BCD 2017

T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 149 PROTECTION & REMEDIAL TREATMENT 4.1 particularly favourable roosting and nesting site, it is believed that the suitability of the structure for roosting or nesting and the lack of disturbance by human occupants is the most important factor. EFFECTS OF CONTAMINATION Physical It is possible for the sheer weight of the accumulations of guano to cause damage to structures, particularly to relatively lightweight structures such as ceiling plaster. However, this generally occurs in conjunction with decay or partial decay. Physical damage may be compounded because inspection and maintenance activities have been curtailed due to the presence of birds or, in particular, bats. This can be a significant problem both on the interiors and exteriors of buildings. In these cases legal restrictions or ‘fear of legal restrictions’ affecting access often hinder maintenance and conservation activities. Significant physical damage may also result from attempts to remove accumulations of guano, or to deter or exclude bird access. For example, unnecessary abrasive cleaning or the damaging installation of fixings for exclusion nets or deterrent spikes. Probably the main cause of physical damage to buildings which results from the accumulation of guano, nesting materials and/ or carcasses is the blockage of roof drainage systems and other services. Blocked roof drainage systems lead to water penetrating structures and providing conditions for chemical and biological decay. Chemical The constituents of bird guano and urine can have direct and indirect chemical effects on building materials. However, these chemicals generally need to be dissolved in water for their direct effects to be significant, and often the most important problem arises from their ability to promote the growth of algae, bacteria and fungi. Guano contains high levels of uric and other acids resulting in a pH of 3–4.5. These acids can attack calcium carbonate and other ‘binders’ in natural stone, artificial stone and even concrete. They can corrode metal building materials such as copper or bronze and attack their protective patina. This can result in aesthetic, and ultimately structural, problems. Although brick and other clay-based materials may be relatively resistant to acids, they are vulnerable to damage from salts. Guano contains high levels of phosphates, ammonium, potassium, chlorides and other materials. These and the salts resulting from acid corrosion can result in severe problems of efflorescence and spalling. Similarly, the effect of the acids and salts on the porosity of masonry and other materials can result in accelerated moisture and frost damage. Bat urine is also acidic and can cause irreversible damage to materials. In historic churches, for example, bat urine can cause staining on brass, marble, slate and polished timber, and may damage porous surfaces such as stone and terracotta. A further potential cause of severe chemical damage to building materials is the use of inappropriate and unnecessarily aggressive cleaning materials in an attempt to remove guano. In the past there has been a particular problem with the inappropriate cleaning of decorative masonry façades, ornamental enrichments and monuments. In these cases, the solution can be worse than the original problem. Biological Algal and/or fungal growth resulting from the additional nitrate and phosphorous components found in guano can cause severe problems for the conservation and maintenance of historic building materials. In particular, the high nitrate content of guano will promote the rapid growth of dry rot ( Serpula lacrymans ) in softwood timber structures, especially where associated with damp masonry or plaster. Although dry rot can grow in the absence of these materials, the growth of the organism is often constrained by the lack of nitrogen as well as the lack of available moisture. Similarly, the presence of nitrogen can promote infestation and decay by wood-boring insects such as woodworm ( Anobium punctatum ) . The increased growth of bacteria, algae, fungi and lichen also causes the production of acids and salts resulting in chemical and physical damage as described above. Aesthetic There is an obvious ‘yuck’ factor associated with accumulations of guano, which can cause unpleasant odours, especially if damp. The colour and appearance of soiling – and any resulting algal growth – may adversely affect the appearance of both historic exteriors and interiors. Soiling may appear as light streaking on a dark background, dark staining on a light background, or green algal staining. HEALTH HAZARDS As well as representing a potential slip hazard, accumulations of guano may contain potentially serious human pathogens such as campylobacter, salmonella, listeria, chlamydia (causing psittacosis), histoplasma, cryptococcus, influenza and other viruses. In addition, the living organism, carcasses, associated parasites such as fleas and mites, and nesting materials are also potential health hazards. In the UK and Northern Europe, guano and related detritus are unlikely to cause serious clinical disease in humans, unless the deposits are grossly mishandled and people are massively exposed to them, either by ingestion or inhalation. Nevertheless, investigations of the immune response of populations indicate that many people who are exposed to these pathogens have developed sub-clinical infections which resulted in an immune response. This might be expected, as many of these infections are described as often causing ‘mild flu-like symptoms’, and only occasionally cause more severe disease. It is therefore probable that these infections are under-diagnosed in the UK and abroad. They can therefore cause serious and potentially fatal infections in people with HIV or under immunosuppressed treatments. Similarly, guano and associated materials can cause severe allergic diseases such as alviolitis, so they should always be regarded as a significant potential health hazard. MANAGEMENT AND REMOVAL The build-up of guano in the built environment is due to a failure to prevent occupancy by birds or animals, and/or a failure to prevent the accumulation of guano. The reasons for these failures are varied but once the decision has been taken to address the problem, the role of the building pathologist and other building professionals is to prioritise the sustainable occupancy and conservation of the affected building or structure, while carefully considering any other factors relevant to the site. The key factors for consideration in this process are outlined below. Control of pigeon/bat occupancy Control may be straightforward in terms of the practical measures needed but it can be very difficult in terms of execution because of the complex social, cultural and legal factors which must be taken into account. As a result, large sums can be spent on pigeon deterrent and exclusion measures which are ineffective and/or harmful to the building. Often the most cost-effective way of controlling pigeon occupancy is to eliminate those birds which have been bred on the building and therefore have a ‘homing instinct’ to the structure. This is best undertaken by a programme of live trapping and culling. However, it may require careful management of interested parties and public relations to Bat guano accumulating on protective sheeting after just a short period of time (Both photos: David Watt) Bat urine damage to a historic church ledger stone