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As the focus on environmental sustainability increases, so does the demand for green roofs. Providing natural insulation, enhancing biodiversity and reducing air pollution are just some of the benefits for your self-build home. However, to make sure your green roof stands the test of time, there are several factors to consider, including the type of waterproofing, installation best practice and future maintenance.
Weigh up your green roof options
The climate, local Building Regulations and building type are just some of the factors that will affect the design of your green roof, including the types of plants and growing substrate. Understanding these elements from the outset is crucial as they will impact the roof build up as well as future maintenance.
If you would prefer a lower maintenance solution, then an extensive green roof would be more suitable. This option is the most common on flat roofs. The system tends to use sedum, groundcover and wildflower-type planting, making it lightweight with less aggressive root systems.
The alternative is an intensive green roof, including trees and shrubs. This is, however, a more complex option as the roof will require more irrigation, fertilisation and maintenance. As the weight will be much heavier, the roof structure must provide a greater level of support and additional protection for the waterproofing system.
Research the waterproofing system
There are different waterproofing systems suitable for a green roof and selecting the right one is essential. The membrane must be resistant to root penetration and strong enough to be walked over when maintenance is required.The RubberGard EPDM and UltraPly TPO membranes, for example, have passed root penetration tests from the German Landscape Research, Development and Construction Society.
We advise using an EPDM single-ply membrane, which is 1.5mm thick and can be fully adhered to the substrate, providing a robust solution. An EPDM membrane that can be installed using large sheets will also limit the number of seams, reducing 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.
Focus on the details
As highlighted by the GRO Green Roof Code, the detailing on your roof can also affect your waterproofing system and its design.
At an upstand, for example, such as 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.
It is also advisable to provide 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. This vegetation-free zone also serves as a fire break.
Ensure a high-quality installation
The waterproofing system must be installed to the highest standard to deliver a robust green roof. Only use a contractor who has been trained to install your preferred waterproofing membrane and is authorised to do so by the manufacturer.
A typical EPDM installation for an extensive green roof would involve laying a concrete deck, which will then be primed before placing a vapour control layer on top. This will restrict moisture vapour 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 should be adhered to the vapour control layer prior to installing the EPDM roofing membrane with a bonding adhesive. The extensive green roof system, including a protection mat, will then be installed over the EPDM membrane.
Prioritise maintenance
A no-maintenance roof doesn’t exist. Even an extensive green roof will require a maintenance plan.
While the green roof is establishing just after installation, there will be specific watering guidelines to follow, provided by the vegetation provider. Regular maintenance after this point will usually involve an annual soil test in the spring. If the soil pH drops under 6.5, many vegetation providers suggest using fertilisers. In our experience, a slow-release fertiliser is most effective and should only be used when needed.
It is also vital to control weeds, as weed killer is not permitted on a green roof. This can be achieved by increasing plant density, using self-mulching plants and removing parent plants before the seed can germinate.
As part of the maintenance plan, a post-installation inspection should be carried out annually for the first two years.
Partner with a specialist
Whether you are installing 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 your requirements and is built to last.