DRY ROT – ARE YOU TREATING PROPERLY?

The mass irrigation of masonry adjacent to a dry rot outbreak is still commonly used by very many contractors and one has to ask why? For years BRE Digest 299 and other authoritative publications have cast doubt on this technique because the benefits are questionable and it brings with it other significant attendant problems. The primary control measure for dry rot is to identify and eliminate the source of moisture that caused and is sustaining the outbreak and then to promote rapid drying. Why therefore is it considered sensible to introduce gallons and gallons of extra water into walls in the form of a waterborne masonry fungicide thereby considerably delaying the drying down process - perhaps by years?

'Dry Rot Treatment' – What does this term really mean?
 
The term ‘Dry Rot Treatment’ usually means the treatment of masonry in which dry rot is considered to be present with a biocide by a process commonly referred to as ‘masonry irrigation’. Masonry irrigation means drilling holes into a wall, usually in a 230 mm diamond pattern and the injection into those drill holes a masonry biocide. How much is injected or is deemed to be necessary it not usually known!
 
However for masonry irrigation to be 100% effective total saturation would have to be achieved but if this were to happen the damage caused by introducing this volume of fluid into a wall would probably be greater than that caused by the dry rot! It would almost certainly take years for the wall to dry down and it would cause considerable surface efflorescence of salts containing a biocide, which is a hazardous waste. Subject to the porosity of the masonry, the amount of masonry biocide required to totally saturate one square metre of solid 9’’ brickwork is between 70 and 110 litres and for one square metre of 13½’’ brickwork between 100 and 160 litres! If anything less than these volumes is being used then 100% effective mass irrigation is not taking place –only the motions are being carried out. But for the reasons already mentioned please do not permit this volume of material to be introduced into a wall! There is perhaps some psychology attached to the sight of walls being drilled and fungicide being injected since it looks and sounds impressive!
 
An important point to realise however, in conjunction with other Dry Rot control procedures, anything less than total saturation when dealing with a well established mature Dry Rot outbreak is likely to cause rapid stress induced growth of Dry Rot in the short term. This usually shows itself as the formation of small Dry Rot fruiting bodies on the surface of masonry despite it having been treated with a masonry biocide. Contractors sometimes panic at this sight and think that their 'Dry Rot treatment' has not worked but since they are unlikely to have achieved 100% saturation (thank goodness!) it is to be expected. Rapid stress induced growth should be interpreted as a positive sign since the fungus has detected that its future survival is at risk and it naturally seeks to reproduce itself before its eventual demise. The use of Borate (Boron) based materials significantly reduces this phenomenon.
 
Why is ‘Mass Irrigation’ still used?
 
Efficient surface applied masonry biocides replaced the hazardous ‘blowlamp surface sterilisation’ procedure over thirty years ago but despite this it is still not unusual to see blowlamp treatment popping up in specifications every now and again. Drilling a wall to enhance the penetration of the biocide was a natural progression and no doubt commercial practice played its part in establishing this technique in what many still regard as the ‘norm’ when dealing with a Dry Rot outbreak.
 
It is also true to say that a large proportion of operative and surveyor training in the preservation industry is conducted in house thus it is inevitable that bad or usual practice, which has been ‘the norm’ for years, is handed down without it ever being questioned. Another reason that no doubt influences the decision to use this process is that it is something substantive for the specialist contractor to do who may be required to issue a long term guarantee.
 
‘Toxic Box’ – ‘Cordon Sanitaire’ – What do these mean!?
 
The preferred course of action when dealing with masonry adjacent to a dry rot outbreak is a thorough surface application of a masonry biocide - preferably a water soluable one. However there are circumstances when it is impractical to protect adjacent areas during the drying down process and in such situations the use of  what many refer to as a ‘Toxic Box’ or ‘Cordon Sanitaire’ is justified.  These terms are often incorporated into specialist contractors reports but the reality is that they probably mean very little to those outside the industry. So what do they mean? Toxic Box means a four sided containment and Cordon Sanitaire means one, two or three containment barriers
 
Both of these treatment processes attempt to achieve  the same - the containment of dry rot mycelium within an area of  wall by the introduction of a masonry biocide through the full thickness of a wall. This would be  combined with a surface application of a biocide over the area contained by the perimeter. The purpose being to contain any viable fungus remaining within the masonry whilst the infected area dries down thereby protecting adjacent unaffected areas. Toxic is an emotive word and since non of the masonry biocides designed for use when dealing with Dry Rot are classified as toxic thus it is prefered to see the term ‘Perimeter Box Treatment’ used. The advent of water soluble masonry biocides with differing levels of active ingredient have now made this treatment technique very effective indeed.
 
