DRY ROT DEVELOPING A LESS DISRUPTIVE APPROACH TO DIAGNOSIS AND TREATMENT
(Presentation to Glasgow West Conservation Trust Masterclass Series)
DRY ROT what is it?
The term dry rot is a mis-nomer in that it implies that there is decay without moisture. This is simply not true and it should be clarified that all wood decay requires moisture. Wood decay is correctly described as of three different types BROWN ROT WHITE ROT SOFT ROT each decay type differs in the way in which the breakdown of wood takes place. This represents
major differences in the enzyme processes by which wood decay is accomplished.
Dry rot and most wet rot decay in house timbers is of the brown rot type. The brown is attributed to the colour of the wood that remains after decay has taken place.
Essentially what is known as dry rot is the damage caused by one particular fungus Serpula lacrimans
A fungus is a type of plant. They differ from other green plants like grass and trees in that
fungi are unable to manufacture their own food. So they rely for food on what they find all around. They live on other dead plants o r decaying plant and animal matter Biologists call them saprophytes. They are very important to us they help to recycle carbon which is fundamental to continuity of life on our fragile planet. Wood decay fungi of which the dry rot fungus is one are essential in this recycling. Wood decay and the growth of the dry rot fungus must be seen in this context.
Wood decay outwith buildings is a rapid process. On the forest floor fallen trees and other
organic materials are quickly broken down by a wide variety of bacteria fungi and insects. Part
of man's success in the world has been his ability to
control this process. It is of benefit to man
to control the process when the value to him of the commodity the log is greater than that of the foodstuffs contained within. This is the way a biologist might look at the decay of wood in a house. Wood decay does not normally take place within the house because conditions do not permit it. The wood is generally kept dry
and well ventilated. The majority of decay organisms
will not grow to any significant extent under such circumstances.
We can see therefore that conditions are are important so what are the conditions for dry rot development ? and of course what therefore are the conditions for dry rot prevention ?
The basic growth requirements of the fungus are similar to most living organisms
· Source of inoculum
· Acidity / Allkalinity of substrate
Wood decay in buildings can be initiated by the germination of spores or by the vegetative development of hyphae.Spores are always present in the air. Hyphal fragments can be spread around during repairs. Hyphal invasion from an adjoining affected area can introduce decay . Decay can regenerate from bits of fungal hyphae left in wood or in walls so long as the conditions are right for growth.
From the point of view of the fungus wood has been described as a series of nicely orientated holes surrounded by food.
So in a building there is plenty of it. The food is accessed by the fungus by means of a complex biochemical process-
which I have to tell you is not yet fully understood.
Moisture is a key requirement and is necessary for both spore germination and hyphal colonisation.
Oxygen will normally be present within air, so is not a limiting factor for dry rot
Growth will cease outwith the temperature ranges the fungus can tolerate. These are below 3
and above 27 C. Survival of
the fungus is well beyond these temperatures. The lower limit is uncertain, however it is well established that temperatures above 50 C have been shown to be lethal
The substrate supporting growth must be at a suitable pH (that is a level of acidity or alkalinity.) The fungus can alter this
to some extent after growth commences.
There is potential for control of fungal growth in the management of any of the growth parameters. It is interesting that all strategies for decay control until recently have focused upon chemical poisoning.
There is a wealth of published literature on the growth and development of the dry rot fungus. I wish to highlight the factors I consider to be of fundamental importance in its control
Wood will not be colonised by the dry rot fungus if the moisture content is lower than 20 % The optimum moisture content
of wood lies between 30 and 40 % These are average figures and have been derived from studies in the laboratory. It is also true that more moisture is required for spore germination
than to sustain hyphal growth Once established in wood fungal hyphae
may survive at moisture contents below those required to initiate growth. The moisture relationships
between the fungus and wood are complex and made increasingly so by the fact that the situation is rarely static in
buildings where moisture contents (both of wood and of air humidity) can fluctuate. The consumption of wood by the
fungus also produces moisture which may have the effect of increasing the available moisture and thus bringing up to a suitable moisture content for decay wood which was below the
threshold. This does happen but the effect is marginal and
will only be of significance in the' statically dry' situation, described by Savoury,
in which no moisture is being removed from the system. In most real life situations the
the building will produce a dynamic situation in which moisture
is removed from the system. In such cases the fungus cannot provide enough moisture to sustain itself. Some other additional source of moisture is normally required.
