Bats in Churches

Bats in Churches: Challenges, Solutions and the impact upon Stained Glass Conservation

Fig. 1. Bats in Churches Logo

When approaching any stained-glass conservation project, it is important to consider all environmental factors and agents of decay. One of these contributing factors is the presence of bats. In this article, Rachel Arnold, heritage advisor for the Bats in Churches project, explores the challenges that bats present while roosting in churches, the impact this has on the stained-glass conservation profession and possible solutions to save both our natural and built heritage.


The Bats in Churches project launched last year to find solutions for some of the challenges presented by bats roosting in churches. While bats are an important part of our biodiversity, their presence can be a concern for anyone who cares for historic churches and their contents. Droppings and urine can cause significant damage to historic fabric and furnishings and are a cleaning burden to volunteers (Figs. 2 and 3). The presence of a roost and location of bat access points can disrupt conservation projects, building works and maintenance plans. However, like our vulnerable historic artefacts, bats are also under threat. Large-scale loss of their natural habitat has resulted in a dramatic reduction in bat numbers.1 They have become increasingly reliant on church buildings as places to roost and are now protected by law. Balancing the need for the preservation of bats and of church fabric is a challenge, but one that the Bats in Churches team are tackling.

Fig. 2. “Some churches struggle with the mess caused by bats roosting in them. At St Andrew’s Old Church, Holcombe, covers are draped over the pews to protect them. This is a common sight in several churches” (Image © Barry Cawston)

Fig. 3. Although stained glass is usually set back in its’ window aperture it can still suffer the affects of a bat presence. This extreme example of a soiled windowsill is in St Moran’s church, Lamorran, Cornwall. The build-up of droppings is likely to harbour damp and be a breeding ground for mould and pests.” (Image © Bats in Churches / Diana Spencer)

Staffed by team members from the project partners: the Churches Conservation Trust, Historic England, Natural England, Church of England and Bat Conservation Trust, the Bats in Churches project brings together experts from a variety of fields to find solutions to the unique challenges presented.

The project is carrying out ongoing research throughout its five years of activity and as part of this we are looking to find out more about how bats affect windows, glass and stained glass.

The Importance of Churches and of Bats

The importance of churches should not be underestimated. The buildings themselves contain a wealth of history and artefacts, including stained glass, but also are a place for people’s personal histories and play an important role in the local community. Through stained glass for example, we are able to trace the changing tastes of a specific area and also the changing landscape of popular religious devotion through symbolism, styles and ages.

Unfortunately, the sustainability of churches is at risk, as smaller congregations struggle to cope with their care, and reduced funding means that essential repair and conservation projects are often put off. The presence of bats can add to this problem by adding a further barrier in the process of carrying out essential works, creating a cleaning burden and causing, sometimes irreparable, damage to ancient artefacts.

Bats are a fascinating and vital part of the natural environment and are a key indicator species that signals a healthy eco-system.2 Bats are a natural pest controller as they forage at night for insects. The common pipistrelle bat can eat as many as 3000 insects in one night.

Bats are an ancient group of mammals and have been around since the cretaceous period. Originally, their natural landscape would have provided ample shelter, but due to habitat destruction for development and farming, they have used buildings like churches, barns and houses to roost in for centuries.

Changes in rural land use, increased urbanisation, the re-development of old buildings such as barns and other factors, like pesticides in farming and wind turbines, have put bat populations at risk. In the last 100 years, for example, the lesser-horseshoe bat has reduced in number by 90%. The Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017 now strictly protects all British bat species, their breeding sites and resting places. Causing damage to a roost site or bat is against the law and punishable by fine or imprisonment. As a nation that wants to protect our environment, we have a responsibility to look after our bats.

Why do bats use churches?

