kill the fungus, and a remedial industry began to develop in order to provide this and other decay treatments as a service. The 'full dry rot treatment' had come of age, including the removal of all infected timber within one metre in each direction of any visible growth, as well as the irrigation of walls. The result was massive destruction, which far surpassed anything of which the fungus was capable. In the 1960s J G Savoury of the Forest Products Laboratory suggested that the treatment seemed to work best where it was not required. He showed that walls were rarely homogeneous, and that saturation with the fungicide was rarely achieved. The industry took no notice, and wall irrigation is still an integral part of most dry rot treatments.
The purpose of this historical sketch has been to show that dry rot may differ from some other decay fungi in its secondary dietary requirements, but it does not do so in its requirements for water. Confusion resulted because the origins of the term 'dry' became obscured and misinterpreted. Once this is accepted then it is clearly of primary importance to remove the source of water and to dry the building. This is not achieved by pumping the walls full of water-based chemicals, making them far wetter than they were to start with.
The fungus may continue to grow a little after water penetration is halted because the walls will take a time to dry, but it will not rampage into dry areas of the building. If there is an obvious previous repair and the walls are now dry then the fungus will probably be dead. Exposure work may need to be extensive if the damage is extensive, but more frequently the fungus is contained around a readily identifiable fault and only the minimum of treatment (if any) is required.
Where dry rot is found, the best advice we can offer is to seek at least three specifications and quotations from remedial companies, and if these all seem excessive or unconvincingly justified then seek independent advice.
Dr Brian Ridout MA, PhD -'An introduction to timber decay and its treatment'