Roof membranes and ecological impact

Among the building components that make up a structure, the roof is perhaps the one most exposed to harsh weather, and its defects can lead to serious problems indoors.

Traditionally, roof waterproofing solutions were built using natural materials such as stone or tiles for pitched roofs, or lime and clay for flat roofs.

With the emergence of synthetic materials, these systems have largely been replaced by roof waterproofing membranes—an economical solution that ensures complete watertightness with a minimal slope, long service life, and minimal to no maintenance.

However, roof waterproofing membranes are synthetic elements whose processing has a high ecological impact, with no natural alternatives available, and they raise red flags in any life-cycle assessment and environmental impact analysis for a project.

For this reason, we propose the need to assess the different options available in terms of technical performance and ecological impact, comparing the various types of waterproofing membranes on the market by environmental impact, performance, and durability.

ROOF WATERPROOFING MEMBRANES

The term “terrat” or “terrado” was traditionally used to refer to the flat roofs of village houses, where activities such as hanging out the laundry or meeting with neighbours took place, and which were protected with a layer of compacted clay that ensured waterproofing and provided a firm surface for accessibility.

In regions with high rainfall, measures based on geometry were adopted, in which the roofing material has lower waterproofing requirements by shedding most of the water via a steep slope—such as thatched or plant-based roofs in Nordic countries, or traditional tile or stone roofs.

The roof is one of the most heavily stressed parts of a building envelope, both due to water in the form of rain and snow, and due to temperature variations between day and night, which affect not only maintenance but also the stability of the indoor thermal environment.

In addition, if the building design does not allow proper access, the roof risks not receiving even minimal maintenance, and any issues may go unnoticed.

Due to its direct exposure to both rain and snow, one of the greatest risks for a roof is water infiltration—a defect that has been largely forgotten since synthetic waterproofing membranes appeared on the market in the early 20th century.

The availability of a new flexible, waterproof material that allows us to protect roofs with all kinds of solutions while guaranteeing watertightness was a disruptive innovation that completely changed the way roof elements are designed and built.

But the fact that synthetic materials have almost completely displaced the rest of the traditional solutions made with natural materials such as lime or tiles is not only an aesthetic issue, but also an economic one, since waterproofing membranes streamline and reduce the cost of roof waterproofing.

Roof types

Not all roof membranes are suitable for all roof types. Therefore, before defining the different types and selecting roof membranes based on their performance and ecological impact, we will start by defining the most common roof types: according to the materials they are made of, their geometry, use, and the order of the different layers.

Typological classification of roofs

In the classic classification, roof types are usually distinguished by their geometry. Flat roofs are typically defined as having a maximum slope of 24%, and pitched roofs from 25% up to 100%.

The origin of this distinction lies in traditional roofs protected with stone or ceramic materials, with the Arabic tile system as the most representative example. This geometric arrangement requires a minimum slope of 24% so that, by geometry, water does not penetrate, bearing in mind that these are porous materials.

Once we move into low-slope or flat roofs, a higher degree of waterproofing is required, so we must start working with non-porous materials. These are roofs that were traditionally built with clay or lime, and nowadays are made with metals or waterproofing membranes.

A second typological classification distinguishes between conventional and inverted roofs depending on the position of the insulation layer, or between ventilated roofs (also called cold roofs) and non-ventilated (warm) roofs—so, in this case, the criterion is energy performance.

Finally, we can classify roofs according to their use: walkable, non-walkable, technical, flooded, or green roof.

It is important to note that the choice of a roof system will largely influence which roof membranes are suitable, since poor roof performance affects indoor comfort, but also the durability of elastic materials. The expansion and contraction of a material subjected to constant thermal changes can lead to rapid degradation.

Analysis criteria

Throughout this article, we propose an analysis for selecting roof membranes based on performance and ecological impact, which we divide into 4 main types:

• Bituminous membrane
• EPDM
• PVC
• TPO
• Liquid-applied membranes

For each of these, a technical and ecological footprint analysis will be carried out.

Criteria for evaluating the ecological impact of roof membranes

Before going into detail on selecting roof membranes based on performance and ecological impact, we will review some key concepts related to the environmental assessment of a product.

– Safety Data Sheet: this is the second document, after the technical data sheet, that must legally accompany any product, although it is not always easily accessible publicly and in many cases it must be requested from the manufacturer.