Fig. 4. “Bats like to roost in cracks and crevices in historic buildings like churches. This cluster of Brown Long-Eared bats have found a cosy corner in which to roost.” (Image © Hugh Clark / Bat Conservation Trust)

The nature of traditional buildings lends itself to the needs of bats. Over many years, worshippers have extended and adapted churches to meet their needs. Inevitable cracks and crevices appear in the stone and wood, which are perfect for bats to crawl into, and they find voids within which to roost (Fig. 4). Historic buildings are breathable, maintain a steady temperature and have long open spaces, like a nave roof, which are all things that bats like. Modern buildings are less suited to bats because, being built for human habitation, they have good insulation, few gaps in the walls, good lighting and can be noisy.3

Windows themselves offer very good access points for bats. They will use open windows, ventilation gaps, areas where there are missing pieces of glass and any other tiny little holes in and around the window. These locations are very important access points for bats and in some cases, they may need to be preserved in order to maintain access to the roost. Depending on the situation and size of hole, this is not always an acceptable outcome.

A wonderful example of bats using an access point in a window, or being given an access point, is at St Michael’s church in Bramcote, Nottinghamshire. This church has created a new window that incorporates an access point for bats into its design. The bats already used a similar hole in the previous window as their entry point to the building.

The exterior of the church is also very attractive to bats. Churchyards often have lots of wildflowers and grasses. Sometimes, there is an area which has been left to grow wild. This means that the churchyard can be the perfect place to forage for insects.

What effects do bats have on churches?

The environment can affect everything inside a church, from monumental brasses to painted medieval glass. The most evident impact on the churches from bats is the physical presence of bat droppings on floors and other surfaces, which presents an increased need for regular cleaning. This is visible to churchwardens, visitors, conservators and bat specialists alike, but the residual effects of the presence of bats in churches go much deeper, presenting long-term, visual and chemical change in the surfaces of historic fabric.

Fig. 6. “Bat urine is corrosive to metal and monumental floor brasses are particularly affected. The blue spots are evidence of this damage and is disfiguring in the face of the brass in St George’s church, West Grinstead.” (Image © Bats in Churches / Rachel Arnold)

Over the last thirty years, a number of research projects have been carried out to investigate the effects of bat dropping and urine on various materials. The most recent and in-depth study was by Dr James Hales, who analysed the content of bat droppings and urine and tested their effects on materials, both in situ and in a controlled laboratory environment.4 His experiments have led to a deeper understanding of the situation and the extent to which the damage resulting from bat droppings and urine is permanent or reversible. Hales mainly focused on stone, wood and metal. An earlier study, by Stephen Paine in 1993, investigated the impact of bat excreta on medieval wall paintings.5 Paine investigated how the chemical content and physical presence of droppings and urine can affect the wall paintings.

From these papers, and other additional research, it is clear that droppings are made up of indigestible insect exoskeletons and are very dry. They look a lot like mouse droppings, but while mouse droppings are hard, bat droppings crumble into dust under pressure. The content of droppings alone is relatively harmless, but small amounts of nitrogen, fats and oils can cause staining. This staining is worsened when droppings are mixed with water and/or urine deposits, especially on light-coloured, porous materials like marble. Droppings can also adhere strongly to surfaces, which is problematic for vulnerable or friable areas, such as wall paintings or stained glass with flaking paint.

Bat urine contains 70% urea, which dries to form ammonia. This strongly alkaline substance is chemically aggressive and causes etching and staining in various materials, especially brasses and woodwork. Urine deposits are of a higher cause for conservation concern because they cause more damage than droppings on their own and are harder to detect. Bat urine can sit undetected on a historic object for a long period of time and cause visible and chemical long-term damage.

The table below summarises the effects of bat excreta on various materials found in historic churches. It draws primarily upon the research carried out by Hales and Paine and also the visual evidence and light-touch inspections of churches carried out by the author.


Visual change when in contact with urine or droppings

Further details



White powdery residue on surface

After urine evaporates a small amount of white powder is left on the surface of the stone.

Urine is drawn into the porous substrate of the stone prior to evaporation.

Droppings alone can result in staining, especially on marble, but it is more likely when droppings and urine appear together.

Monuments are made from marble and alabaster which are particularly affected. The results can be visually disturbing as well as damaging.

Untreated wood

Spots of watermarks


The areas which have been exposed to urine droplets will appear lighter than the rest of the wood having been effectively bleached.

If droppings have mixed with water on the wood surface a darker stain may occur.