Those of you familiar with building structure will be aware that in our climate most building timbers that are in air dry equilibrium will have a moisture content at around 16 to 18 %. Clearly it does not take much moisture to increase such timbers to at or around the threshold for decay. Especially if there is little or no ventilation.
There are many lists of the principal sources of moisture causing dry rot in buildings in the literature. The most popular are Rainwater, Groundwater, Plumbing leaks, Condensation. I don't regard any of these lists as complete
If you can think of a source of moisture you can be sure it has caused dry rot somewhere. I remember yeas ago reading
an article about the incidence of Dry Rot in church buildings. The reporter noted that 70 % of the incidences were found in non-Catholic churches 30 % in Catholic. A curious statistic indicative of usage I think rather than Divine intervention.
The ability of the fungus to survive and remain viable in wood with low moisture content has
been shown to depend on temperature. There is little consensus in the literature on how long viability can be maintained. Viability has been demonstrated in dry wood for periods in excess
of eight years
I suggested that wood breakdown is a complicated matter. This is definitely true and I would
say that the total processes
are not fully understood. Most constructional timbers are softwoods and softwoods are broadly speaking comprised of 60% cellulose and 30 % lignin. Brown rot
decay degrades the cellulose component leaving the lignin mostly unaltered. Cellulose break
down is achieved by the action of enzymes. Recent studies into the breakdown of cellulose
have shown that the process is not entirely an enzymic one and that the enzymic process is initiated by by an acid-catalysed hydrolysis, a chemical rather than biological process This seems to modify the cellulose in a way which makes it amenable to enzymic breakdown. This is quite a significant finding and it offers an explanation for many of the observations other researchers have made regarding the growth and development of the dry rot fungus. In particular it has
been reported that Serpula will increase the acidity of a substrate across which it grows. It has
also been reported that dry rot decay is never found unless
in association with some sort of masonry plaster or brickwork and mortar. This is relevant in that the biological process the enzymic breakdown will not occur except in a fairly neutral environment i.e. neither acid nor alkaline. The alkalinity of the masonry components may be used by the fungus to generate or restore these neutral conditions and allow the cellulose breakdown the take place.
To what extent the masonry can provide an additional source of nutrient is unclear. It is likely that minerals and trace elements can be derived from this source although without wood growth will not be sustained. I know from my own experiences of timber decay in this city that the source of initiation of dry rot is very often the deafening or sound insulation between floors in a tenement which has been soaked with water. This is slightly at odds with the accepted technical literature which suggests that spore germination takes place on wood. I confess I have never carried out a full analysis of deafening to try to explain this, although I will be pleased to do so for a very modest fee.
Temperature and humidity
The consensus of reports from the literature indicate that Serpula will grow best at around 22 C. Growth will not take place
at 3 C or below, nor will growth occur above 28 C In the UK the temperature within buildings is not likely to lie outwith this range and so for practical purposes we should consider that the fungus can grow most of the year round . It is also true
that the optimum growth temperature of 21 C - 23 C is quite high and this is not likely to be sustained
for any length of time. It is however important in a situation where heat is applied as a means perhaps of drying the structure. In the
absence of adequate ventilation this will only serve to increase the rate of growth.
I mention temperature together with humidity since humidity is a crucial element and is of course temperature dependent. Very high levels of humidity are required to permit and sustain Serpula growth (Relative humidity in excess of 90%) This is one of the main reasons why growth
generally takes place in areas where we can't see and consequently we can't find it
start looking for it.
