Monday, April 28, 2014

Dry Rot – An ‘Intelligent’ Fungus requiring intelligent diagnosis

The reason that dry rot is often so devastating is because of its ability to travel long distances as it searches for more and more timber to remove moisture from

As a Building Surveyor there are certain words that you know, just by saying them will strike fear and panic into the majority of members of the public.  Words such as subsidence or asbestos are examples which regularly appear in the news due to significant cost or health implications. In many cases however, potential subsidence or the identification of asbestos often result in relatively simple and cost effective solutions although it seems to be a natural human reaction to automatically think the worst.  The ‘term’ dry rot is also generally well known by members of the public, however unlike subsidence and asbestos the implications of the discovery of dry rot are often serious, depending upon the stage at which it is actually identified.

I was recently watching a well know property renovation programme on TV a few weeks ago where the Presenter had identified what looked to be dry rot on the ground floor of semi-detached three bedroom residential property.  Although, I agree that what he was looking at did appear to be dry rot, his description included, ‘feeding off concrete’ which is completely inaccurate as well as some of his terminology being confusing and wide of the mark.  TV programmes should be mindful of the information that they provide, as it is possible, dare I say likely, (just by the nature of the amount of viewers), that someone will act on what they are being told which could result in loss/damage.  In order to identify if or where dry rot may be present, it is useful to understand the conditions that dry rot needs to grow and thrive. 

Dry rot is a fungus, often referred to as an ‘intelligent fungus’.  The reason dry rot is referred to as intelligent is because of its ability to travel across non-timber surfaces and to take moisture from timber.  What is left behind is dry friable timber that can easily be broken up with moderate hand pressure.  The reason that dry rot is often so devastating is because of its ability to travel long distances as it searches for more and more timber to remove moisture from. It is worth noting that when the dry rot fungus travels across non-timber surfaces such as bricks, mortar, concrete etc. it is just using these as a route or bridge to find other timber.  Dry rot does not ‘feed’ off these types of surfaces but carries moisture with it in strands which allows it to grow and spread. If left untreated dry rot has the ability to affect vast amounts of timber within a building, often resulting in extensive specialist remedial works which are not cheap.

Dry rot is a living fungus which will continue to grow by feeding off timber, which it will completely destroy be removing all of its moisture.  Without being too technical, there are four primary stages in the dry rot lifecycle.  The first stage is Spores. The spores are constantly present in the atmosphere however are only activated in certain conditions, which include timber and moisture.  For dry rot to thrive it’s ideal environment will include timber with a moisture content of between 22% and 25% warm humid temperatures of between 240 and 300, poorly ventilated areas and dark concealed spaces. This is why dry rot will often spread undetected in basement, floor voids, roof voids, behind plasterboard in timber stud walls and the like. As the spores start to become more concentrated they develop into small white strands known as Hyphae, which look a little like small white cob-webs.  These are reasonably easy to identify and a good indication of dry rot.

As the hyphae feeds off the timber it will extract further moisture from the timber as it continues to grow and become more concentrated in volume to a point where the hyphae mass develops into the next stage of the dry rot cycle, know as Mycelium. Visible large white mycelium strands can travel large distances in search of more timber and as previously stated can travel across non-timber surfaces in order to find new timber. In suitable conditions, mycelium will continue to exist and grow at a considerable rate within a building. Fungi prefers dark and damp areas with little or no air movement, therefore where these conditions change and threaten the fungus; its natural response is to create a Fruiting Body (Sporophore), and this is the final stage of the dry rot lifecycle.  Visually the fruiting body can take a number of forms, however will generally appear in ‘mushroom like’ form. The fruiting body is the fungi's response to a threat to its survival and its reaction is to throw out spores into the air which can be transferred to other vulnerable areas within the building, which allows them to germinate and create a new attack of dry rot, thus restarting the dry rot life cycle right from the very beginning.

