Understanding Breather Membranes in Roofing: Why They May Not Be Necessary for Historic Buildings
In modern construction, breather membranes have become a common component of roofing systems. These materials can offer benefits such as weatherproofing, moisture management, and improved thermal efficiency. However, when it comes to historic buildings (and sometimes modern buildings), the use of breather membranes requires careful consideration. In this post, we'll explore the purpose of breather membranes and why they may not be the best option for introduction in historic buildings.
What Are Breather Membranes?
Breather membranes are lightweight, flexible sheets used in roofing and wall constructions to provide a weather-resistant yet “breathable” barrier. They are designed to prevent water ingress from external sources while allowing moisture vapour to escape from within the structure. This dual functionality helps to manage condensation and maintain a healthy, dry environment in the building envelope. This “breathability” operates at the microscopic level, much like a breathable raincoat, keeping water out while supposedly letting sweat escape. However, many of us have experienced how “breathable” such a jacket is during a hike, as they can become rather damp inside. This is because they cannot move large amounts of water as vapour, unlike a base layer or wool, which wicks moisture away as a liquid and dries through wind drying and evaporation.
Evolution of Roofing Membranes
Roofing membranes were initially introduced to make roofers' work easier by adding roofing felt, which eliminated the need to mix large amounts of lime mortar or cement to bed and torch tiles. Roofers could simply lay roofing felt, which also acted as a weatherproof shield while the roof covering was off, and then batten and place tiles. This greatly sped up the roofing process, making it more profitable but ultimately deskilling the craft of the roofer.
With bitumen-type felt, wind-driven rain would be caught and fed to the gutters, so the roof covering did not need to be as watertight as it once was. Bitumen 1F felt decays over time, often rotting at the gutters and wherever light hits it. Additionally, where water gets trapped beneath and underneath through condensation, it causes decay of the timbers. Companies tried to overcome this through the use of plastic membranes of various types throughout the latter part of the 20th century and into the early 2000s.
Breather membranes were mainly introduced to stop or mitigate the decay caused by the non-breathable nature of 1F felt. These membranes prevent liquid water from passing through from above while allowing water vapour to pass the other way. However, this only works under certain conditions, similar to the Gore-Tex jacket analogy. Despite the advancement of membranes, issues have arisen with breathable membranes as well. In dusty and old buildings, roofing membranes can clog over time, reducing their breathability. They also continue to trap water as they cannot transfer it across the membrane in the liquid phase.
Key Functions of Breather Membranes
1. Weatherproofing: Breather membranes act as a secondary layer of protection beneath the primary roof covering, such as tiles or slates. They prevent rain, snow, and wind-driven moisture from penetrating the roof, safeguarding the underlying materials from water damage. The effectiveness of this weatherproofing layer relies on the proper installation and maintenance of the membrane, ensuring it functions as intended throughout the roof's life.
2. Breathability: One of the primary advantages of breather membranes is their ability to allow water vapour to escape. This breathability helps prevent condensation buildup within the roof space, reducing the risk of mould growth and the deterioration of insulation materials. The breathability of these membranes is particularly important in modern homes with higher levels of internal moisture due to activities such as cooking and bathing.
3. Thermal Efficiency: By helping to manage moisture levels, breather membranes can help maintain the thermal performance of insulation. Dry insulation is more effective at regulating temperature, leading to improved energy efficiency and comfort within the building. This contributes to reduced energy costs and a more sustainable approach to modern construction practices.
Traditional Roofing Techniques: Torching
Before the introduction of modern membranes, traditional roofing techniques relied on a process known as torching. Torching involved applying a layer of lime mortar to the underside of roof slates or tiles. This could have been applied afterward to the backside of the roof covering but was often combined with the laying of the roof. When tiles and slates were bedded on lime, the bedding material would squish through and then be either left, in the case of something agricultural, or dressed to the batten or lath, or added to the underside of the roof to create one homogenous finish from lime plaster to lath, to slates, in which case it was called a sheeted roof. The method of torching served multiple purposes:
· Waterproofing: The lime mortar acted as a buffer, preventing water from being blown through the gaps between slates or tiles. It sealed the roof against external moisture while maintaining a natural equilibrium with the environment. This technique provided robust protection against wind-driven rain and other environmental factors.
· Moisture Absorption: Given that lime can absorb water very well, the torching acted as a poultice or sponge, absorbing moisture in both liquid and vapour phases. This water absorption acted as a buffer until the weather was dry, at which point the lime would dry out. If the weather was continuously wet, the lime would reach a saturation point where no more water could be absorbed, thus acting like a cork plugging the gap.