Waxed Wood



White bloom developed within wax coating

The wax provides a protective layer on the surface of the wood and a white powdery layer remains on the surface after the urine evaporates.

The wax develops a white bloom within the wax layer itself.

Wood coated with shellac

White powder on surface and some blistering

A complete shellac coating does not allow the urine to penetrate through but instead the urine site on the surface, evaporates and leaves a white powder.

If the shellac is cracked or urine (and other moisture) can get underneath the surface it will cause the shellac coating to blister and lift.

Tiles (coated/encaustic)                 

Significant staining and white deposits are visible on the tiles that accelerate the degradation and process. This can cause cracking and blistering of surface finishing


Tiles (uncoated/pantiles)

Significant staining and white deposits


The porous surface will absorb the urine and its damaging chemicals into the surface. This can be damaging in the long term, but no research has been carried out.







Delamination of vulnerable paint surfaces, acceleration of corrosion process but no obvious visual effects in the short term

If left in contact with a stained glass or plain window for a long period of time the urine and droppings will accelerate the degradation and corrosion process. Droppings can adhere to the surface and may cause vulnerable painted surfaces to delaminate (Fig. 5).

Metal (copper alloy)

Significant staining

The presence of urine on the surface of the metal can result in a chemical change which leads to permanent visual change e.g. etching of brass monuments (Fig. 6).




Light staining



Staining, structural degradation stickiness of fibres

Drops of urine are visible on the surface of textile and cause significant staining. If the pile of the textile is particularly deep, the fibres are often congealed and stuck together. Once the urine soaks into the fabric it will begin to chemically degrade the fibres leading to structural issues.

Wall paintings

Staining, physical degradation of the substrate, flaking and, in extreme cases, delamination

The surface of wall paintings is porous and readily absorbs urine. This leads to staining.

Droppings can adhere strongly to the surface of the wall paintings. If they expand by taking in moisture and shrink when they dry out again small particles of painted detail can be pulled off the wall.

Other painted surfaces

Risk of adhesion of droppings to vulnerable painted surfaces causing delamination and loss.



The summary of this background material displays a clear gap in the research. Materials that have not been covered with thorough research over time or in a control environment are glass, textile, plaster and painted surfaces on wood and glass.

Effects on glass and stained glass

Fig. 5. “This stained glass window at St Mary Magdelene’s church, Brampton has bat droppings stuck on the surface. It is harder to spot droppings and urine on the surface of stained glass.” (Image © Bats in Churches / Rachel Arnold

Stained glass windows are usually set back in the aperture of a window so, are less likely to receive the same covering of droppings and urine that a flat surface in the main body of the church might. However, where bats are using the window as an access point the glass is very likely to receive the full impact.

Based on our knowledge that glass chemically reacts with its environment, especially when in contact with moisture, it is undoubtable that glass will react and deteriorate if droppings or urine are present on the surface (Fig. 5).

The most obvious ways that this could happen are:

  • Lifting of paint layers
  • Destruction of paint layers
  • Chemical reactions on the glass surface
  • Corrosion of the glass surface
  • Attraction of dust and moisture on the surface


Yet there has been no robust research or thorough investigative work done into the effects of bat excreta on glass and stained glass. The Bats in Churches project aims to plug gaps in the current field of research by collecting evidence of bat damage and encouraging further research into the degradation phenomena of bat excreta on various fabric, including stained glass. It is important that we understand how bat droppings and urine affect glass and the severity of the problem, so that we can recommend the most appropriate approaches to bat mitigation that will support the preservation of historically significant stained glass in situ.

Further impacts of a bat presence

The presence of droppings and urine and the damage caused present an increased need for cleaning. Even when cleaning carefully with conservation approved materials and approaches, historic fabric is at greater risk. Regular cleaning and well-intended maintenance can result in damage, for example the loss of paintwork and abrasion of metal surfaces.

