May 6, 2019
Last Updated on May 12, 2023
Chartered surveyor Stephen Boniface clears up the mystery around rot, including what it is, how it works and what you should do if it is discovered by a building surveyor.
In recent articles I have considered the problems of damp and in this article, I turn to consider the allied issues of timber rot and infestation.
The Fear of Rot
Perhaps the one thing that causes most fear to a client is when a surveyor declares that the property has a ‘dry rot’ outbreak. Over the years I have had clients who have been under the impression that if they have dry rot in a corner of the building today then within a short period of time the whole building will have collapsed due to the rot spreading and causing destruction throughout. This is, of course, completely wrong and in this article, I will attempt to explain why and what can be done when problems do arise.
What is Rot?
We commonly refer to dry rot or wet rot, both being misnomers that are misleading. Each are fungal growths that have adapted to live off wood and therefore take their nutrients from wood. In so doing they each live off and destroy specific (different for each) parts of the wood, thus causing it to lose its strength and general integrity.
When thinking of a fungal growth it is often useful to think of the mushroom. This requires very specific conditions to grow and in the right conditions it will flourish. When the conditions are not appropriate it will not grow at all.
Better descriptions of these rots are brown and white rot. Dry rot is a member of the brown rot family whereas wet rot is a member of the white rot family. Because each of these attack different parts of timber they have different characteristics. Nonetheless, they are both a form of fungal attack and have some similarities. Particularly, they require timber as a source of nutrient and there has to be a source of moisture; without both the spores will not germinate. However, they also benefit from other conditions including humidity level and amount of light.
How Rot Functions
It is not the purpose of this article to provide a scientific treatise on the subject but to help homeowners understand the problems and allay some fears they might have.
With regard to the source of nutrients we are surrounded by timber in some form or another. These fungal growths perform a very useful task in woodland and forests because they help to break down timber, particularly on the forest floor where the timber has fallen and become damp. Without the help of such rots and infestations breaking down the timber our woods and forests would be full of fallen trees. However, the one place we do not want these rots to flourish is in our homes.
It is often said that if we sample the air about us, we would find spores from these rots because they are all around us in the atmosphere. If this is so and if wood is so abundant the question has to arise: why do we not see much more rot?
The answer is simply because the spores are not easy to germinate. They require a high level of moisture (despite the term ‘dry’ rot) in addition to a source of timber. A piece of timber that is dry, occasionally gets wet and then quickly dries out again, a piece that has a low level of moisture, or even a piece of timber submerged in water, will not have the right level of moisture or the right conditions for the spores to germinate.
I have heard it said that dry rot can transport its own moisture to ‘wet up’ dry timber, but this is simply not true.
As the rot germinates and starts to grow it sends out its mycelium (or roots) in search of nutrients. Whilst these will have an amount of moisture in them, they do not transport dampness to ‘wet up’ dry timber. Rather, if the mycelium encounters timber that is wet they can develop (take root) and take nutrients from that timber thus causing the outbreak of rot to spread.
If it were true that dry rot could transport its own moisture to cause timber to become sufficiently damp, we would see far more rot around us and have a far more extensive problem than exists.
Some years ago, I was asked to inspect a mediaeval house that was timber framed but had sadly been left unoccupied for approximately thirty to forty years. Over those years a roof valley had leaked quite seriously in one corner of the building; there was a very serious and extensive outbreak of ‘dry’ rot to that corner. As I neared the outbreak, I wondered how close I would be able to get before I might fall through the floor! In the circumstances (a very dilapidated building) one would have expected that this rot would have more or less destroyed the building over a period of thirty to forty years. In fact, I was able to get to within 1m of the outbreak before the floor started to deflect. The rot was not found elsewhere despite the extensive amount of timber.
The conditions in this property were apparently ideal for the rot to cause widespread damage but why had they not done so? The simple reason was that the rest of the building was dry, and the rot had occurred and flourished only in the area that had become damp. The rot was unable to transport its own moisture and only caused localised damage to the area where the timber had become damp; in this case because of a leak.
Some years ago, I attended lectures by Brian Ridout who had undertaken much research into rots and infestations. A matter raised at the time was that of chemical treatment for rot outbreaks. What was explained at that time is that many of the chemical treatments we use to try to combat rot have an active chemical that is intended to kill the rot, but it is transported by water. Therefore, when we spray the chemical onto a piece of timber we are introducing further water to that situation. Rather than kill the rot some chemical treatments might help it. Indeed, I recall seeing a photograph of a trial whereby Brian had been able to germinate rot spores in the chemical intended to kill the rot. What Brian explained that day explained what I had witnessed prior to then and many times since; that rot had returned to treated timber where the damp problem had not been adequately resolved.
What to do with Rot
So, what should we do if we encounter an outbreak of rot?
