Mothballing buildings: Proactive maintenance and conservation on a reduced budget
Tim Hutton MA MSc Vet MB MRCVS
Huw Lloyd BSc AIWC


In these days of financial stringency and a sluggish property market there are many buildings awaiting renovation or development. In this building equivalent of the ‘poverty trap’, maintenance programmes are often curtailed at the very time when reduced occupancy puts the building at a higher risk of problems such as water penetration and poor ventilation. These conditions often result in chronic problems of timber decay and especially dry rot Serpula lacrymans. In these circumstances, it can be difficult to motivate property owners and managers to carry out remedial works, as they will wish to minimise expenditure which might be made irrelevant by future developments. At the same time, it is important to prevent rampant timber decay developing; as this devalues the property, and increases the cost of eventual renovation

Examples of the conundrum discussed above can be found in every sort of property portfolio from Grade 1 listed historic buildings to offices and domestic accommodation. Similarly, the owner may be anyone from a government department to a private individual and from an occupier to an offshore based speculator. This may raise an additional problem of conflict of interest if the owner is tempted to let a listed building decay so that it can be demolished

The result of this situation is that many basically sound buildings, are allowed to decay to a stage where it is expensive or even impossible to refurbish them. In the authors opinion, this represents a negligent waste of money and valuable resources. This state of affairs is even more reprehensible because there are a few simple and cheap measures that can be taken to prevent further timber decay in such buildings. These measures should be understood and advocated by any professional involved with building conservation, so that owners can preserve their property by a policy commonly called ‘Moth Balling’ when applied to other situations.

Factors in the spiral of decay and neglect

As discussed in a previous article (1); timber decay occurs when environmental conditions in the structure become suitable for the growth of timber decay organisms. These are generally adapted to survive in the damp airless conditions found in the forest floor. This applies to both wet rot and dry rot fungi and to the wood boring insects such as woodworm and deathwatch beetle. The environmental conditions required for such organisms to flourish are very specific and narrow (9). However, the major factors are moisture levels and ventilation rates (8). These in turn are affected by many factors including building defects, maintenance programs and occupant activities.


A major problem for conservation is the difficulty in getting small maintenance tasks done in an unoccupied building or a building under reduced occupancy. This is not just a result of budget constraints but may also be due to organisational and contractual problems.


Reduced occupancy often means tenants on short term ‘licences to occupy’ or even squatters. Such people have no interest in maintaining the building so far as cleaning drains, stopping plumbing leaks or even preventing overflowing baths! Another factor resulting from such styles of occupancy is the reduced levels of structural heating and ventilation that tend to be used. In an attempt to save money, the occupiers will often use intermittent air heating systems and block all chimneys, windows and ventilators. This results in condensation problems and moisture build-up (4).

Malicious damage

Vandals are an important factor in the structural damage that they can cause. This may be a serious problem independent of timber decay. However, damaged windows and doors can also allow water penetration. Arson and its consequences are also, sadly, only to common. Another ‘human’ factor is that common pest Plumbus larcenus or the ‘common spotted lead nicker’, whose theft of even small amounts of lead from valley gutters, parapet gutters and flat roof areas can result in massive water penetration into very vulnerable areas. The measures taken to try to control vandalism can also be very damaging. Thus, the locking or screwing up of windows and doors restricts ventilation and access for inspection. The restriction of ventilation is especially severe if windows are tightly boarded up.

Roof drainage

The most important sources of moisture causing timber decay in buildings are those associated with defective roof drainage (4). This may be as a result of leaks. For example, as caused by loss of lead guttering described above or as a result of the blockage and overflow of roof drainage systems. Of these, the latter can be the most dangerous, as blocked drainage can result in the pooling of water in the system. This then acts as a long term reservoir of moisture for decay organisms as compared to leaks which may only be intermittent. Common causes of blocked roof drainage are leaves and the activities of feral pigeons. However, it is surprising how often detritus such as tennis balls and plastic bags are found blocking hopper heads even on the highest of roofs. Similarly, ice and snow can block drainage systems, especially on the roofs of country houses or churches which often have roofs designed for regular cleaning by permanent staff.

Pigeons and plants

Feral pigeons are a special problem as they are particularly attracted to unoccupied buildings for nesting. Not only can their carcasses, feathers, faeces and nests block roof drainage systems, but they can also force their way into roof spaces by displacing slates (2). Creepers and other plants are obviously the source of the leaves that commonly block roof drainage if not cleared out regularly. In severely neglected buildings, they can also grow into roof drainage systems, blocking them with their stems and roots. Their roots can also grow down into pre-existing fissures in and open up leaks to masonry. Extensive creeper growth also restricts ventilation around infested structures.