In a product’s Safety Data Sheet, it is mandatory to indicate all components in the mixture that may be considered toxic, provided their percentage of the total is at least 0.1%. This means that some toxic products may be present in minimal concentrations that do not have to be declared.

Ideally, a full component declaration would be available, but since many manufacturers do not want to disclose a product’s complete composition, numerous ecolabel certifications have emerged in recent years. These independently analyse and validate whether a product’s composition meets certain quality standards regarding its impact on the environment and human health.

In addition, the Safety Data Sheet includes information on handling and disposal methods to minimise toxicity for both users and the environment.

– LCA: Life Cycle Assessment is one of the most widely used methods today for evaluating a product’s environmental impact. Through this method, environmental impact is considered beyond just calculating carbon emissions. To do so, various impact categories are assessed, such as resource depletion, ozone layer impact, water acidification, eutrophication, as well as human toxicity and ecotoxicity.

As it is a relatively recent method, it is still difficult to find LCA documentation for many industrial products. Hopefully, in the near future, comprehensive assessment methods like this will become mandatory for all manufacturers. LCA is regulated by the European standards ISO 14040 [2] and ISO 14044 [3].

– EPD: From a Product Life Cycle Assessment, we obtain the EPD (Environmental Product Declaration). An EPD is an increasingly widespread document developed by the product manufacturer. An EPD is classified as a Type III ecolabel and is governed by ISO 14025 [4].

There is a platform called OpenDAP that collects the largest number of declarations in Spain, allowing free and open access.

EPDs refer to the origin of the raw material, its extraction and processing, but above all they assess CO₂ emissions. Therefore, the figure we will use is the product’s emission factor.

Selection of roof membranes based on performance and ecological impact

Bituminous membrane

Bituminous membranes, also known as bituminous sheets or polymer-modified bitumen membranes (PMBM), are the most widely used system for roof waterproofing. This term refers to the fact that the asphaltic material used in these membranes has been modified with additives or polymers to improve certain characteristics, such as weather resistance, elasticity, and adhesion. The material composition usually includes, among others: natural asphalts, penetration-grade bitumens, oxidised bitumens, tar, and pitches.

First, let us clarify the origin of all the materials mentioned. Bitumen is a petroleum-derived product, while tar and pitches are coal-derived. Asphalt, in turn, is a mixture of aggregates with bitumen that is normally used for road construction.

In addition, the bituminous membrane starts from a base material that provides support and strength, which used to be asbestos or natural fabrics. Today, new backing materials are used: some years ago they were aluminium or copper, and now they are polyester, polyethylene, or fibreglass.

The manufacturing process for asphaltic material consists of the following steps: extraction, transport, preparation of the asphalt mix, rolling, application of the finishing material, cooling, and winding. [5]

Oil extraction is normally carried out through deep wells, requiring a large amount of energy in the process. Spain is characterised by high energy dependence, as it has only 0.4% domestic oil production. As a result, most imports come from Mexico, and must cross the entire Atlantic by ship, entailing an extremely high ecological cost in transport.

Once at the plant, crude oil is refined to obtain products such as petrol, diesel, gases, etc. In refineries, crude oil undergoes different distillation and separation processes, with temperatures varying depending on the process and the specific unit used. However, in general, some units operate at relatively high temperatures, reaching between 300°C and 400°C or even higher in some cases, to carry out distillation and separation of the different components of oil. This process generates a by-product: bitumen. In other words, it is a residual product that is used after oil refining for products such as bituminous membranes. Subsequently, this bitumen must be mixed with a specific proportion of aggregates to obtain asphalt.

The malleability of bitumen is closely related to its temperature, and at ambient temperature it is in solid form. For this reason, it must be handled at 150°C at all times, even during storage, when it is typically kept in fixed tanks equipped with a large boiler to maintain heat.

The final cooling process also involves high energy consumption, since it must bring this 150°C down in a very short time to maintain the material’s geometry.

On top of this, additives are often added to modify the material’s behaviour. To increase maximum elongation, elastomers are added—thermoplastic polymers or rubber derivatives. To improve tensile strength, plastomers or thermosetting polymers are added, which enhance its strength.

An important feature of this type of membrane is its flexibility at low temperatures. A material’s flexibility refers to its ability to adapt and conform to the roof’s surfaces and configurations, even when exposed to cold temperatures. When ambient temperature is low, asphalt and bituminous materials tend to become more rigid, which could make handling and application difficult in certain cold-weather conditions. However, bituminous membranes with good low-temperature flexibility remain more pliable and workable even in cold environments.

