Layers of functionality: what can go under EPDM?

A flat or low slope roof assembly consists of several components that need to work together efficiently and consistently to offer a long-term, sustainable solution. While the roofing membrane is often the top and therefore most visible layer, what lies under it has a significant impact on the overall roof performance. Choosing the right materials (and ensuring their proper installation) is what makes a roof stand the test of time and be ready for the challenges of the future.

“A roof above one’s head” is the first line of protection against the elements. From simple roofs to more sophisticated ones, they all share the same purpose. In recent years, however, there has been a clear trend to go beyond this basic function: a roof should not just be easy to install, maintain and repair, but also energy-efficient, with an overall positive impact on the environment. This idea is often reinforced by sustainable construction practices. A considerable part of the building envelope, roofs have a significant impact on the flow of energy in and out of buildings. This is why layering the roof assembly according to its specific functions is so important. Let’s take a look underneath the waterproofing layer of EPDM to discover a variety of possibilities.

Choosing the right system

Does your project consist of a brand-new roof or an existing one that needs renovating? What is the roof deck made of? How much weight can the roof structure hold? Is the roof accessible or non-accessible? What is the local climate like? What are the local regulations? Will the roof need to support a green roof system or a photovoltaic installation? Do you want to manage rainwater or collect it on the roof? These are just some of the questions that need to be properly addressed in the design phase in order to choose the most suitable roofing system. 

EPDM roofing membranes offer a variety of installation options to best meet the specific requirements of each project. They are installed in a single layer and without the use of flames, either using contact adhesives (fully adhered system), plates/bars and fasteners (mechanically attached system) or simply a ballast layer such as river-washed gravel or pavers (ballasted system).


Insulation plays an important role in sustainable construction. It is the most practical and cost-effective way of making buildings more energy-efficient and keeping them cooler in summer and warmer in winter, which reduces heating and cooling costs. Proper thermal insulation is also one of the key requirements for passive buildings, a construction concept aimed at minimizing the building’s overall ecological footprint. 

When it comes to flat roofs, the “warm roof” concept is the most common. This means that the insulation layer is placed above the roof deck and below the waterproofing membrane. In Europe, the most commonly used types of flat roof insulation are polyisocyanurate (PIR), polyurethane (PU), mineral wool, expanded polystyrene (EPS) and extruded polystyrene (XPS).

Although EPDM roofing membranes can in principle be used with all these types of insulation using different membrane installation methods, PIR insulation boards offer the best performance when used in combination with EPDM membranes on flat or low slope roofs. In addition, EPDM roofing membranes can be adhered directly on top of PIR boards, something that is not possible on standard mineral wool, XPS or EPS insulation.

Rigid PIR boards are also the most efficient in terms of thermal performance in relation to thickness. This means that a thin and lightweight PIR board is able to achieve the same insulation value as thicker and heavier insulation products.

Cover boards

Cover boards are semi-rigid, relatively thin boards that are usually installed over the insulation layer to provide added impact protection, separation and support to the roofing membrane. They are available in a wide variety of materials, such as PIR-based, gypsum-based, cement-based, wood-based, etc. 

Including a cover board in the roofing assembly enhances the performance of the whole roofing system and helps it last longer. Although not yet very common in Europe, cover boards are particularly useful when the roof has additional functions, performs in extreme weather conditions (such as hail or heavy snowfalls) or is exposed to frequent foot traffic (for example during installation or maintenance works). This can potentially damage the insulation layer, resulting in reduced thermal performance, which would in turn cause an increase in energy costs. From a life cycle cost perspective, it is therefore a smart move to protect the insulation layer. 

Cover boards can be used in all types of roofs, both in new build and refurbishment projects. In renovations, they can be placed on top of the existing roofing system, providing a suitable substrate for the installation of the new membrane.

In combination with quality insulation, cover boards also provide other benefits: they increase moisture protection, improve acoustics and can even help achieve a better wind uplift performance and enhanced fire resistance. 

Vapor control layer

A vapor control layer (or barrier) is a thin film or sheet that primarily serves to control the migration of moisture through the roofing assembly. It eliminates the risk of moisture build-up from condensation within buildings, which can lead to the development of mold, damp and rot. A properly installed vapor control layer also contributes to the airtightness of the roof and assists in controlling energy losses, ensuring the thermal performance of the insulation.

Vapor control membranes are usually placed directly on top of the roof deck and below the insulation layer. The use and type of vapor control layer depends on climatic conditions and the function of the building. The most commonly used on flat and low slope roof assemblies are made of either polyethylene (PE) film, aluminium foil or are bitumen-based.

Adhesives, fixations and beyond

Other roofing components such as primers, adhesives, plates, fasteners, etc., also play an important role in the overall performance of the roofing system. Their main function is to secure the EPDM roofing membrane in place so they are intrinsically linked. If they fail, the whole roofing system will fail. 

At the end of the day, as Harvey S. Firestone put it: “success is the sum of details”. The key to the success of a roofing system is to choose the right products that are not only compatible but also guarantee its long-term performance. This means that each layer that is used under the waterproofing membrane matters, together contributing to the overall goal and function of the building project.

There is a lot to
discover about EPDM


EPDM roofing membranes last for at least 50 years and their structure and elasticity remain unchanged. They can be easily repaired, which allows them to last even longer. No doubt: EPDM is the most durable roofing material available.


Sustainability certifications create new standards for  the future. As a material that significantly reduces environmental impact, EPDM actively contributes to achieving highest ratings in BREEAM and LEED programs.