The constantly changing environments within buildings are crucial in this regard, as are the microclimate effects observed
as you move around within parts of a building.
I have read often that dry rot does not affect roof timbers to any great extent because the temperature is too high and humidity is too low. Most Glaswegians can tell you what nonsense this is. It comes about in my opinion from concentrating too much on averages and ignoring the little damp dank unseen parts these investigators fail to reach.
It is not uncommon for example and I have measured this myself to have a typical roof joist
at a moisture content of 18% sitting on a typical wallplate the upper surface of which is also at 18% the lower surface, however, is in contact with a damp wall head and decay is only revealed when the wallplate is broken away. The relative humidity at the point of contact between the wall and the wallplate is 100% although the average relative humidity a few feet away is 60%. These micro- environments are of great importance in the diagnosis and treatment of decay in buildings. Wood in contact with wet masonry or built into wet masonry is in a totally different environment and can be likened in some ways to wood buried in soil.
I would like to point out a further complication which can arise as a result of the constantly changing conditions of temperature and humidity and one I have observed frequently. It is not uncommon for more than one type of timber decay to be present within the same piece of timber. The decay may be seen as and identified as a type of wet rot due to the
of moisture, and the visible damage to timber. This may mask an earlier dry rot problem, not active at present. because the higher levels of moisture have suppressed growth. The problem will typically be dealt with by a contractor as a wet rot problem and at the same time the source
of moisture is eliminated as of course it should. It is often also true that the contractor may only be instructed to repair the roof or source of moisture - not dealing with the rot problem at all. As
the structure begins to dry the levels of free moisture fall and the humidity increases. Once
again conditions for dry rot growth are restored and dry rot develops again resulting in a costly dispute. This I believe highlights the need for a full
and accurate identification of the problems. and regular routine inspections to be incorporated as a part of general maintenance.
In summary of this it is clear that the conditions for decay to take place ought not to be present in buildings. When they do occur it is usually the result of some breakdown in maintenance.
The detail of construction is important in determining whether any particular incident will result in rot, and how severe it
might become in particular the presence of timbers built into or in direct contact with wet masonry.
The distribution of moisture within the building and the accessibility of wood for nutrition are the key elements that will determine the spread of dry rot and are the key features in its control and eradication.
SECOND SECTION DIAGNOSIS AND TREATMENT
Having looked at some of the features of the biology of this unusual fungus one might ask why it proves to be so difficult
to eradicate in the light of the apparent sensitivities.
I think it is worth spending a few minutes examining the historical development of the remedial industry since I think this goes some way towards explaining how the current customs and practices were established.
Dry Rot has existed in the UK for a long time but it is only since the second world war that it has become a specialised
niche within the construction industry
It was partly the structural damage brought about during the war that allowed so many buildings to suffer from substantial rainwater ingress. This together with the details in construction and blocking up of vents to prevent gas brought about a large increase in numbers of the buildings affected in the 1950s.
Cosequently a large number of contractors emerged to satisfy the demand for repairs. The total volume of work has
reduced since then but the awareness of the problem continues and at present there are around 1000 contractors offering this type of service.
What I find interesting is that the specialist remedial contractors have for years been considered part of the wood preservation industry rather than the construction industry, although they adopt the contractual procedures common within the construction industry. The contractors have depended for technical information on the wood preservation industry and
the suppliers of wood preservatives. The industry concerned with wood preservation is heavily dependent on the use of chemical preservatives for very good reasons. These are beyond the scope of this presentation. The information advice
and products they provide for remedial contractors are therefore based upon the use of chemical preservatives, or formulations derived from chemical preservatives.
It is true that these wood preservative formulations and various derivations have served the remedial contractors well for many years. However the dependency on this imported technology has tended to make remedial contractors dependent on a single strategy for rot eradication -
the use of pesticides - to the exclusion of all others.