Droppings also act as a source of nutrition for bacterial and fungal growth which causes decay.6Large accumulations of droppings are a welcome area for general dust, dirt and humidity to form (Fig. 3). This presents the added problem of sulphuric and nitrile acids which attack surfaces, alongside the presence of the droppings.7

Finally, the physical movement of bats across historic surfaces can cause abrasion and discolouration. Bats clambering over a door or across the top of carved wooden rafters when entering and exiting their roost will slowly wear this surface down and cause it to darken after years of abrasion by fur and claws.

The presence of bats can also complicate the process when churches are applying to undertake urgent repair work, for example after lead theft, through the Faculty system (or equivalent consent processes for non-Church of England places of worship). The timing of the work is dependent on the type of roost present, so the Parochial Church Council (PCC) and architect need to plan the faculty application to take account of the time-limited window for the works or they risk being delayed considerably. This is where the Bats in Churches project, described in more detail below, is hoping to combine the specialist knowledge of their various partners to provide guidelines for churches looking to navigate this procedure and to update the standard procedure itself.

What does this mean for stained glass conservation?

As mentioned previously, bats are protected by law. It is punishable by fine or imprisonment to do any of the following:

  • Deliberately capture, injure or kill bats
  • Damage or destroy a breeding or resting place
  • Obstruct access to their resting or sheltering places
  • Possess, sell, control or transport live or dead bats, or parts of them
  • Intentionally or recklessly disturb a bat while it is in a place of shelter


Although not all conservation or building projects will affect the bats or their roosts it is best to call the National Bat Helpline for free advice, just in case. You may be eligible for a highly trained volunteer (a Volunteer Bat Roost Visitor) to come and carry out a free assessment. It is important to contact the Helpline as early as possible to benefit from this service. Following the site visit, you will receive specific written advice relating to your church, the planned works and any important information to help the works progress. In some cases, if the scale of the works is outside the remit of the volunteer service, or is very complex, you may need to consult a professional ecologist for more surveys and even a licence.

The surveys and licenses are something that need to be organised by the client and, if a license is sought, it must sit with the client or site. That is not something the conservator need worry about applying for, but something to be aware of nonetheless.

A lot of the conservation projects and small works that a stained glass conservator takes on are unlikely to be affected by the presence of a bat roost. General in situcleaning and small repairs can carry on as normal and are not likely to need a bat licence. However, for some works it is important to check whether the church or site has had a bat survey and that your work will not impact a roost, these include:

  • High-level work that requires the erection of a scaffold. If the scaffold is erected close to an access point to a roost it may confuse a bat and obstruct their access.
  • Replacement of broken panels. If the broken piece of glass has been used for a long period of time as an access point and there is no other way to gain access to the roost then you will be causing an obstruction.
  • Large-scale projects involving the removal of whole lights, windows or panels. Again, the erection of a scaffold or the prevention of entry could obstruct the access to a roost.
  • Being part of a large-scale building project, within which the cleaning or repair of glass is a minor part. Although the minor stained glass work is unlikely to impact a roost compared to the large-scale activity that is taking place, it is good practice that you make sure all of the correct licenses are in place before taking on the work.


Often works will be restricted to certain months of the year, when the bats are least active on the site. But this is something that changes site-to-site depending on the roost that they have. The most likely outcome, however, is that there is a maternity roost using the church. The bats in this kind of roost are active between April and September, so works are often restricted to between October and March.

To conclude, it is important to check that the work you are being commissioned to do by a client will not impact upon a bat roost and, if it might, that a survey has been done and, if needed, a licence has been obtained.

How does the Bats in Church project work?

The development of a new license by Natural England and funding from the National Lottery Heritage Fund made the Bats in Churches project possible. The Bats in Churches Class License (BiCCL) allows highly skilled ecologists, with years of experience working in churches, to trial novel solutions to reduce the damage being caused by bats in churches. By bringing together church communities, ecologists and heritage professionals, the project is helping to find the most appropriate solution for each project church, and to monitor its success over a 5-year period.

One early case study that has had some success already is All Saints’ church, Braunston-in-Rutland. This Grade II* church is home to a large maternity roost of Soprano Pipistrelles and was struggling to keep on top of the situation. The fifteenth-century wall paintings, pews, monumental brass and a twelfth-century font were all being damaged, and the strain of cleaning and unpleasant smell had left the PCC believing that they would have to close the church.