Depending on the membrane’s composition and additives, we distinguish several subtypes:

• Oxidised asphalt bituminous membrane
• Modified oxidised asphalt membrane
• Elastomer-modified bitumen membrane
• Plastomer-modified bitumen membrane
• Extruded polymer-modified bitumen membrane.

The manufacturing process for so-called oxidised asphalt stands out. In recent decades, it has been the most widely used material for producing bituminous membranes. The difference is that, while the asphalt is subjected to high temperatures, air is blown through it to oxidise it, resulting in a hard yet highly elastic material. [6] Since 2013, the use of OA-70/40 type oxidised asphalt in roofs has been explicitly banned in public procurement and in other construction contexts in general, such as cycle lanes, because its tensile strength does not perform well under the UNE 104202:1992 standard. [7]

As an alternative, membranes with SBS elastomers and membranes with APP plastomers are used today, also known as polymer-bitumen membranes. In most cases they can be used interchangeably, but in warm-climate countries, APP plastomer membranes are easier to apply, since penetration at high temperatures (25°C or more) tends to be lower. Lower penetration means these membranes are less prone to softening excessively and losing their mechanical properties in hot conditions, which facilitates installation and prevents deformation-related issues during application.

In terms of classification, there are two types of bituminous membrane depending on their finishing layer: self-protected and unprotected. Self-protected membranes have an integrated mineral or metallic finish, while unprotected ones are given a simple plastic film finish and must therefore be protected with gravel or paving.

Depending on their composition, membranes are classified as follows: single-layer, double-layer, or enhanced single-layer. The single-layer system consists, as the name indicates, of a single layer of the roll material. For this reason, overlap between adjacent strips is important to ensure waterproofing, no less than 8 cm, used especially in self-protected membranes. By contrast, the double-layer system consists of two layers made up of overlapping strips, where the next layer is laid to cover the previous overlap joint, in both cases perpendicular to the line of maximum slope. Finally, enhanced single-layer bituminous membranes are similar to single-layer membranes, but include improvements in their composition to provide additional performance characteristics.

The bonding system may be fully bonded, partially bonded, or loose-laid. Fully bonded means the waterproofing is adhered to the substrate over its entire surface, offering better watertightness by acting as one with the base substrate. The partially bonded method is attached at points over 15% to 50% of its area—a technique used to distribute vapour pressure. Finally, the membrane may be left unbonded using the so-called loose-laid system, except at the perimeter, at wall junctions, and at singular details. In this case, the layer can move more freely and will therefore be subjected to less structural stress.

The jointing technique between membranes is usually done with a torch; it can also be reinforced with cold adhesives, or even carried out solely with mechanical fixings or hot asphalt.

The Spanish standard governing waterproofing in above- and below-grade building works with modified bituminous membranes, UNE 13707:2014, recommends polymer-modified bitumen membranes among these systems. The standard also specifies that membranes must have an appropriate reinforcement to provide the required strength and stability. Some of the recommended reinforcement materials include fibreglass felt, polyester felt (non-woven) unreinforced or reinforced with mesh, polyethylene felt or other polyolefins, and polyester film or polyethylene film. Self-adhesive polymer-modified bitumen membranes are also covered by the standard, with fibreglass felt and non-woven polyester felt reinforcement. [8]

EPDM – Ethylene Propylene Diene Monomer

Ethylene Propylene Diene Monomer membranes are waterproof sheets made of synthetic rubber. Natural rubber is an organically sourced polymer extracted as a milky emulsion from certain tree species, known as latex.

However, synthetic rubber only shares the name with this other material, so it is presented as natural when it is not. It is manufactured by mixing a variety of monomers—some of organic origin but many derived from petroleum itself—such as ethylene, propylene, and diene, which are used in plastic production. On top of this, a series of impurities or additives are added to give the product its optimal properties.

Another option is vulcanised rubber membranes, a derivative of natural or synthetic rubber, depending on which material the vulcanisation process is applied to. Vulcanisation is a process in which raw rubber is heated while being mixed with sulphur. This process gives it greater stability, hardness, and cold resistance without losing its natural elasticity. It also transforms the material’s sticky finish into a smooth surface that does not adhere to materials it comes into contact with.