· Adhesion: When working with natural materials such as stone or slate, or early fired tiles that were often unsquare and twisted, the use of a bedding material greatly helped with the adhesion of the roofing material to the roof. Old roofs would have been laid upon riven lath, rather than sawn batten, which came about in the late 19th century. The lime torching would grip around the lath, like a lath and plaster ceiling, and adhere to the undulations of the roofing material, cradling it and ensuring maximum contact with the materials below. Getting the lime in the right place and in the right amount was key; if it was too low down the roofing material, it would start sucking water in. It is only there to act as a binder and to block the water that is driven in by the wind.
· Control of Condensation: As described above, lime has the ability to absorb both liquid and water vapour. It is well known that a room plastered with lime will feel warmer than one plastered with gypsum or cement because it can regulate the humidity of the space. The majority of pre-1919 buildings, where solid wall construction was used, allowed moisture to be pulled through the wall from areas of high concentration to low. However, we need to consider not only solid wall construction but solid roof construction as well. As mentioned above, with sheeted roofs, having a large area of lime plaster torching on the underside of the roof worked as a moisture sink. Since it was homogenous with the roof covering, the wind drying, which pulls the water molecules away from the lime torching at the top, continued to pull water through the lime deep into the materials due to strong covalent chemical bonds on a molecular level, like pulling a series of water molecules all chained together. That is why the roof will continue to shed water even when the weather is not hot and sunny. This is not through evaporation but through the process of wind drying.
· Protecting Timber: As the lime torch acts as a poultice, it wicks water away from the surrounding timbers of the roof, drying them out and keeping them safe from wet rot. As you will see in walls, timber that gets sealed with cement and plastic paint will rot at an accelerated rate, whereas timber packed with lime mortar will protect the surrounding timber by keeping it dry.
Challenges of Modern Breather Membranes in Historic Buildings
1. Potential for Moisture Trapping:
One of the critical challenges of using breather membranes in historic buildings is the potential for moisture to become trapped. While these membranes are designed to allow water vapour to escape, they can also inadvertently create moisture traps for liquid. The efficiency of vapour transfer can also be reduced as the membrane becomes clogged or damaged over time, leading to moisture accumulation within the roof structure. In historic buildings, this will exacerbate issues such as timber decay and structural damage.
2. Compatibility with Traditional Materials:
Historic buildings were constructed using materials and techniques that inherently manage moisture effectively. Stone slates, clay tiles, lime mortar, and other traditional materials are naturally breathable, allowing moisture to evaporate and preventing condensation issues. Historic materials, such as stone slates and handmade tiles, naturally have larger gaps, allowing more water to penetrate the roof covering. Short of reducing head laps, a lime torch or head bed allows the areas to be plugged and the roof to be bedded to the lath.
Adding a modern breather membrane can disrupt this balance, as it may have to work harder due to the increased amount of driven rain. Additionally, there have been no studies on the effect of dissolved stone acid rain and how materials such as calcium carbonate in solution would affect breathability. The concern is an incompatibility between the two techniques, leading to moisture being trapped in the structure. This can result in the deterioration of the building fabric and loss of historical integrity. If the membrane is applied to a new roof with interlocking concrete tiles as part of a complete system, it makes sense. However, a membrane is part of a system being added to a historic roof, much like a dry ridge system, would never be used in this situation.
3. Preservation of Historic Character
The introduction of modern materials like breather membranes can alter the appearance and character of a historic building. It is essential to preserve the original materials and construction techniques that define the building's historical and architectural significance. Maintaining the authentic look and feel of the structure is crucial for preserving its heritage value. Introducing new materials can sometimes compromise the building's aesthetics and authenticity, which are important aspects of its cultural and historical importance.
One of the worst things to see in a historic building is modern coloured roofing battens and roofing membranes, which completely change the feel of the building and are alien to its landscape. We wouldn’t put plastic windows in a historic building or concrete floors with damp-proof membranes. We have learned that these are detrimental, but there seems to be a sensible lack of understanding regarding roofing.
4. Skilled Craftsmanship
Traditional roofing techniques require a high level of skill and craftsmanship. Roofers working on historic buildings must have a deep understanding of traditional materials and methods, ensuring that any repairs or restorations are sympathetic to the original construction. The use of modern membranes may reduce the demand for skilled craftsmanship, potentially leading to a decline in the quality of repairs and the preservation of traditional skills and the wrong roofers being assigned to the job.
The more we look to the past, the more we understand that people were not ignorant; they didn’t live in cold houses with leaking roofs. Crafts were skills, and the more we understand and re-educate ourselves with the knowledge that has been lost, the more we see that these roofs were passive in their ventilation, controlled the moisture, used eco-friendly materials, and lasted. The old roofs that survive and are doing well in this country are those that have not been changed or altered and are just functioning as intended. However, we are seeing many roofs being recovered where a roofing membrane was introduced in the 20th or even the 21st century.