The dry rot lifecycle described above demonstrates how the fungus can spread so quickly and how much damage that can be caused if left undetected. It is possible to treat dry rot however this requires specialist knowledge and something that should not be attempted ‘on the cheap’.  If all traces of dry rot are not dealt with then all that will happen is the fungus will continue to grow and spread and start to affect any new timber that may have been installed. The steps below provide an indication of remedial works to deal with dry rot, however please bear in mind that this is indicative only and specialist advice should be sought in all situations:

1.   Deal with the moisture source
2.   Brush down any exposed masonry to remove visible surface fungal growth
3.   Deep-drill masonry at regular centres and irrigate with fungicidal wall solution
4.   Sterilise all exposed masonry surfaces with fungicidal wall solution
5.   Remove all affected timber including a minimum of 1m past the last identifiable location
6.   Dispose all affected timber from site
7.   Provide new treated and primed timber where previously removed
8.   Spray all new and adjacent timbers including cut ends with fungicidal spray
9.   Re-plaster where required using a cement and sand render mix
10. Ensure adequate ventilation is used

Dry Rot is easy to misdiagnose and I would always recommend that you engage the services of a professional such as a Building Surveyor for advice and guidance.  The short video below provides a good example of dry rot occurring in a residential dwelling and is well worth look. Many thanks to Bryan Hindle who has provided his kind permission for use of the video. Please also take a look at Bryan's excellent article on the following link

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  1. Excellent article and very informative.

  2. Thanks for the article which provides a good insight into the key considerations regarding dry rot. If I may be so bold as to contend your comments regarding remedial action. From my experience of dealing with dry rot I have come to the absolute conclusion that as long as conditions are made impossible fir dry rot to exist then it will not. This may seem silly but I refer to your comment above about the need to apply chemicals in buildings. The dry rot always needs water to survive. Remove all water sources and the dry rot will not survive. For example, repair all leaking gutters/roof parts and the dry rot will not return. The remedial action that is needed is to firstly prevent all moist timber sources, then cut out and replace damaged timber and then ensure the building parts are sufficiently dried out. Adding chemicals means you are adding more water to the building fabric (bad), and then spraying chemicals everywhere (bad). If chemicals are sprayed everywhere, and subsequently the building is fully repaired and all moisture sources rectified then the chemicals serve no useful purpose as it is impossible for dry rot to exist.

  3. I agree with Tom 100%

  4. Hi Gary,

    On the whole a very good article and thank you for using my little video, I am glad you like it.

    I'd say though, that the extent to which sound timbers should be removed is excessive and really, providing care is taken in diagnosis, including tracing the full extent, sound timber should be left in place and just treated topically.


    Whilst a purist will say 'just dry out the building and the rot will die', in practice that approach is rarely acceptable in every day situations IMHO.

    A home buyer, property developer or estate manager will usually want the dry rot damage rectified and will need some sort of guarantee that the problem will not come back any time soon. Dry rot stores a considerable amount of energy in it's strands and mycelium and once it is stressed, by having it's head cut off and the gutter fixed, it doesn't just give up the ghost immediately. During this period more growth will sprout from the walls and the odd spectacular fruiting body may emerge also. This impacts on the tenant, owner or 'god forbid' the buyer and all hell is let loose.

    Responsible application of safe and proven chemicals is a wise and proper thing to do and has it's place. Mass irrigation with hundreds of liters of water based biocides doesn't.

    All in all a good article Gary and, well done for taking the time and effort to put your knowledge on line for others.



  5. Stop all water sources well you better make the building air tight, Dry rot can live of moisture and no matter how hard you try to banish it from a building you cannot eradicate it fully. Don't forget under a floor void the earth can be damp from the ground water.

    I am hands on and been doing this work for over 18 years now and dry rot is the toughest problem to eradicate in a building.
    The stuff does not need a lot of water to survive and one of the main ways to stop it is ventilation more than anything.
    I personally go over the top when treating it as I have seen other contractors fail because they think the magic spray cures all, yes you have to ensure the conditions don't exist for the dry rot to thrive, but you have to do it in a way that ensures no further outbreaks can happen.

    Don't forget dry rot can live of condensation as a water source, I have seen it growing on wall paper so it must have been absorbing the condensation.