Perimeter  Treatment Procedure:
 
It is our opinion that the most effective material to use for perimeter box treatment in masonry is a Borate based material, commonly referred to as ‘Boron’. Borates are water soluble which is a distinct advantage when dealing with moist fungi in a damp environment. There is the added benefit that if ever the masonry were to become damp again in the future the Borates would dissolve again and significantly disrupt the ability of Dry Rot to use the treated masonry as a means of spreading. We have suggested a perimeter box treatment procedure below that assumes Borates will be used. Please check that the material you intend to use has the appropriate Health and Safety Executive (HSE) clearance for this purpose. Be aware that not all Borates and Borate Gels are cleared for use on/in masonry. At all stages the appropriate personal protective equipment must be worn. Fully read the label of all pesticides before use and only use in accordance with the instructions.
 
Perimeter Treatment Preparation:
 
Use a surface moisture meter and visual appraisal to ascertain the area of wall in which it is believed viable dry rot fungus is present – this will define the perimeters of the  treatment procedure. From the wall area to be contained within the designated perimeters hack off existing plaster back to the masonry substrate and clear away resultant rubble. Wearing goggles and a dust mask use a stiff brush on the exposed masonry surface to remove any remaining dry rot growth and loose residual surface plaster/mortar.
 
Perimeter  Treatment Drilling:
 
In conventional brickwork along a horizontal perimeter use a 10 mm drill bit and drill two rows of holes at 70-mm horizontal centres into two adjacent horizontal mortar beds to within 30mm of the opposite face (28/30 holes per linear metre). For vertical  perimeters again drill two rows of holes at 70 mm horizontal and vertical centres which in conventional brickwork is the top and bottom of a vertical mortar joint (perp) and 70 mm to the side of this. For other building materials try adhere to this drilling pattern as best able.
 
Perimeter Box Treatment Hole Filling:
 
Have available on site a ready to use Borate solution or dilute a Borate material in accordance with the mixing instructions to form its ready to use state. This type of material is usually a 10% Borate solution. Thoroughly flush out each perimeter drill hole using a 10% Borate solution. Whilst the drill hole is still damp/wet following flushing insert the nozzle of a Borate Gel cartridge (usually a 50% Borate material) into the full depth of the drilled hole. Gently squeeze the trigger of the skeleton gun and slowly withdraw the nozzle to completely back fill each perimeter drill hole with Borate Gel. Extension nozzles are necessary for this operation. Do not make extension nozzles any longer than is necessary as this creates back pressure and may make the material more difficult to exude. Make certain that each hole is still wet/damp prior to the introduction of Borate gel. If necessary repeat hole flushing prior to introduction. Be aware that the glycol used in the manufacturer of Borate Gels readily absorbs moisture. On occasions this will cause an initial swelling of the Gel causing a small amount to run out of the hole prior to it diffusing into the substrate. Surface efflorescence is to be expected.
 
Perimeter Box Surface Application:
 
Apply to the surface of the exposed masonry contained by the perimeter drilling a 10% Borate solution using a coarse low pressure spray. The usual recommended application rate for this procedure is in the region of 1.5m² per litre. Always read and adhere to the recommended application rate stipulated on the label of the material being used.
 
Never rely on ‘Dry Rot Masonry Treatment’ alone
 
It should always be remembered that the use of masonry biocides when dealing with a Dry Rot outbreak is  a secondary measure. It is almost certain that if used on its own such treatment will very probably not eradicate a Dry Rot outbreak. The single most important control factor for dealing with a Dry Rot outbreak is to identify the source of moisture that caused it, rectify that cause and promote rapid drying. Refer to BRE Digest 299 ‘Dry rot: its recognition and control’ for greater detail about dealing with Dry Rot.
 
P. Mac PGA 06.07

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