The first thing to remember is that a rot outbreak means there must be a source of moisture. In fact, it is not just a small amount of moisture but significant levels. This might mean a sudden amount of moisture being introduced such as a flood or when a fire is put out, but it might be a small leak that has accumulated to become a lot of moisture over a period of time. Therefore, the most important action is to find the source of moisture and deal with it. No amount of chemical treatment will deal with the rot if there is still a source of moisture; the rot might be temporarily halted but will return.
That said, there may be an appropriate time for some treatment. In a situation where the source of moisture has been eliminated and the building fabric is being dried out (but taking some time), there may be a situation where a piece of timber needs temporary protection during that drying out process. It might be appropriate to undertake some carefully targeted treatment to provide the additional protection that timber might need for a limited period. However, this must be very carefully considered bearing in mind that the treatment could introduce water to the timber. Often, rather than such treatment it is better to monitor the levels of moisture over time to ensure the timbers are drying.
Because moisture is the key to the problem of rot, where there are timbers that might be vulnerable it is sometimes necessary to monitor a situation. This might mean taking regular readings of the moisture content of the timber to establish whether the levels are at a point where the timber might be vulnerable. In some situations, you might need to think about installing a data logger. This is a system whereby a sensor or reading equipment is installed in a permanent or semi-permanent situation and the readings can be taken remotely to plot the moisture content over a period. This information can be gathered over time. From this a suitable method of treatment or repair can be devised, or it may be that a decision is made for monitoring to be continued and intervention only undertaken if the moisture levels increase to a point whereby they might cause an outbreak of rot.
In some situations, I have seen brick and stone walls irrigated with chemical treatment in the hope that this would prevent a rot outbreak. As already mentioned, because the chemicals themselves are usually water based such irrigation can often increase the level of damp in a wall and increase the risk of rot.
I have heard it said that rot will grow through and feed off masonry. Again, this is untrue. Whilst the mycelia will grow and try to find a source of nutrient and water these could grow through cracks and gaps in masonry. However, they do not cause the masonry itself damage.
Another problem I have come across in the past is a belief that where there has been a rot outbreak the timbers need to be cut back at least 1m beyond the last sign of the outbreak. This often results in unnecessary and extensive work. Of course, if rot has caused a piece of timber to lose its structural integrity then that timber needs to be removed and a new piece installed. In so doing it is necessary to check the timber to ensure that any rotten timber is properly removed and replaced. However, it is not necessary to remove dry timber that has not yet been affected by rot.
In some situations, timber will have to be installed in a wall or positioned within a property whereby it might become damp in the future. It is sometimes appropriate to install an isolating membrane around the sections of timber that are likely to be in contact with a wet surface. This will mean there will be some separation between the damp material and the timber itself. This should help prevent the timber from rotting. However, part of the process should also ensure that the timbers are as well ventilated as possible because air circulation around the timber should help to remove moisture and reduce the risk of problems arising.
When dealing with a rot outbreak it is important to identify the specific nature of the rot because this helps to understand precisely what damage it might be causing. However, the basic treatment is much the same regardless of the nature of the rot. The source of moisture must be identified and removed or at least reduced (if the source of moisture cannot be eliminated). The vulnerable timbers should be adequately protected and where timber has already been degraded it will need to be removed and replaced. Whilst chemical treatment might be appropriate as a temporary protective measure in certain specific circumstances chemical treatment itself is not an appropriate remedial measure for rot outbreaks. Monitoring moisture levels in vulnerable timbers might be appropriate. Anyone with a rot outbreak should be extremely wary of any contractor wishing to treat with chemical but not deal with the other issues of moisture etc. as outlined above.
How trees work
When considering timber and the fact that the piece of wood in your property started life as a tree it is useful to remember that when growing as a tree it transported moisture up its trunk from bottom to top. When we convert a tree into timber for use in buildings, we can see the grain. This grain is the natural route for moisture up a tree and the end grain that will more readily absorb moisture than the side of a piece of timber, i.e. across the grain. Therefore, wherever end grain is exposed to moisture it is more vulnerable to absorbing moisture and then for rot or infestation to occur.
The importance of maintenance
Most rots prefer darkness and do not usually thrive in light. This causes us a problem because it means that rot is more likely to be in the hidden areas of a building. The first we know of a rot outbreak may be when a timber begins to fail. Often the rot will cause a lot of damage behind the surface before it becomes visible on the surface.
If we maintain our properties in good condition this will reduce the risk of water ingress and therefore the risk of dampness getting into hidden areas and causing concealed problems.
The number of times we have had to deal with serious outbreaks of rot costing thousands of pounds to remedy and repair when the simple act of clearing a gutter or roof valley would have prevented water ingress and therefore the outbreak of rot, is just incredible. Maintenance of a property is often the key to preventing such problems from arising.