In both cases, we are talking about membranes made of a plastic material or one that has been removed from its natural origin, reinforced with a polyester scrim, which is also plastic. Once again, we face the issue of this country’s lack of raw material self-sufficiency: the material is already polluting because it is fossil-based and requires a lot of energy to extract, and it must also be transported long distances to reach the factory.

In Spain, EPDM membranes are manufactured using the calendering technique, in which the material is passed between two heated metal rollers that rotate in opposite directions, and the final sheet is cut with a blade. It is at this stage that the material may undergo vulcanisation.

By definition, it is a non-breathable material. However, in recent years a number of techniques have been developed that allow moisture from inside the building to escape while still preventing water ingress, thus avoiding condensation that can lead to damp problems. This involves adding up to 4 layers with different geometries: an unbraided textile layer, a diffusion film, a PE reinforcement fabric, and a final unbraided textile layer.

It is supplied in rolls, reaching up to 9 m wide by 30 m long. The bonding or jointing techniques between sheets include hot air, which activates the adhesive properties of the material itself, or cold adhesives, which offer certain advantages by not subjecting the material to high tensile stresses during installation.

A minimum level of protection is necessary, and it is not possible to leave the membrane exposed. This protective layer may be heavy, using rounded gravel or paving set in mortar—in which case we will place a geotextile layer—or, alternatively, light, using a light-coloured elastic paint to minimise solar exposure.

At the end of its service life, EPDM is highly recyclable, although approximately 10% of the material is lost due to prior bonding with other sheets. For recycling, the material undergoes mechanical shredding and can be reprocessed into other construction products, as well as sports applications such as track surfacing or play areas.

PVC

Polyvinyl chloride (PVC) is a very common type of plastic in construction, used for roof sheets, water pipes, enclosures, carpentry, and fencing, among other elements. It is a hazardous material for both the planet and users, posing a serious problem of pollution and toxicity in its production process. In fact, among the polymers declared most dangerous, PVC is by far the most used in construction. [9]

PVC is unique in its technical advantages within the broad spectrum of plastics because it is a chlorinated plastic. Basically, its components come from 43% crude oil and 57% chlorine from salt, which means it is the plastic with the least dependence on crude oil and the easiest to produce in Spain, thus reducing costs. In fact, the chlorine industry holds significant weight in the Spanish economy, being the 6th largest producing country in the EU.

All this does not mean that its manufacturing is ecological; quite the opposite. In fact, it still requires transporting oil from Mexico or Saudi Arabia, while Spanish chlorine operations exploit salt flats, transport this salt to a factory where, through electrolysis, it is progressively converted into brine, caustic soda, sodium hypochlorite, and chlorine. These are four complex chemical reactions that consume energy and release toxins into the environment.

Chlorine and ethylene combine into a single molecule called a polymer, a process known as polymerization, which can be produced by four different methods: suspension, emulsion, bulk, and solution. The suspension process is the most commonly used and is carried out in stainless steel reactors operating at high pressure or high temperature.

However, the chlorinated chemistry of PVC is responsible for a variety of environmental and human health hazards: from the beginning of its life cycle, where the material itself is a known human carcinogen, to the release of organochlorine substances when burned in fires or incinerators, emitting dioxins that are highly environmentally toxic and another human carcinogen. In fact, it also releases large quantities of dioxin throughout its manufacturing process.

PVC sheets are fully synthetic waterproof sheets, making them not at all breathable.

Its presentation system consists of rolls 1.4 meters wide by 20 meters long, which presents problems due to the need for a large number of joints. For this type of sheet, the fixing technique can be mechanical, hot air, or adhesives.

It has the advantage of being able to be left exposed, which is why manufacturers offer different colors of sheets, as its chlorinated nature results in low incidence of solar radiation. Similarly, it allows for the placement of any type of top paving, enabling roof trafficability.

Finally, the possibility of recycling PVC is very low, as the quality of the material is greatly reduced, making it necessary to always use more than 50% virgin material.

TPO

TPO, or thermoplastic polyolefin, is one of the most innovative compounds on the market, growing in popularity in diverse sectors such as automotive and waterproofing. In the last 20 years, its commercial growth has been so significant that it now accounts for more than 50% of waterproofing installations on flat roofs.

Its composition is based on polypropylene with elastic ethylene copolymers, typically polyolefin elastomers, and other additives. These additives may include plasticizers, antioxidants, and other components that give it special properties, such as high resistance to chemicals, UV rays, moisture, and heat. Additionally, TPO does not tend to contract or expand easily and exhibits good resistance to microorganisms.