Plumbing leaks can be an important source of chronic moisture penetration into the structure of unoccupied and partially occupied buildings. Burst pipes, as a result of freezing, cause the most severe water ingress and may commonly remain undetected long enough for large quantities of water to build-up in the structure. Minor leaks also typically occur around lavatory systems and the stop cocks of central heating radiators and these commonly cause localised timber decay.

Moisture reservoirs

Large quantities of water introduced into the structure of the building at any time, can form a reservoir of moisture in the fabric which will be available to timber decay organisms for many years. Such water penetration is commonly the result of catastrophic roof drainage leaks or gutter overflows, or of plumbing problems as described above. The most severe manifestation of this problem is seen after fires. As previously discussed, these are relatively common in unoccupied buildings and when buildings are not properly maintained. Many thousands of litres of water may be introduced into the building in the process of extinguishing such fires and roofs are commonly destroyed, leaving the building exposed to the elements for some time.

Poor ventilation

The major factor in the development of timber decay in unoccupied or partially occupied buildings is poor ventilation. In a well ventilated structure, timber decay may be limited even if it has been left roofless for several years. However, timber decay will quickly set in if such structures are subsequently roofed and boarded up. Factors affecting structural ventilation internally are doors or shutters, furnishings, stored goods, general detritus and floor coverings. The latter are especially important and fitted rubber backed carpets or linoleum commonly result in decay of floor timbers, because they restrict ventilation of sub-floor spaces. Linoleum, in particular, provides an ideal impermeable surface for the spread of foraging dry rot mycelium at its interface with floorboards.

Timber preservation

Previous chemical timber treatments have little effect on the development of timber decay in unoccupied buildings or buildings under reduced occupancy. This is because of the long time scales involved, compared to the very short duration of protection they provide in conditions of chronic water penetration or long term high moisture levels. The only effective way to control timber decay is, therefore, by altering those factors affecting moisture penetration and ventilation.

Proactive maintenance and conservation

Having considered the factors leading to timber decay in unoccupied buildings or those under reduced occupancy, it is possible to identify the minimum, cost effective, proactive measures that can be taken in each case to conserve the building. The aim of such measures is to set up the building structure and its management so that the environment is unfavourable to decay organisms. This involves attention to moisture sources, moisture reservoirs, moisture sinks and structural ventilation. If properly set up in this way, the system can be very robust and tolerant of major problems of water ingress and will even control pre-existing decay problems. The optimum combination is a self correcting balance of passive ventilation and moisture control measures. These should put the building environment and structure in to a ‘virtuous spiral’, whatever state it is in. By maintaining the building in such a ‘virtuous spiral’, the time remaining before refurbishment is usefully used in drying the structure and in eliminating decay organisms. This time then, becomes an asset for the buildings’ conservation and a financial advantage to the owner

Moisture sources

The commonest factors precipitating the growth of timber decay organisms in buildings are associated with roof drainage problems. Although roof surface defects and leaks can cause local problems, it can be surprising how a well ventilated structure will tolerate these without significant structural decay. Far more important are defects in the roof drainage system because water has been concentrated at these points. Gutter soles and hopper heads are also commonly associated with less well ventilated parts of a structure.

Special attention is therefore required to minimise defects, blockage and leaks. This will require regular inspection and maintenance. Measures that will help safe and easy access to the important parts of the roof drainage system are, therefore, well worthwhile, especially when time and money are restricted. Temporary repairs to roof drainage systems need not be difficult or expensive. However, in some cases it is cost effective to by-pass blocked or defective systems. The ultimate measure of this type is the installation of complete temporary roof structures. Less drastic measures are the installation of overflow spouts from gutters and the by-passing of blocked hopper heads or down-pipes by spouts draining water clear of the building. Such measures can be especially useful, as the effect of most roof drainage systems is to concentrate water and hence, multiply the size of the potential problem when a leak occurs.

It should not be forgotten that all such temporary roof and roof drainage measures will require the same attention to inspection and maintenance as the main systems. Similarly, balconies and the roofs over bay windows and porches require the same consideration. Water penetration at ground level should also be minimised. This is made far easier and cheaper if all rubbish and vegetation can be cleared from around the base of the walls and external ground levels can be reduced to below internal floor levels.