Associated with rot and dampness are beetle infestations. There are a number of beetles in the UK that can destroy timber but the two most frequently mentioned are common furniture beetle and death watch beetle. As with rot, in the right conditions these can cause a lot of damage but otherwise cause little harm.
In both instances the beetles themselves are small and their jaws are smaller still. Quite simply they do not have the strength in their jaws to eat through well-seasoned dry timber. Therefore, for a beetle infestation to flourish there needs to be a certain moisture level in the timber and the timber fibres need to be breaking down to be soft enough for the beetles to consume.
Common Furniture Beetle
Common furniture beetle has a life cycle of approx. three years or thereabouts. The adult beetle will lay eggs and the larvae burrow into the timber for a few years before exiting and becoming adults. The cycle then repeats. They primarily attack soft woods but can attack the sapwood of most timber if it is soft enough, i.e. through damp and rot. It usually eats its way out of the timber and its emergence creates what we commonly call a flight hole. Over time a heavy infestation can weaken timber and cause it to fail.
If there is an outbreak of common furniture beetle it is an indication that there is dampness around. This could be atmospheric damp such as condensation but there might be other forms of dampness and this needs to be addressed. Properties (or areas within) that have high levels of humidity are vulnerable to infestation because the timbers generally have elevated moisture levels. Control of humidity and dealing with condensation will go a long way to help reduce the risk of infestation.
Many timbers will suffer from infestation in their early years (whilst they are still drying out) and before they become fully seasoned. With modern properties many structural timbers are pressure-treated to provide some protection whilst they season and dry out. In older properties the timbers are already seasoned and infestation occurs usually only if timbers become damp.
If active common furniture beetle is identified targeted chemical treatment to surfaces can be effective. Note the use of the word targeted because widespread treatment simply for the sake of it is unnecessary and a waste of money.
Death Watch Beetle
Death watch beetle has a lifecycle that can be around ten years or thereabouts. Like common furniture beetle the adults lay eggs that form larvae, which then eat through the timber to eventually emerge and become adults to repeat the cycle. Death watch beetle tend to attack hard woods and is not usually found in soft wood. Even so, it generally only attacks the sapwood and is rarely found in the heartwood unless the heartwood itself is deteriorating through damp and rot.
Death watch beetle does not always create its own flight holes but can sometimes enter and emerge through cracks and gaps in the timber. Because of this, and its long-life cycle, surface treatment is rarely effective and is not something I would recommend.
With death watch beetle it is far more important to identify precisely where the outbreak exists to undertake specific targeted treatment and repair. The problem here is finding out precisely where they are. In some instances, the damage that might be caused in finding the outbreak could be greater than the outbreak itself is causing. If the outbreak is limited to a couple of mating pairs then it may not be appropriate to undertake any work at all but to monitor the situation.
How to tackle beetle infestation
Where there is a significant outbreak it is first and foremost an indication that there is damp rotting timber. As with the issue of rot it becomes necessary to deal with the dampness as the primary method of remedying the problem. Dealing with the damp will often then expose the infestation allowing it to be removed, and, if necessary, very specific targeted treatment undertaken. Of course the damaged timber will need repair and/or replacement.
With historic timber framed buildings, where the frame is covered in part or fully by render, plaster etc., the first sign that there is a concealed damp and perhaps rot problem is usually the emergence of death watch beetle. In such situations it is essential that advice be obtained from someone who can consider the problem holistically and advise on a bespoke solution appropriate for that property and that situation. Although chemical treatment might be part of that solution it will not be sufficient without dealing with the dampness and rot.
Identification of activity
Identification of activity is of course important. I have already mentioned flight holes and for common furniture beetle these are usually approximately 1mm in diameter but for death watch beetle they can be up to approx. 3mm in diameter. When looking at or into the hole if you see a dark dirty interior it will be an indication that the hole is old. However, if the edges of the hole are crisp and just inside the hole appears to be clean fresh timber it could be an indication that the hole has recently been formed and that there is activity. If you suspect activity a quite simple solution to help identification is to use flour paste and paste a piece of tissue paper over the suspect area. If there is activity the beetles will emerge through the tissue paper and holes will be seen in it.
These beetles are generally active in the spring to early summer period and this is when you need to look out for evidence of activity.
The most important thing to remember is that dry timber will not suffer from rot or infestation.
The key to understanding and dealing with these problems is an understanding of dampness, how this can affect timber and how to deal with the damp.
In this article I have discussed in general terms the issues of rot and infestations. If you suspect any such problems in your property you need to seek specialist advice from those experienced in dealing with such issues but also from those who are able to take a holistic approach and are not wholly reliant on chemical solutions as a remedy. Here at Whitworth we are able to provide independent advice on rot and infestation issues.
About the author
Stephen Boniface is a Chartered Surveyor at Whitworth. Stephen has extensive experience in historic buildings and conservation work, and on a daily basis carries out a variety of work from surveys to expert witness.