Waterproofing Green Roofs: Key Considerations

Carl Bailey, Regional Technical Manager for Elevate (formerly Firestone Building Products), explains what to consider when specifying and installing a waterproofing system for a green roof.


As the focus on environmental sustainability increases, so does the demand for green roofs. Homeowners and developers are beginning to recognise the value a green roof can bring to the environment as well as to people and their wider communities.

The benefits are vast, including providing natural insulation, enhancing biodiversity, reducing air pollution, improving stormwater management and mitigating the ‘urban heat island effect’ by returning moisture to the environment through evaporation.

The installation of a green roof system also generates points in certification systems such as BREEAM (Building Research Establishment Environmental Assessment Method) and LEED (Leadership in Energy and Environmental Design), improving the environmental credentials of a property.

Creating a resilient roof

There are two main types of green roofs: extensive and intensive. An extensive roof uses sedum vegetation and is the most common on flat roofs as they are lightweight and low maintenance. Intensive green roofs are more complex, including trees and shrubs requiring irrigation, fertilisation and maintenance.

Selecting the right waterproofing system for either an extensive or intensive green roof is a critical element of the specification process. If the waterproofing layer is not specified or installed correctly, this will affect the performance of the roof and could cause it to fail.

The waterproofing membrane must be resistant to root penetration and strong enough to be trafficked when maintenance is required.

The RubberGard EPDM and UltraPly TPO membranes, for example, have successfully passed the EN 13948 – resistance to root penetration test – and the FLL root penetration-resistant test from the German Landscape Research, Development and Construction Society.

We advise using an EPDM single-ply membrane that is 1.5mm thick, and can be fully adhered to the substrate, providing a robust solution. Specifying an EPDM membrane that can be installed using large sheets will generate further benefits, including a faster installation and fewer seams, lowering the risk of any water ingress.

Studies have shown that EPDM membranes can last for more than 50 years. Their flexibility can accommodate expansions or contractions due to temperature changes, and they don’t contain any chemicals that can affect the membrane’s characteristics over time. Rooftop vegetation also offers additional protection for an EPDM membrane, further extending its service life.

Best practice installation

The waterproofing system must be installed to the highest standard to deliver a robust green roof. This demands a contractor who has been trained to install the specified waterproofing membrane and is authorised to do so by the manufacturer.

A typical EPDM installation would involve laying the concrete decks to falls to achieve a minimum finished slope as per local requirements and encouraging efficient roof drainage.

The concrete deck is primed before placing a vapour control layer on top, which will restrict moisture from entering the insulation, where it could condense and cause damage.

An insulation board with a high compression strength will be required with an appropriate thickness to achieve the required roof U-value. This will be adhered to the vapour control layer prior to fully adhering to the EPDM single-ply roofing membrane with a bonding adhesive. A green roof system, including a protection mat, will then be installed over the EPDM membrane.

During installation, it is also vital to protect the waterproofing membrane from other trades and for the work to be completed in one phase within the shortest time possible to prevent any moisture from affecting the system. For the same reason, the substrate board, roof insulation and cover board must be kept dry while installation is underway.

Focus on the details

As highlighted by the GRO Green Roof Code, another key consideration when designing and specifying the waterproofing system is the detailing on a roof.

At an upstand, for example, whether that is a wall abutment or penetration through the roof, the waterproofing system must extend to a height of at least 150mm from the finished surface level of the green roof.

Further waterproofing considerations at an upstand include the provision of a vegetation-free zone around the perimeters of the roof using pebbles. This helps to prevent the roots from encroaching where most of the critical details occur in the waterproofing system, such as the drains and other service penetrations.

The vegetation-free zone also serves as a fire break. The GRO Green Roof Code recommends that this should be 300mm in width and increased to 500mm where there are openings to the building.

Partner with a specialist

Whether you are specifying an extensive or intensive green roof, it is important to partner with a specialist provider of roofing waterproofing systems from the earliest opportunity. Their knowledge and technical advice will ensure a green roof meets all specification requirements and is built to last.

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