5. Ecology
A historic and uneven roof provides the perfect space for crevice-dwelling bats to live, and they are often found in historic buildings for this reason. The addition of 1F-type felt has probably created even more habitats in roofs for bats as it provides a warm pocket behind the roof covering and above the felt. However, when using a modern breathable membrane with a historic roof covering, bats can unfortunately become trapped in the spun woven fabric and die. Therefore, when adding a roofing membrane, we are often requested to use a 1F bitumen, which will work with a cold roof that can be well ventilated to the underside, but not a warm roof. There are “bat-safe” breathable membranes, but our ecologist has advised against these, as it turns out they are just as harmful as a standard breathable membrane in trapping bats.
Case Studies: Impact of Breather Membranes on Historic Buildings
Several case studies illustrate the potential challenges and pitfalls of using breather membranes in historic buildings. By examining these examples, we can better understand the importance of careful consideration and adherence to conservation principles.
Case Study 1: Orleton Manor
Orleton Manor, a 16th-century timber-frame manor house in Herefordshire, had a roof covering of 24x12 sized Welsh slate. The roof slates were clearly late 19th century, based on the material type and tied with other alteration works to the building from the Arts and Crafts period. The roof originally would have been stone, but the stone slates in Herefordshire are of poor quality and have a very limited lifespan, so it would have made sense to reroof with the abundance of slate available at the time.
The roof covering at Orleton had lasted around 120 years, although it was now coming to the end of its lifespan. What was interesting was that the roof slates, when they were laid, were on a very stretched batten gauge with very little headlap. Normally, this would be a concern and, although not good practice, it had not caused much issue in this instance. Why? Because the roof slates had been torched internally with a half torch, where the gaps at the batten were torched along with the side lap perp joints. What made it even more interesting was the fact that when we removed a few slates, it was apparent that they were reused and had been holed multiple times before being laid. Not only was the roof at a stretched gauge with little headlap, but the slates were peppered with holes. Yet, the roof structure was bone dry!
The torching, in this instance, was just enough. After 120 years, it was coming to the end of its lifespan as a sacrificial element, much like pointing. From tens of thousands of cycles of wetting and drying, the torching had started to become friable, and much of it had dropped off, but the roof had performed incredibly well.
The roof covering had to be removed for repairs to the wall plates and trusses, which were suffering structural issues. We did not feel the slates would be good to be holed and reused for a third or fourth time. So we decided on a roof covering between stone and slate, opting for a hand-riven Welsh slate, up to about 10mm in thickness, and laid in a diminished course with Welsh swept valleys. The plan for the roof, much like the rest of the building, was to ensure it lasted for several hundred years, so the addition of a roofing membrane that would trap moisture was unacceptable from both a longevity and heritage point of view.
The new roof was laid with a head bed, allowing the lime material to squidge through to the backside of the roof. In this instance, we did not use a full torch but just dressed off the material that came through for the head bed. The bed of the slates, along with a mechanical fixing, allowed good bonding to the uneven slate below. The roof is a resounding success and elevates the building, transforming it from a flat, uninteresting roof to something that brings character and interest back to the structure.
The roof has not yet been insulated, but the plan is to insulate between and below the rafters with wood fibre insulation to achieve a U-value of 0.20 W/m²K, which is quite impressive for a Grade II* building using natural materials, without losing the special architectural interest of the building. It would be possible to get down to 0.15; however, we did not want to cover up the principal roofing structure that would detract from the understanding and appreciation of the building.
Case Study 2: Broad Street, Leominster
This roof was recovered in the mid-20th century with a 1F type bitumen felt using a mixture of handmade and concrete plain tiles. The roof is post-medieval, probably from the late 17th century. However, it appears that the majority of the roofing materials, including the rafters, were reused from elsewhere in the building during an extensive restoration at this time. Much like Orleton Manor, as mentioned above, the roof was re-laid with a very stretched headlap, possibly due to cost-saving measures or a roofer trying to increase his profit margins.
Unlike the roof at Orleton, which lasted for about 120 years, this roof has lasted around 60 years. Upon uncovering the battens, it shows they have suffered severely from water ingress and moisture being trapped, exhibiting signs of wet rot with white mycelium all over them, necessitating replacement. The underside of the roof also shows signs of mould growth on the medieval reused rafters.
Yes, there has been more water ingress due to a stretched headlap, but the felt concealed this mistake for many years before it was known that it needed to be addressed, resulting in significant decay. Installing a roof on a building should not be for 20 or 30 years but for the life of the roof covering. There seems to be an attitude of "Ah, well, 60 years from now, I won't be here, and it will be someone else's problem," but that is not the attitude to have on a historic building. We are custodians and should do what we can to keep them ticking over. The use of the membrane allowed for a small saving and to cover the lack of skill of the roofer, but at what detriment to the building?