Ultimately, if drains are inadequate or cannot be entirely kept clear, the digging of temporary surface water drains is worthwhile. It is important that water overflowing from roof drainage is prevented from splashing back onto the walls. This may be done using temporary spouts and gutters as described above. Obviously, the most economical way of minimising moisture sources inside the building is to turn off and drain any plumbing or heating systems that are not required. Those that are still in use will require inspection and maintenance

Moisture reservoirs

A number of the factors that favour timber decay organisms act as moisture reservoirs, providing a long term and continuous supply of moisture for their growth. If such moisture reservoirs are identified measures can be taken to isolate them from valuable structures and to allow them to be dried out in the long term. This will usually only involve minor exposure work and attention to ventilation, though in some cases more active measures are useful. As a first step, if building rubbish, furnishings, or other movable items have become wet and are acting as a moisture reservoir, they should be removed from the building. Building materials such as insulation material, plaster, pugging or other infill material can be similarly disposed of if they become wet. This is the especially useful if they are in sealed cavities as is case with the masses of building rubbish that can be found in most sub-floor spaces

Moisture sinks

In a building under reduced occupancy and maintenance, it has to be accepted that there will be some moisture sources and even occasional catastrophic water penetration into the structure. Given this situation, it is necessary to arrange adequate moisture sinks for all foreseeable moisture sources or moisture reservoirs. Ideally these should be passive, should require minimal intervention or maintenance, and be self correcting. As water tends to percolate downwards through a structure the provision of good drainage for the foundations is useful. As described above, the physical removal of wet materials from the building is also cost effective. Mechanical dehumidifiers can sometimes be useful and may be cost effective in some circumstances. However, the most important moisture sink is provided by ventilation


All structures and cavities should be provided with adequate through ventilation and anything that tends to restrict this should be removed. A through flow of fresh air not only acts as a moisture sink but also has a direct effect in limiting the growth of decay organisms. The dry rot fungus Serpula lacrymans particularly, is unable to grow in these conditions and is the major cause of structural decay in neglected buildings (6). As a first step, all windows can be fixed slightly open and any protective boarding perforated or spaced away from the window frames. Similarly, all internal door hatches or cupboards should be fixed open. It is most important to allow adequate ventilation into the top and bottom floor of the building. This takes advantage of the ‘stack effect’ to increase through ventilation and also protects the two areas of the building where most water penetrates or gathers. Any fireplaces or chimneys should be opened up to allow through ventilation of the structures involved. This also takes advantage of the stack effect to ventilate the rooms they serve

Any furniture, rubbish, or other stored goods can restrict ventilation and is best removed. This is especially important with fitted floor coverings. These should be removed wherever possible or at least rolled back from external walls to allow some ventilation to ‘at risk’ floor timbers. The next step is to carry out minor exposure work to allow through ventilation of any sealed cavities. Such works need to be carefully carried out and supervised in order to prevent damage and to maximise their effect. Examples of such measure are the raising of floorboards overlying joist ends in external walls, and the opening up of window soffits, shutters or reveal boards. Where moisture sources or moisture reservoirs are found, extra ventilation may be introduced. For example, by the cutting of vents in lath and plaster to allow a through flow of air behind, or by the stripping of plaster overlying built-in timbers. Insulation materials and pugging can also be removed from around ‘at risk’ structures to allow proper ventilation. It is important that any of these works are carried out carefully so as to preserve any materials removed that may be used on renovation. For example, floorboards and window shutters should be carefully identified and stored. If timbers removed in this way are damp or partially decayed, they should be stored so as to allow adequate ventilation and drying of the individual elements. In this way, considerable savings of historically important materials can be made and the cost of renovation significantly reduced

In some cases, it is cost effective to continue heating an unoccupied building. Ideally, this should employ low level structural heating techniques to increase the movement of moisture from the structure into the air. A similar effect can be achieved by careful use of pre-existing heating systems run continually at a low setting. However, intermittent heating, air heating systems or heating without proper structural ventilation is to be avoided. These tend to promote condensation and may even increase the rate of growth of some decay organisms. They are thus, at best, a waste of money and may be counterproductive.