Professional practitioners within the industry (remedial contractors technical or surveying staff)
see the solution to timber decay problems as a wood preservation problem where the use of
wood preservatives can be applied to building timbers to stop them from rotting, and that the fungi present in wood or in walls can be killed by treating the structure with pesticides.
Changes have come about in recent years.
In recent years legislation with respect to the choice use and application of pesticides has
become more onerous e.g. CPR COSHH etc. and it seems inevitable that this will continue and that the remedial contractors will come under a more
intensive scrutiny from the legislature, environmental groups and the media.
To date the wood preservation industry has responded by formulating alternative and generally safer pesticide products to comply with the more stringent legislation.
I believe a better response is to seek alternative ways of dealing with the problem of dry rot especially ways which minimise or avoid the use of pesticides altogether.
The procedures by which dry rot problems are currently dealt with are very much that of a small building contract. A client
(a householder an architect or property manager) will instruct a contractor to give him a report and quotation for a rot problem. Typically this may have been discovered when the piano has sunk into the floor.
Three or more contractors will visit and quote for the job. This is basically a tendering procedure except that the contractors' quotes will vary widely in scope and in price and are based on what the company's representative guessed will be the extent of the repairs and treatments necessary from his short inspection. All will be based on the standard accepted wisdom of cutting out beams stripping plaster and applying pesticide products to the walls and remaining timbers. The successful quotation might be the lowest in cost- it often is- and it may be adequate -it may not and may be followed up by a supplementary quotation to cover further areas of rot not included during the original guesswork.
A long term guarantee will be provided by the contractor and accepted by a client in good faith
in place of his own lack of knowledge or inability to obtain quality independent advice.
If professional advice is available at all it is most likely to come from someone familiar with wood preservation technology and probably trained within the industry to deal with timber decay in a conventional way using pesticide products. He will
see the problem as one which requires the application of chemical pesticides and the stripping out of timber and plaster.
He is unlikely to seek an individual solution to any individual problem
This is not good enough ladies and gentlemen The approach needs to be wider than this and the philosophy needs to
focus on the health of the building rather than the control of the disease within the structure.
The trouble is no-one can determine the extent of a dry rot problem in one short - perhaps 30 minute visit.
What can be done to change a most unsatisfactory situation?
Anyway I don't intend to dwell on the past or even the present. I believe that attitudes will
change and I believe I can
offer a better approach even now and this will be improved as the technology improves.
What we have to consider what is going on in the building and look at the building as a whole.
Dry rot needs to be regarded as a symptom of a problem with the building as a whole. A building whose internal ecology is out of balance. The approach to dealing with the dry rot should tackle the building as a whole and attempt to restore the balance of the internal ecology. It is not simply a wood preservation problem. It is not enough just to kill - or more correctly - try to kill
the fungus with pesticides.
I mentioned earlier the piano sinking into the floor might be the first indication of a problem of dry rot. It is a useful analogy and highlights that there are two considerations to be taken into account. There is the fungal problem to be
tackled and there is the structural damage it has caused. Both must be taken into account, and I will discuss them separately although they need to be tackled together.
It goes without saying that an accurate identification of the problem is a pre- requisite to any approach to dry rot. This of course seems obvious but it may amaze you how often an inaccurate diagnosis is made.
It is fairly easy to identify dry rot when confronted with large fruiting bodies and lots of cotton
wool like fungal growths.
These are the stuff of text-books and technical advertising literature, and even in many cases training literature. What is
to be found and seen within the building may bear no resemblance to these fine photographs and drawings. Such evidence that is present may be slight, not readily seen, and difficult to distinguish from other benign artefacts such as spiders webs with a liberal coating of plaster dust. A good working knowledge of the way the buildings are constructed is useful to predict where timbers are likely to be present. This is especially important in the tenement buildings we have in this city where it is common to have timbers built into the fabric.
I will describe a typical scenario.
It is very common for slight evidence of dry rot to be found within the roof. This however is often the tell tale sign of major structural decay in lintols and the floors below.
