Dry rot treatment refers to techniques used to eliminate dry rot fungus and alleviate the damage done by the fungus to human-built wooden structures.
Dry rot (Serpula lacrymans) is considered difficult to remove, requiring drastic action. Remedial timber treatment and damp proofing companies typically recommend stripping out of building fabric beyond the visible extent of the infestation and the use of fungicide. More holistic approaches attempt to eradicate dry rot by controlling the local environment to reduce timber moisture levels and increase ventilation in order to promote drying.
The first priority when treating dry rot is to find and remove the dampness within the building that caused the outbreak, and to promote drying out by taking measures, such as increasing ventilation. Treatment approaches differ after these steps are taken.
Mycology of S. lacrymans S. lacrymans is a form of brown rot, a group of fungi which digest the cellulose and hemicellulose in timber. This particular species poses the greatest threat to buildings since it can spread through non-nutrient providing materials (e.g., masonry and plaster) for several meters until it finds more timber to attack.Palfreyman, J. W. & Low, G. (2002) "Studies of the Domestic Dry Rot Fungus Serpula Lacrymans With Relevance to the Management of Decay in Buildings", Edinburgh, Historic Scotland
Dry rot is spread by spores which are present in most buildings. The minimum moisture content of timber for spore germination is 28–30% (lower than other rots), and the relative humidity must be in excess of 95%. Spores are resistant to desiccation and may still be viable for germination when they are several years old.Ridout B. (2000) "Timber Decay in Buildings, The conservation approach to treatment", London, Spon
If conditions are suitable, the spore will germinate producing microscopic fungal threads called hyphae.
Minimum Optimum Maximum Temperature (°C) −5–+5 15–22 30–40 Timber Moisture Content (%) 22–25 20–55 55–90
Table 1. Environmental conditions for mycelial growth following germination
The average moisture content of modern softwood timbers in dry buildings is generally in the range of 12–15%, and heating systems may reduce this to a far lower level. There is, therefore, no prospect of a dry rot infestation developing in a building that has been properly designed, constructed, and maintained.
Once the environment starts to dry out, the rot will become dormant and eventually die. The length of time that the fungus can remain dormant under dry conditions seems to depend on the temperature, with approximate times of nine years at 7.5 °C and one year at 22 °C being quoted.Building Research Establishment (1993) BRE Digest 299 "Dry rot: its recognition and control", Watford, BRE
Most fungi will only thrive in acid conditions, but dry rot will remain active in alkaline conditions. This gives it the ability to grow through damp mortar, masonry, and plaster, infecting other areas of the building.Ridout, B. (1985) "Dry Rot an Alternative Approach". Architect’s Journal. 18 September 1985, pp. 69 – 72 Thick, conducting strands known as rhizomorphs, are produced to cross inert surfaces and penetrate masonry.
A fruiting body (sporophore) may develop naturally or in response to unfavourable conditions of humidity, temperature or exhaustion of nutrients.Lloyd, H. & Singh, J. (1994) "Inspection, Monitoring and Environmental Control of Timber Decay". In: Building Mycology – Management of Decay and Health in Buildings, ed. J. Singh, pp. 159–186 London, Spon Often the stress that provokes this is the exposure of the infestation. The fruiting body will produce millions of rust brown spores.
One belief held about dry rot is that, once established, it can survive by producing water by breaking down the timber if the original source of water is removed. Laboratory experiments conducted in 1932 on timber samples in unventilated glass jars showed that significant quantities of water were indeed produced. However these experiments do not replicate the "real world" environment of a building where processes of evaporation and capillary action in the wood will be removing moisture from the area faster than it can be produced by the fungus.
Another misconception is that dry rot can transport water from a source over considerable distances to an area of dry timber so that it can decay there.Berry, R. W. (1994) Remedial Treatment of Wood Rot and Insect Attack in Buildings, Watford, BRE While the mycelium strands do conduct a nutrient solution around the fungus, it has been shown that any ability to transport water to "wet up" dry timber is limited.
Despite being a successful coloniser of buildings, for reasons that have not yet been fully explained, S. lacrymans is rarely found growing in the wild. Occasional specimens have been found in the foothills of the Indian Himalayas, Mt. Shasta in California, and woodlands in Czechoslovakia.
Treatment methods
Introduction
The first step in any course of treatment is to make the necessary repairs to the building defects (overflowing gutters, blocked airbricks, missing slates, etc.) that allowed the ingress of dampness. The treatment methods described below assume that the dry rot has been positively identified, the full extent of the rot ascertained, and that the building is now water tight.
A number of methods of attacking dry rot have been developed which can be classified as follows:
Orthodox – emphasis on the use of chemical fungicides Environmental – emphasis on controlling the fungus by controlling environmental conditions Heat treatment – exploiting the fungus' sensitivity to heat Biological treatment – use of competitor organisms
The latter two methods are included for completeness as they are currently not widely used. The main purpose of this article is to compare the orthodox and environmental approaches.