Monitoring and maintenance can be the most important and the most problematical part of conserving a neglected building. This is not just because of financial constraints but is due to the difficulty in organising appropriate small scale inspections and works. For this reason, a specialist consultant and contractor team are required. This ensures that information is not lost, that continuity of policy is maintained and that fixed costs are minimised

The most cost effective interval between inspections will vary with the complexity of the building and its state of decay. Once the environment has been stabilised by appropriate measures, less frequent inspection may be required, but in all cases there should be at least two thorough inspections per year. Inspections should access all the relevant factors described above and be followed by appropriate maintenance. Any measures that can be taken to help this process can be very cost effective. For example, the provision of ladders and temporary access hatches to facilitate access are especially important

Some techniques have been developed specially for inspection and monitoring work, to make it as cost effective as possible (7). For example, the use of Rothound search dogs is generally useful because of their ability to quickly search a large building and indicate the extent and degree of dry rot activity. This gives a timely indication of critical areas for maintenance work and picks up any residual ‘hot spots’ of fungal growth from previous problems which require further measures (5). In some cases, electronic monitoring and alarm systems are useful. These can allow the rapid reassessment of inaccessible structures and can even be connected via the telephone system to allow remote monitoring and inspection of buildings. Such remote monitoring and alarm systems can save travel expenses and high cost expert time. They also allow the earliest possible detection of problems in unoccupied buildings. The most useful sensors on such systems are timber moisture probes and roof drainage overflow sensors. Standard burglar alarm sensors and systems for the early detection of the activities of anyone trying to steel roof lead can also be installed


The existing stock of buildings should be conserved, not for cultural or historic reasons, but because they represent a major national asset. This is because they are a repository of irreplaceable natural resources, especially high quality timber from the virgin forests that we have now destroyed.

Similarly, the craftsmanship and artistry that went into the design and construction of these buildings is no longer available. It is to be hoped that we are moving from a society based on extravagant consumerism to one aiming at sustainable development. These assets are thus becoming increasingly important to our future prosperity and quality of life.

The combination of design and material found in traditional buildings has often evolved over many centuries, to give an optimum solution to a particular social and environmental requirement which we discard at our peril. This principle does not just apply to the prevention of damp and decay. It is equally relevant to environmental health, where the importance of ventilation was well understood and far better managed than today. Similarly, those readers who have experience of open fires will appreciate that ‘energy efficiency’ was a real necessity, not just a slogan, when people had to carry energy into buildings by hand in the form of solid fuel. Because of this, the solutions that were evolved by our predecessors involved structural storage heating and the management of microclimates to a level that we are only now beginning to appreciate

If we accept that buildings should be conserved as discussed in this article, then there are six very simple and cost effective measures that can be taken in unoccupied buildings and those under reduced occupancy whatever their current state of neglect. They can be summarised as follows:-

  1. Inspect and clear roof drainage systems at least twice a year
  2. Ensure ventilation through all windows, internal doors and hatches
  3. Remove all floor coverings and rubbish
  4. Raise floorboards along external walls and in damp areas
  5. Turn off and drain all unnecessary plumbing
  6. Open all window shutters and reveals and soffits if damp

Even the partial application of these measures, in the absence of advice from expert consultants, will significantly reduce the damage caused by damp and decay. In the opinion of the authors, there are compelling financial, environmental and cultural arguments for all professionals involved with conservation to undertake an urgent campaign to bring these policies to the attention of property owners and the building industry generally.


Hutton T.C., Lloyd H. & Singh J. (1991) ‘The Environmental Control of Timber Decay’. Structural survey, vol. 10, no. 1, pp 5-20
Hutton T.C. & Dobson J. (1992) ‘The Control of Feral Pigeons: An Independent Approach’. Structural survey, vol. 11, no. 2, pp 159-167
Hutton G.H. (1989) ‘Building Defects Leading to Bio deterioration’ in ‘Proceedings of the first international conference on Building Pathology 89’ (Eds Bans et al), ISBN 0907 10102X
Garrat J. & Nowak F. (1991) ‘Tackling Condensation’ Building Research Establishment Report BR 174, ISBN 0 85125 44446
Hutton T.C. (1991) ‘Non Destructive Testing’. Building Research and Information, vol. 19(3), pp 138-140
Jennings S.D.H. & Bravery A.F. (1991) ‘Serpula lacrymans Fundamental Biology and Control Strategies’. John Wiley & Sons, New York, ISBN 0471 93058X
Singh J. (1991) ‘New advances in identification of fungal damage in buildings’. The Mycologist, vol. 5(3), pp 139-141
Singh J. (1989) ‘The ecology and environmental control of timber decay in buildings’. Construction and Repair, vol. 3, number 3, pp 26-30
Bravery A.F., Berry R.W., Carey J.K. & Cooper D.E. (1987) ‘Recognising wood rot and insect damage in buildings’ BRE, ISBN 085125 2443

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