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Dry rot - with an attitude?

For some time now there have been reports of significant failures in so called 'dry rot treatments' - by this we mean masonry irrigation/sterilisation. Following treatment dry rot has reported to have gone wild, popping out of the irrigation holes and growing furiously over and through supposedly treated material. There are many horror stories of comebacks and retreatments of supposedly sterilised areas; some have cost £1000's, and even expensive litigation. And, of course, this always happens on the most expensive prestigious jobs. Murphy's Law in action again!

But why? A super strain of dry rot, mighty rot, faulty chemicals or what? Well, actually this phenomenon is not new. It's been occurring as long as masonry treatment have been in use. However, the industry was not so knowledgeable as it is now, and the public not so critical and litigious minded. Probably they accepted the contents of the dry rot guarantee that it was only valid if the building remained dry (and as rot will not attack dry timber then what was the value of any chemical treatment - none!)



Dry rot treatment??
So what is 'dry rot treatment'? One thing it certainly isn't and that is masonry sterilisation! Dry rot attacks wood - it only uses masonry as a medium through and over which it can grow - and then only if the substrate is damp. Dry rot will not grow over and/or through truly dry materials. Therefore why do we dry rot mycelium over wallstill pursue the practice of masonry sterilisation? Basically it is historic, past down from the days when people thought that if you threw enough chemical at the rot then it would kill it - the mentality was that only chemicals could kill the growth. But by research and a lot of common sense it was clearly demonstrated what was already obvious - rots need water to initiate, grow and survive - without it rots will not develop and they will die!

Masonry sterilisation became and always remained fashionable because it is supposed to kill the mycelium within the masonry thereby stopping it spread to other timbers. But the mycelium will only grow and spread if it is fed - no food, no grow - and there must be water present. Clearly, the current (and indeed historic problem) is that the mycelium is not being killed - and if it continues to spread then there is a food source. The top photo shows dry rot growth over a wall following masonry irrigation - the lower photo shows the irrigation holes found beneath the sheet of mycelium as illustrated by the sticks.

The chemicals used are not a problem; most have been laboratory tested, and under these conditions they are shown to be very effective. But a laboratory is not a site where a full wall, etc, has to be treated. In practice, however, this is not possible; the fluid does not saturate a wall, large areas remain untreated. Furthermore, the volume of fluid required to fully saturate a wall is enormous: this problem is discussed in BRE Digest 299. "Dry rot; its recognition and control". The other irrigation drill holesmajor problem is that the volume of sterilant injected may even cause more damage than the dry rot itself. As stated above walls require a huge amount of liquid for full irrigation, and this has got to dry out. This significant wetting of masonry has, in many cases, caused considerable damage, sometimes more than that caused by the dry rot.


'Toxic boxes' and spray:
So let's talk sense about masonry sterilisation. If there is a requirement for a treatment and one doesn't feel yet that one can give up irrigation then a far better practice is to form a 'toxic box' where perimeters and surfaces of walls are treated, hopefully to prevent it from breaking out from the treated 'box' and spreading to adjacent timbers. This is the most extreme form of treatment that will ever be required. But remember, rots will only grow if they have food (wood) and it is damp!

In most cases, however, surface spray will suffice, and if the face of a wall is to be cement rendered it then don't even bother with that; the high alkalinity of the new cement will prevent dry rot from going near it. Furthermore, the render will be far too dense for dry rot hyphae to penetrate. As far as sterilisation goes, one world renowned expert on dry rot is quoted as saying, "masonry irrigation works best where it is not needed" that about sums it up. Quite simply, you're better off to just leave the masonry and concentrate on the real control processes, preventing water ingress, drying out efficiently and preventing wood from becoming damp.


So back to the failures. In very broad and general terms, and this is very much a hypothesis, most recent dry rot fluids are micro emulsions containing a normal solvent soluble fungicide suspended in water. It is possible to some extent that these materials are not taken into the fungus so readily as truly water-soluble materials. Indeed, there seems to be a greater degree of success with water-soluble materials than the micro emulsions. I am personally aware of one case where the mycelium kept emerging from a wall and the investigator couldn't find a food source. Irrigation was tried twice with no success, even full irrigation. Then it was noted that a borate timber preservative gel was slowly dripping from some timbers recently treated above. The borate had dripped down the face of the wall and this was simply brushed into the surface to clear it up. But to everyone's amazement the growth never returned! This perhaps does support the hypothesis above with reference to water-soluble materials, and it is well-known that water-soluble borates are extremely effective fungicides, especially when formulated with glycols.

So it is highly unlikely we have a 'super strain' of dry rot; it is just we continue to rely on materials whose mode of application and on site efficacy maybe wanting. Mass application of chemical is not the treatment of dry rot - good building practice is: this is described in the BRE Digest. Yet still much reliance is put on masonry irrigation which is laughingly called 'dry rot treatment' probably because one still hopes to rely on that probably unfair guarantee clause with reference to keeping the building dry. And all this risk with the potential of more expense for only a small percentage of the price as irrigation fluid. The real control lies in good building practice including building hygiene, and the proper and selective use of preservatives.

Nevertheless, there may be times when some reliance must be placed on chemical treatment. In these cases one must be very selective on what and how the fungicide should be applied, and where. If, after consideration, some at masonry treatment is deemed necessary then choose the right material - probably purely water-soluble.


The conclusions:
To conclude, there are no 'super rots', the basic problem in the control programme is that too much reliance is placed on chemical treatment, especially masonry treatments, rather than good building practice. Where you may have to rely on preservatives then it is necessary to carefully target the treatment and choose and apply them wisely. If the industry continues to rely heavily on chemical treatments then risks of failure will continue to be high.


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