If we look at this very common example it can easily be seen how this puts both the contractor and his client in difficulty in trying to offer a quotation for repair.
The extent of damage needs to be confirmed
How can this be done ?
In exceptional circumstances a client (or more commonly his agent) having realised that a major problem might be
present, may instruct a disruptive survey . Typically in such a procedure some plasterwork will be removed to allow concealed timbers such as lintols to be inspected, and floorboards may be lifted to allow inspection of the joists below. Lintols may be drilled in an attempt to locate internal decay. This certainly helps to establish the extent of the problem especially if exposure is continued into the floors below. These procedures are disruptive and require the attendance of tradesmen. So they also cost money .
The Industry has become accustomed to providing free "surveys" for rot, and their clients have become accustomed to getting them for free. So why should anyone pay for something they can get for free.
Why indeed ?
Only I don't think what you get for free is the same thing.
What you get for free is a quotation or maybe only an estimate for a specific amount of treatment and repair work.
What is in the estimate or quote is what the remedial company's representative has guessed is the likely area requiring
the conventional strip out and replace treatment as his training has taught him.
It may be accurate and appropriate it may not but at least you have a price- to compare with another contractor's guess
Even a disruptive survey can fail to satisfy.
They are however not foolproof since the dry rot fungus can be unpredictable in the routes it selects for growth particularly within the walls. Visual inspection of timbers does not reveal
internal decay and drilling does not indicate their presence of fungal growth within the timbers which will result in decay
Is there a better way ? I believe so.
A more comprehensive more accurate and less disruptive approach is to prepare a moisture
map, and to take drillings from at risk timbers and test for the presence of fungal growth.
We discussed in the earlier section of this presentation how moisture and nutrition are the key elements in determining decay within the building. The distribution of moisture is the key to identifying the spread of fungal growth and in defining the area within which the repair specification must be applied. Hand held moisture meters are helpful in the initial stages
but the best way of measuring moisture from the point of view of timber decay is to directly measure moisture by installing probes. This eliminates the need for most of the disruption at least at the investigation stage and enables a
comprehensive repair specification to be prepared The moisture map will define the source and distribution of moisture and establish the
dry perimeter beyond which the repairs need not proceed.
Experience has taught us that after a period of about 10 days they will have come to equilibrium and the moisture content
of the wall can be measured with a conventional hand held meter.
At this stage a decision will have to be made about suspect timbers whether they should be removed or left in place.
Drilling will locate decay if decay is present but a much more precise indication of decay or fungal colonisation can be by combining drilling with analysis for the presence of fungi and fungal viability
These techniques allow a comprehensive and accurate assessment of an active dry rot outbreak to be made.
It is minimally disruptive and comparatively inexpensive.
What about the treatment and repairs what can be done ?
I will deal first with the controlling the fungus then discuss the repair of decayed timbers.
Since dry rot is initiated by the ingress of moisture to the fabric it is theoretically possible to
arrest the outbreak simply by eliminating the moisture and rapidly drying out the structure. Indeed such practice is not uncommon and there are several advocates of what is called ‘environmental control' who do just this.
I advocate this approach to some extent but I feel that its general application requires a very high degree of management which is nearly always absent except in high value public buildings with full time caretaking staff.
In most real life situations however I feel a little more is required since there is a high degree of risk in implementing this strategy in isolation.
It is necessary to move on from the standard approach, and the off-the-shelf solution to each and every dry rot outbreak.
I don't think I have ever seen two dry rot problems I would describe as the same. They are all individuals and they all need to be dealt with in an individual way. A cursory examination of the advertising literature literature from a chemical supplier selling pesticides for dry rot treatment
will show that they all tell you to go about tackling dry rot in the same way. Hack off plaster cut out timber burn the decayed timber on site irrigate the walls.
This standard approach has become so commonplace as a procedure that it is written into almost every contractor specification and these are copied and passed on without any critical reflection regarding there inherent necessity or value.