TPO has a similar quality to PVC but offers some complementary benefits. One of the greatest advantages is that it withstands the effect of almost all chemicals, has high resistance to ultraviolet rays, and high resistance to moisture, heat (as it does not contract or expand easily), and microorganisms.

Its manufacturing is usually done by injection molding into the desired article, although other processes such as thermoformed sheets or extrusion profiles are increasingly used. The overlaps of the sheets are welded with a hot air welder to form a homogeneous joint, and in this case, the necessary temperature is higher than that used for the installation of PVC sheets. For mechanical fastening of the membrane, the installer can use traditional fasteners with screws and metal or plastic washers in the overlap area with the adjacent sheet, or use induction fasteners.

Depending on the thickness, it can reach a length of 20 to 30 m and a greater width compared to PVC, up to 3.65 m. [10]

Liquid-applied membranes

The last type of roof waterproofing sheets we will analyze are the so-called ‘membranes’ or liquid waterproof sheets sprayed in situ: bituminous products, acrylics, polymers, synthetic resins, or polyurethanes.

The main characteristic of the in-situ spraying application technique is its ability to adapt to complex geometries, forming a continuous skin without joints, which avoids joint problems. Within this group, there are many options corresponding to very different materials, including materials as new to the market as nanotechnological waterproofing agents. Below, we will list the different types, providing a brief summary of their manufacturing process and the ecological implications involved.

Depending on the membrane’s composition and additives, we distinguish several subtypes:

• Waterproofing agents with nanoparticles
• Polyurethane membrane
• Resins
• Acrylics

• Waterproofing agents with nanoparticles:

This is a membrane formulated at an atomic scale that achieves total surface watertightness while maintaining its breathability for up to 10 years through a process called photocatalysis. However, the use of nanoparticles, typically titanium dioxide (TiO₂), can lead to a series of pulmonary pathologies from inhalation. Due to their infinitesimally small size, they diffuse through the circulatory system to reach various organs, inducing diseases. [13]

It can be applied by brush, roller, or spray gun and has the ability to waterproof while allowing the base material to breathe, in addition to adding a biocidal effect that prevents the appearance of living organisms on the roof. Its nature as a heavy material makes its ecological cost high, since the extraction of materials, their transformation process using blast furnaces, and their complex technical process in their transformation into nanoparticles are very energy-intensive.

• Polyurethane membrane:

Polyurethane is a plastic material derived from petroleum. It is a polymer, meaning small molecules connected to each other, used in different formats for various purposes. Especially in foam form, in the manufacture of insulation, or directly as an elastomer in the case of liquid sheets.

It is basically formed by the gaseous chemical reaction of two compounds, a polyol and an isocyanate, although its formulation needs and admits multiple variants and additives. This reaction releases CO₂, which forms bubbles as it is a reaction with expansion of the liquid volume. Thus, it entails a high emission factor due to its direct relationship with petroleum and its need for transformation through processes with a high carbon footprint.

Its greatest advantage lies in its ability to be applied cold and have a service life of up to 50 years if properly protected from the elements. However, its breathability is very low and usually involves a prior layer of plastic asphalt primer, asphalt paints, or asphalt emulsions.

• Resins:

In the case of resins, durability will vary depending on the type.

Thus, both polymeric, acrylic, and epoxy resins do not originate from natural resin extracted from trees but mostly come from petroleum derivatives. This is why we are again talking about a high global warming potential, compounded by the need to transport crude oil from abroad.

• Acrylics:

Acrylic paint is a type of paint made from plasticized material, an emulsion of an acrylic polymer (PMMA) manufactured by a process known as bulk polymerization or cast sheet. This involves taking the pre-polymer in liquid form and ‘casting’ it between the plates that form the mold.

Additionally, these paints have thermoreflective properties due to their white color, which reduces roof insolation, thus minimizing the effect of material expansion and contraction. An interesting characteristic for their application on metal roofs, especially galvanized sheets, to which they adhere particularly well.

The acrylic membrane is applied cold and dries very quickly. Despite being water-soluble, once dry, it becomes water-resistant and highly elastic, giving it a lifespan of up to 10 years.

For all these products, the difficulty of recycling is added due to the liquid nature of their application, which makes their separation from the roof’s finishing layer for reuse extremely difficult. However, all of them can be left exposed or have subsequent coatings applied.