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Treating Dry Rot in Historic Buildings

Treating Dry Rot in Historic Buildings

A Review of the conference first published in "Preserve"

Organised jointly by English Heritage, ICOMOS and Historic Scotland, who are all keen to introduce and promote the use of less disruptive techniques in dealing with dry rot in buildings of historic or architectural importance, the seminar provided a forum for the supporters of environmental control of dry rot to illustrate their school of thought, and to draw comment from the industry practitioners and their clients.

Brian Ridout's historical perspective was so enjoyable that I, and many others, bought his excellent book on the subject (highly recommended (Timber decay in Buildings Brian Ridout Published by E & FN Spon ISBN 0-419-18820-7). 

Brian described and illustrated some early recorded incidences of timber decay, and from this traced the development of
the industry as we now know it.Timber decay has been with us for a very long time and I have always been intrigued by the historical reports of dry rot in wooden ships, especially since more recent investigators tell us that the minerals found in the building fabric are actually a requirement for dry rot growth.The term was probably more loosely applied in those days and any form of rot was described as dry rot.Such mis-diagnosis would never be made nowadays (would it?)

There are many interesting issues around the environmental control debate.Essentially, supporters advocate no intervention and no treatment. “Just ventilate the affected areas” we hear them say “and the dry rot will die”.This can be demonstrated in theory since the dry rot fungus is a very sensitive plant and only thrives under specific conditions of temperature and available moisture.The difficulty arises in practice.How do you ventilate some parts of a building?How do you deal with the fact that the parts that are difficult to ventilate are usually the parts affected by dry rot.And of course the £1m question, when will the dry rot die?

There are various reports in the literature of how long the dry rot fungus can remain viable under adverse conditions.Many years seems to be the answer: and even then, how do we know it is dead and not simply dormant?The concept of death
has intrigued medical scientists for centuries.It defies accurate definition.I expect that most contractors and specifiers would not be comfortable about leaving dry rot alive in a building, especially after they had signed over their long term guarantees.

James Simpson, on the other hand, was not worried by this at all.He took the industry to task and suggested that the issue of guarantees and insurance, diverted money from essential maintenance.He made some good points about conserving valuable building artefacts, which are often lost by the traditional approach to dry rot repair.He supported this with photographs of Arniston House and described a typical repair scenario carried out over forty years ago.He singled out one contractor for criticism, perhaps rather unfairly, since they were only doing what any contractor would have done at that time.Things have moved on a lot in the last forty years!

It was a relief that the focus of argument from the environmental control supporters on this occasion centred on the issues of conservation and restoration.The arguments I have seen in the past have usually been associated with the intemperate (and inaccurate as Graham Coleman reminded us) scaremongering around the use of “toxic chemicals”.Graham makes the point that you will often find more hazardous products under thekitchen sink, than are used in remedial work.Remedial chemicals nowadays are classified as “irritant” not “toxic”, and then only in the concentrated form.

There are many good reasons for adopting a less intrusive approach to the repair of dry rot affected buildings.It is a pity that the debate often becomes wrongly focussed and becomes polarised as a result.

Geoffrey Hutton made some interesting points about buildings, and traditional building methods.He stressed that the building should be regarded as a whole.He also stressed the importance of the role of sympathetic materials and methods in the practice of conservation.He suggests that the presence of dry rot should be seen as a symptom of something else wrong and the remedial exercise directed towards correcting the cause of the rot and not just the rot itself.He maintains (incorrectly in my view) that the use of remedial chemicals is never necessary, and that the environmental control principles are sufficient in themselves.The presence of rot was often (as we know) the result of neglect or the introduction of
ill-advised alterations or extensions. He did make a valid point about the reference by many organisations to “superseded standards” which is one very good reason why practice is slow to change.

It is of course of great importance to understand the biology of dry rot so that it can be dealt with in a more sensitive manner.David Dickinson reminded us of some of the biological facts about dry rot and showed some slides of a badly affected church.He has been involved in the European standardisation of wood preservation treatments and in defining the appropriate hazard classes defined in EN335 (European Hazard Classification for Building Timbers) He showed a proposed standard test method for masonry biocides.

Putting flesh on the bones of a better understanding of the dry rot problem is currently being actioned at Abertay University.John Palfreyman described what he called a “knowledge based approach” in which the information collected by scientists in the field is brought together with the understanding and experience of practitioners working in the field.

Some of the more practical considerations were highlighted in a good-humoured manner by Peter Ross of Ove Arup.He reminded us that the engineers are left with the task of making sure the structure remains standing whatever way we
decide to tackle the rot.Whether we tackle the problem by killing the dry rot, or not killing it, it is nevertheless essential to carry out structural repairs.These have to be done properly, and achieving this is often the reason for the removal of plasterwork and panelling.

Chris Wood described some of the problems encountered with dealing with dry rot in listed buildings making the point that sometimes the repairs are to destructive that they might be considered as demolition.Michael Carden described some real life experiences of applying conversation principles and the difficulty of getting the details right.He illustrated some of the horrible problems when we get the details wrong.

If there was a paper missing from the seminar I think it was one on the engineering of structural repairs.There have been many advances in repair technology in recent years.It would have been useful to have some of these presented and discussed.

Overall the presentations were in favour of a less intrusive approach to dealing with dry rot.The integration of moisture control with effective treatments is essential in most cases .  There is, no longer much support for treatment practices such as mass wall irrigation except in the limited circumstances spelled out in the BWPDA Code of Practice. It seems likely that contractors will move towards reducing chemical treatments where possible. 

This of course also depends on those specifying the treatments preparing tender documents which incorporate what is considered to be good treatment practice

I think it is fair to say that there is scope for a more sensitive approach to dealing with dry rot in buildings, and not just those of historic interest and importance.It seems clear however that there are major differences between working on
historic buildings and working on a typical domestic or commercial property.One major and important difference is the expectation of the client, and another is the degree of management available to monitor the building during the crucial drying out period.Those caring for historic buildings perhaps will be content to regularly monitor the dry rot over a lengthy period, and be prepared to accept that a breakdown in maintenance might at some point lead to a recurrence.There is of course no opportunity to do this in a typical domestic or commercial property and a recurrence of the rot may result in a costly legal action against the Specifier or Contractor, and a Contractor would not guarantee something he could not control.There are practical difficulties also in applying the environmental control approach generally.Many situations do not allow for the introduction of ventilation to the affected areas and it is therefore not a practical proposition.Flats or terraces
for example where adjoining properties are under separate ownership prove the case.

There is a need however for the industry to respond to an increased concern for the environment and the public will expect more environment friendly approaches in the future.There is no reason why the industry should not respond to these demands 

Some of the criticism directed at the industry is fair.There has been a tendency over the years to adopt a single approach towards all dry rot problems, which comes from the perception that dry rot in buildings is a wood preservation problem. It is not.

It is a building ecology problem.

1. Dry-rot

2. Fruit-bodies

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