These in turn become bills of quantity sent out to specialist contractors for costing. Prices are asked for spraying and irrigating walls for spraying wood preservatives over existing timbers. Supervising offices will inspect remedial works to ensure that the required areas of plaster have been stripped off that the requisite holes have been drilled for wall irrigation, dyes are often requested to be added to the fungicide fluids to ensure that the areas of treatment can be easily seen .and that the timbers have been cut back to the required length. The contractor must comply with the specification in order to be paid for his work. This adherence to out-of-date practice helps to maintain the status quo and progress is inhibited.
Much if not all of this chemical usage is of no value in dry rot treatment.
The rigorous adherence to standardised approaches of cutting out timbers is unnecessary. It is often argued that these standardised procedures ensure that a complete and rigorous specification is implemented in order to ensure a successful treatment. It is simply not so.
It is important to understand the degree to which chemical applications will be applied
successfully to timbers and walls
in a building.
The surface application of wood preservatives to wood within a building will achieve only minimal penetration of the outer surfaces of wood even under ideal conditions. The ability to sterilise or prevent reinfestation is confined therefore only to
the outer surface millimetres. This is often further reduced on site because conditions are far from ideal and the timbers may be wet and dirty. Complete penetration is impossible. Paste products do penetrate better depending on the moisture content species of timber and access being gained to all external surfaces. Diffusible products are better but more expensive and can be leached out.
The whole concept of wall irrigation is fundamentally flawed because the route which a fluid will take when introduced into
a wall is that of the least resistance to fluid flow through voids and open pores making saturation impossible. Since the wall is already wet there is little point in introducing more moisture into the system making it take longer for the wall to dry out
In any case the fungus growing within the wall will not necessarily follow the same paths as fluid flow resulting in areas of non-treatment. This in fact can be counter productive since some diffusion will take place after treatment which can result
in dilution of the fungicide within the wall. This can produce a sub lethal dose of fungicide which has been shown in some laboratory studies to stimulate fungal growth rather than prevent it
I would argue that the standard procedures of wall sterlisation and on site timber treatment for controlling dry rot have played little or no part in the success of the overall specification.
The effective control was achieved by the other building repairs. These building repairs involved repairing the leaking roofs and pipes, and carrying out structural repairs on the decayed timbers. These were the real factors which controlled the dry
rot in that they deprived the fungus of its essential requirements for growth and development
The crucial factors in sustaining dry rot in a building are moisture and nutrition of the fungus.
The crucial factors in its eradication are the same factors. Dry rot control will be achieved by the elimination of moisture and removal of the sources of nutrition.
I briefly mentioned earlier that I believe this needs to be supplemented in most cases (unless there is a high degree of supervision) and the reason for this is the practical difficulty in applying moisture elimination and drying. In particular, the length of time taken to dry the structure may be many years. There are obvious difficulties in incorporating such a timescale into any kind of repair contract. It is during the drying phase that the risks of reinfestation and spreading of the decay are greatest.
In recent years some investigators have suggested interesting ways of controlling dry rot during this crucial phase.
Heat Treatment of walls is a fairly well established procedure in Denmark. Although expensive there is little doubt that it is successful. The procedure involves wrapping the external walls in insulating materials and introducing hot air to raise the
wall temperatures to 50 C. Maintaining this temperature for 2 hours will certainly kill the fungus. Microwave technology has also been tested and also works however the mineral content of walls can produce dangerous side effects. We have considered introducing hot air into Scotland as an alternative. We are concerned that there are differences in construction details that have not been properly taken into account in testing which might result in damage to walls and ceilings
The fungus can be fed with dietary supplements to prevent it decaying wood until such times as
it dies from lack of moisture. Dr Sara Watkinson of Oxford University published some interesting work involving the use of alpha amino iso butyric acid
Our own studies at Glasgow University have indicated that biological control may be a useful way of supplementing the environmental control procedures. The attraction is that a parasitic organism may prove to have the ability to seek out
and destroy the dry rot fungus like a magic bullet. This work is ongoing at present.