If this had been a breathable membrane, we might not have had as much mould on the underside of the rafters, but given that it was a well-ventilated cold roof, the moisture was likely in liquid form, so it would not have passed through anyway. As for the top side, a more suitable headlap would have reduced the water ingress, but given the uneven nature of the tiles, driven rain would still have made its way in. There is no doubt that the addition of a membrane increases the rate of decay of timber compared to a torched finish.
Conservation Principles and Best Practices
Conservation best practices, such as those advocated by the Society for the Protection of Ancient Buildings (SPAB), emphasise minimal intervention and the use of traditional materials. The goal is to repair and maintain historic buildings without introducing modern materials that could compromise their integrity. By following these principles, the historical value and authenticity of the building are preserved for future generations.
Key Conservation Principles:
1. Minimal Intervention: The focus should be on repairing rather than replacing historic materials. Any interventions should be as minimal as possible to preserve the building's original fabric.
2. Use of Traditional Materials: Traditional materials and techniques should be used whenever possible to ensure compatibility with the historic building. This approach helps maintain the building's authenticity and integrity.
3. Reversibility: Any modern interventions should be reversible, allowing for future restoration work without damaging the original structure. This principle ensures that historic buildings can be preserved for future generations.
4. Skilled Craftsmanship: The use of skilled craftsmen who understand traditional construction techniques is essential for successful conservation work. This expertise ensures that repairs and restorations are carried out sympathetically and effectively.
Conclusion
Breather membranes have become a staple in modern construction due to their ability to provide weatherproofing and moisture management. However, their application in historic buildings presents a series of challenges that can undermine the structure's integrity and accelerate decay. The introduction of breather membranes in historic roofing systems, which were designed for lime torching and head bedding, creates an environment where decay is accelerated rather than mitigated.
Historically, roofing techniques such as lime torching provided a breathable yet effective moisture barrier that worked in harmony with traditional materials. Lime mortar absorbs and releases moisture naturally, acting as a buffer that prevents water ingress while allowing water to escape as liquid and vapour. This capability to manage moisture without trapping it has long protected timber structures and roofs from rot and decay. When a breather membrane is introduced, it interferes with this delicate balance by trapping moisture against the roof covering, especially if the membrane becomes clogged or damaged over time. This leads to timber decay and structural damage that was previously mitigated by traditional methods.
Additionally, the compatibility of materials is a critical factor in conservation. Traditional roofing materials such as stone slates, clay tiles, and lime mortar have been used for centuries due to their natural breathability and effectiveness in managing moisture. These materials allow water to evaporate, preventing condensation and maintaining a dry environment within the building envelope. The addition of a modern breather membrane disrupts this natural process, as it may not perform effectively in concert with the historic materials, leading to trapped moisture and accelerated decay.
The impact of breather membranes on the aesthetic and historical character of a building is another significant concern. The visual intrusion of modern materials can detract from the historic appearance, altering the perception and cultural value of the building. It is crucial to maintain the original materials and techniques that define a building's historical and architectural significance to preserve its heritage value.
Moreover, conservation best practices, such as those advocated by the Society for the Protection of Ancient Buildings (SPAB), emphasise the importance of minimal intervention and the use of traditional materials. By following these principles, we can ensure that any interventions are sympathetic to the original construction, maintaining the authenticity and integrity of historic buildings. Skilled craftsmanship is essential in this process, as it ensures that repairs and restorations are carried out effectively and in harmony with the building's original design.
Everyone thought they were doing the right thing in the 20th century by covering buildings in the hardest possible cements to protect them; we now know how wrong that was. The same thinking needs to be applied across the building, including the roof, with an understanding of both solid wall construction and solid roofs. Learning from past mistakes, we should prioritise traditional methods that have stood the test of time.
In conclusion, while breather membranes are valuable in modern construction, they are often unsuitable for historic buildings. Conservation efforts should focus on preserving traditional materials and techniques, ensuring that these architectural treasures continue to stand the test of time. By prioritising traditional methods and understanding the unique needs of historic buildings, we can protect both their historical significance and their structural integrity, allowing future generations to appreciate and learn from these remarkable structures. Through careful consideration and adherence to conservation principles, we honour the past and safeguard the future of our architectural heritage.
Disclaimer
The views expressed in this article reflect the thoughts and perspectives of Montez Architecture. It is important to recognise that each historic building is unique and should be assessed on its own merits and specific conditions. This article is intended as a discussion tool to highlight potential considerations and is not to be taken as definitive guidance or gospel. We strongly encourage consulting with us when making decisions about respair and maintenance on your historic property.
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