One of the specially licensed ecologists, Dr Charlotte Packman of WildWings Ecology, carried out surveys and concluded that the bats did not need to use the inside of the church at all. Instead, they were given access points from the outside of the church straight to their roosts in the rafters and eaves on the south side. The access points on the inside of the church were blocked up in Spring 2019 and the bats no longer fly inside the church, eliminating the mess and disruption that they once caused.

Surveys show that the colony is still thriving and a churchwarden commented: “It’s been a fantastic outcome for us. The church can now be used as it was originally intended.”

Fig. 7. “At All Saints church, Swanton Morley in Norfolk the Bats in Churches project have installed bat boxes at eaves level and blocked their entry points into the interior of the church.” (Image © Bats in Churches Project)

At over 100 other churches, the project is exploring lots of different techniques for bat mitigation. Some of these include isolating the bats from high-risk areas within the churches through roof voids, tower spaces, rafter boxes and external bat boxes (Fig. 7). Each church is unique, as are its bats, so the techniques will vary, but we are very hopeful that we can help as many churches as possible.

Some of our churches are exploring the use of access points as part of the design for their bat mitigation. One in particular is looking at using the apex of a window where the bats enter and installing a discrete bat box above it. This project is still in the planning phase, but after implementation and detailed surveys to monitor its success, I am planning to present the findings in a future edition of Vidimus.

In addition to developing bat mitigation solutions, the project is hosting training sessions, workshops and events, and publishing free online resources to download from the website.

At the end of the project, there will be plenty of well-trained professionals who can employ the techniques successfully trialled at the project churches. We hope that the connections made between ecologists, bat groups, architects, diocesan committees, conservators, and church communities will continue, and that these groups will come together with a deeper understanding of each other’s concerns and the skills needed to protect both bats and churches.

Every church is different, as is each species of bat, and how they use a building. Therefore, approaches to bat mitigation at every site must be tailored appropriately. This is why surveys are so important. At the end of the project, we will have a broad sample range of tried and tested techniques that can be used at different sites.

Get in touch, follow the projects activities and what is coming next

Fig. 8. (Image © Hugh Clark / Bat Conservation Trust)

The project aims to offer advice and support to a wide range of professions. If you would like our help with a conservation project that has felt the impact of bats, please let us know. We can advise upon surveys, licensing, identifying bat damage and discussing how to protect historic fabric from bats. Furthermore, the project is keen to facilitate further research into the damaging aspect of bat droppings and urine and how the presence of bats affect buildings and conservation projects. If you have any information that you would like to share, or if you would like to help us with our research, please get in touch. We would love the project to branch out into the world of stained glass conservation if possible. Get in touch via our website (details below).

To find out more about the project and our case studies visit our website The website has a contact page and a form to fill in to sign up to our mailing list. Or find us on Twitter @BatsinChurches and Facebook @BatsinChurchesProject.


  1. Joint Nature Conservation Committee (JNCC) and Bat Conservation Trust National Bat Monitoring Programme Annual Review 2018(2019). []
  2. Department for Environment, Food and Rural Affairs (Defra) Biodiversity Indicators in Your Pocket(Defra Publications 2009). []
  3. MadeleineRyan, “Bats, Churches and Landscape: Ecology of soprano pipistrelle bats in eastern England”, unpublished PhD thesis, University of Bristol, 2019); Christian Dietz and Andreas Keifer, Bats of Britain and Europe(Bloomsbury, 2016). []
  4. James Hales, ‘Bats in Churches: Objective Assessment of Associated Damage Mechanisms’ in Archaeology International 17 (2014). []
  5. Stephen Paine, ‘The Effects of Bat Excreta on Wall Paintings’ The Conservator17 (1993): 3–10. []
  6. Jonathan Howard and Phil Richardson, Bats in Traditional Buildings (English Heritage, National Trust and Natural England 2009): 56–57. []
  7. Nicola Ashurst, Cleaning Historic Buildings Vol. 1: Substrates, Solvents and Investigation(Donhead 1994): 63. []

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