For the moment we are content to advocate the selective and strategic application of certain fungicides to prevent reinfestation. These are applied locally when we feel it is appropriate. The future is bright because as more research is undertaken into the control of dry rot more options for control will become available.
Although I am dealing with these as a separate section the way in which these are carried out can have an important contribution in the environmental control process.
Please consider again the piano sinking into the floor. When structural timbers have reached a state of collapse there is
little point in discussing conservation. It is of course a facetious analogy but in many cases all too true.
The engineering considerations are of extreme importance in the design of repairs and they
must be fully taken into
account in designing a repair scheme. As a biologist who twenty years ago entered this branch of the construction industry
I owe a debt of gratitude to the many structural engineers (some of whom remain my friends) who have listened with thinly concealed amusement to my suggestions regarding structural repairs to incorporate schemes for extra ventilation. I cannot over emphasise the need for a good design but I would also encourage the need to move on and try new materials especially if their use can limit the need for removal of valuable features such as cornicework and ornamental panelling.
I have been impressed in recent years by the improvements in beam end repairs using epoxy resin systems.
These do need to be carried out with care and the whole process needs to be carefully managed .They offer the opportunity to save valuable ceilings. However it is necessary to ensure that the decay has not spread beyond the repair area and that this is verified at the survey stage by a moisture map and detecting technology mentioned esarlier.
More conventional but still conservation orientated engineering can take the form of steel beam end replacements. These have the benefit of being more readily quantifiable in terms of the engineering calculations. They tend also to be cheaper
to carry out but don't offer the same scope for saving ceilings by virtue of being carried out from above or from the side.
The repair of structural beams has to be carried out in conjunction with a very thorough examination for decay. It is
essential that it is verified that no internal decay exists along the length of the beam especially at the other end which
might not have received as thorough an examination and may be built into the wall. The limitations of successful chemical treatment
must be taken into account.
There is no general guidance to be given about what beams can be repaired and which need to come out. An individual assessment is a must in all cases.
The replacement of structural timbers should always be done using pre-treated timbers. Where possible it is best to order the required sizes to minimise the cutting done on site. Where cutting is necessary it should always be the practice to locate the uncut beam ends on or in proximity to the walls which are affected by dry rot. The reason for this is that the end grain achieves high loadings of preservative and much better preservative penetration. This will give protection against reinfestation as the affected walls dry out, and also in the event of a future breakdown in maintenance.
The decision about how far to strip out plasterwork should not depend on a crude rule of thumb_ for example 1 metre beyond the last visible sign of fungal growth.
The distribution of moisture will determine this together with the detail of construction.
I would advocate the removal of wet lath and plaster from within an area of dry rot activity unless it has particular value.
The frames themselves supporting the lath and plaster are quickly damaged by dry rot and the plaster may fall away. Even the swelling and shrinkage which takes place as a result of wetting can be sufficient to cause the collapse of plasterwork supported on laths. If the decision is taken to leave them in place then some consolidation needs to be considered to prevent future collapse. There are a variety of ways that this can be done.
Removal of plaster on hard is only neccessary where it has become damaged or when it is necessary to get access to concealed timbers behind. The original sand and lime mixes that are often present in tenement or other older properties
are damaged by moisture and can undergo a chemical reversal process. Replacement is the only answer in such cases.
I hope I have been able to present a case for a change in attitude towards dealing with dry rot.
I am confident that as continuing research opens up new possibilities for alternative methods of control public confidence
will follow. In the meantime I urge anyone who has an interest in the subject to think carefully about how the problems are assessed and repaired. Don't just rely on the received wisdom of what passes for standard practice. Each and every dry rot outbreak needs to be approached as an individual problem with individual characteristics and a unique solution. That in alone is a leap forward.
George Mc Gill BSc AIWSc mail to email@example.com
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