Dry Rot - Serpula Lacrymans

The damp industry, headed by the Property Care Association (PCA) promote a morbid fear of dry rot.  Their web pages are covered in photos of dry rot outbreaks, and speak of urgent remedial treatment for 'Serpula Lacrymans' and the Cellar Rot fungus - 'Coniophora puteana'.

We survey hundreds of houses, and very rarely see these fungi.  The reason is very simple - they need an awful lot of moisture in order to thrive.  Commonly, they appear under floors which have had water running under them - for instance blocked drains, or water running into sub-floor vents when ground levels are raised. Brian Ridout, the leading expert in the subject (Timber Decay in Buildings - English Heritage) states: 'In practise, dry rot sometimes attacks timber with a moisture content down to about 22%, but probably only from a far wetter source.  Cellar rot does not normally cause much damage below about 25% moisture content.  A study quoted by Ridout states optimum timber moisture content of 30 to 70% for both fungi.

A dry rot attack then, is usually limited to the zone of damp walls and timber...

Our surveys use various methods of measuring moisture content - and keep very closely to BS: 7913.  We look for humid areas of the building first - using thermo hygrometer probes under floors to establish the moisture content of surrounding air. We have found that an RH of 90% is a minimum level at normal temperatures - between 15 and 20 degrees C.  Above this level, there is a possibility that timbers will begin to experience decay.  Ridout states "... dry rots requirement for water, and its basic intolerance of drying conditions...  The threshold of infection is 90% RH at 20 degrees C, optimum humidity at 99%. These humidities would equate with timber moisture contents at, or slightly below, fibre saturation point, ie about 26 to 30% total moisture".  Our own observations thus confirm those made by Ridout.  

This level of moisture is what you would find in freshly felled timber - and it's hard to get that in your house.

Even more interesting is the remarks Ridout makes about sorting out and stopping a dry rot infestation:

Savory (1971) pointed out:  Effective irrigation (of toxic fungicide) cannot be achieved on brickwork with open mortar joints or on masonry walls with loose infill cores.  The irrigation process will provide an improvement on surface applications only when the treated volume of wall is thoroughly saturated with fungicidal fluid.  This requires the introduction of large quantities of liquid which dries out only slowly and introduces a risk that efflorescence of salts will damage prematurely applied decorations.

Ridout states: It should also be remembered that the primary aim of any dry rot works must be to dry out the structure, and that this process is not assisted by the introduction of large volumes of water-based fungicide.  

One needs also to take into account that dry rot treatment chemicals are highly toxic - Pentachlorophenol, Dodecylamine, zinc oxychloride, and Boric acid are not exactly nice things to have in your home.

Ridout summarises his extensive research into dry rot and cellar rot by stating that environmental control is much preferred. This is also the simplest, and easiest - as per BS: 7913 - dry timber cannot suffer rot, or insect infestation.  Therefore, the simplest solution is to remove the source of water, and dry out the timber!  

Isn't that an elegant and simple solution folks!

We have used this solution successfully in several cases where we have found dry rot and cellar rot.  Just by improving ventilation and stopping water ingress from broken drains, timber has dried out and the rot disappeared.  Of course, some replacement of affected timbers will be needed, but sustained dry conditions will kill any remaining fungus - which research has shown will not remain in any way viable beyond 9 or 10 years, and more likely 1 year.  

So to conclude - the PCA scaremongering campaign about dry rot and cellar rot is really not up to much.  The solutions are simple - remove the cause, and the fungus will go away.  It's always the same with damp issues - remove the cause, and don't treat the symptoms.

So if a London Damp company tries to tell you that Dry rot, or Serpula Lacrymans, together with Coniophora puteana, is a problem, just tell them that your Independent Specialist Surveyor (in the guise of Heritage House or Heritage Consulting - backed by Brian Ridouts work, and BS: 7913) would prefer NOT to use any form of remedial treatment, and opt for stopping the water leak that started it, and drying the place out!

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