"Orthodox" treatment for dry rot The following description for the treatment of dry rot is typical of traditional methods:Hicken, N. (1972) The Dry Rot Problem 2nd Edition, London, Hutchinson & Co.
Cut out all wood showing decay, presence of white mycelium, etc. and all apparently sound timber within a radius of one metre of the nearest visibly decayed timber. Burn all such material. Hack off all plaster/render and remove any skirtings, panelling, linings and ceilings necessary to trace the fullest extent of the growth over or through adjacent masonry, concrete or timber surfaces. Clean off with a wire brush all surfaces and any steel and pipe work within the area up to a radius of 1.5 metres from the furthest extent of suspected infection. Remove from the building all dust and debris ensuing from the work. Apply fungicide to all such masonry, concrete and earth surfaces at the specified rate. Apply two generous coats of fungicide to all timber surfaces to a distance of 1.5 metres from the cutting away. (Allow first coat to be absorbed before applying second coat) Use only fully preservative-treated timber for replacement. Replaster with zinc oxychloride (ZOC) plaster or, for areas not to be replastered, apply two coats of ZOC paint.
As can be seen from stages 1 and 2, this involves the removal of a considerable quantity of building fabric.
The desire to kill the fungal strands within all materials adjoining the affected timber has led to the practice of "wall irrigation" at stage 4. This entails saturating the masonry with a water-soluble fungicide at a rate of about 10 litres/m3. Walls of more than half-brick thickness need to be drilled at spacing to a depth of just over half the wall thickness. Walls of over thickness should be drilled from both sides. Fungicide is then injected into the holes, and the wall surfaces are sprayed.Shaw, G. et al. (1986) "Construction Risks and Remedies Timber Decay Part 2". Architect’s Journal. 15 October 1986, pp. 69 – 83
Ensuring thorough penetration of fungicide throughout the structure of a non-homogeneous wall is extremely difficult. "There is no practical way of ensuring that all dry rot strands within a wall are killed."Hilditch, E. A (1994) "Chemical Control of Fungal Decay in Buildings". In: Building Mycology – Management of Decay and Health in Buildings, ed. J. Singh, pp. 212 – 238 London, Spon
A more recent variation of the practice of wall irrigation is the "toxic box" where the area of irrigation is reduced to forming a margin around the perimeter of the wall thereby containing the fungus within the wall. Here it can do no harm and will eventually die for lack of food.
Application of fungicides to the timber at stage 4 may be by brush, spray or injected under pressure into holes drilled into the wood. Preservatives based on organic solvents are used as these have better penetration into wood than water based solutions. Examples of suitable organic solvents include the following: (--- please provide examples)
Alternatively pastes consisting of a fungicide in an oil/water emulsion may be applied to the timber.
In addition to more conventional fungicides, boron-based fungicide can be supplied in glass-like rods which are inserted into holes drilled into the wood. Boron is commonly available as the laundry supplement Borax, and as boric acid which is commonly available from pharmacists or in cockroach pesticides. Boron rods are soluble and should the timber become damp the rod will gradually dissolve diffusing preservative into the damp area. Their use is particularly appropriate for areas that are at risk but not yet affected.Coleman, G.R. (2005) "Dry Rot and its Control [Internet], Remedial Technical Services". Available from: www.buildingpreservation.com [Accessed 18 January 2008] A surfactant such as dish soap, is recommended in water-based preparations.
It has been reported that boron fungicides react with the cellular structure of the wood whereby the boron is deposited, and it is this process which can to varying extents harden the dry rot, depending on the degree of degradation of the wood. However, structural members should be remediated by sistering in new wood after the fungus issue has been dealt with.
Boron/glycol preservatives consist of an inorganic boron preservative dissolved in a glycol to produce a paste. These are water-soluble and will readily diffuse into damp wood, even from the surface, and therefore offer better penetration than more conventional fungicide products, where it is necessary to penetrate damp wood.
Glycol solutions have an advantage in that they can be applied over paint.
Glycol and boron solutions are hydrophilic (water-loving) and react with the water in wood, making it unavailable for the fungus. This is why dry rot can seem to be a little stringy; cells grown in drier periods are smaller than the larger, "plumper" cells that grow in Spring, as can be seen in tree rings used to tell how old the tree is. This moister Spring growth contains the moisture the fungus consumes, in addition to that available from leaks for example.
Water-based fungicides, because they are water-based, can be washed away in time if the wood they are applied to keeps getting wet. This is another reason why it is important to fix leaks, thus keeping the wood dry in an alkaline environment, and seal the wood (especially the end grain) to prevent ongoing exposure to hungry spores.
Fungicides to defeat brown rot include: baking soda, hydrogen peroxide, tea tree oil, boron solutions, ethylene glycol or propylene glycol, vinegar, etc. Since the dry rot fungus requires an acidic environment from pH 0 to 5.5, certain of these fungicides work because they change the pH.
Recipes for homemade fungicide solutions, wood hardeners and penetrating epoxies have been reported on the web in addition to commercially available products. (It would be appreciated if you could improve this page by providing information about tests showing the efficacy of different treatments.)