Going green on top

Victoria Ramwell from Kemper System discusses specification best practice and the key considerations for green roofs, including design, location and waterproofing aspects

Green roofs are evolving rapidly, transforming from a specialist eco-feature to a more mainstream technology which can be seen on new homes of all shapes and sizes. However, the success of a green roof depends on a number of factors, so self-builders must not only consider the structure itself, but the location, orientation, shading and climate of their site.

Planning at design stage

While it may appear that planting can be decided at a later stage, it is important that these decisions are considered at the design stage as the roof build-up impacts on the planting possibilities. For example, wildflower meadows are a popular choice for large expanses of roof surface, but they need plenty of moisture, which means designing the roof to accommodate the load and a greater depth of reservoir core, as well as necessary growing medium to keep the green roof healthy. Alpine planting however needs much less water to thrive, which influences the load bearing capabilities required from the roof.

A watertight solution

The most vital element of the green roof is the waterproofing membrane which is used to create a watertight barrier between the roof substrate and the green roof system. The membrane should be flexible enough to cope with any post-build settlement, tough and durable enough to cope with the load bearing requirements of the planting medium, and have a sufficiently long service life to make the green roof viable.

A common green roof design would typically consist of an inverted warm roof build-up, applying the waterproofing system to the roof substrate, followed by the insulation, and then the green roof elements. This can add an extra layer of protection for the waterproofing membrane as it is cushioned underneath the insulation.

However, a cold-applied liquid waterproofing solution can be installed either below or above the insulation, depending on the type of insulation used. This is because these systems are FLL certified as root resistant, so there is no risk of root damage to the integrity of the waterproofing membrane, even as plants mature and their root size increases.

Consulting the waterproofing membrane supplier for technical support can help determine the right specification in this regard.

Location is key

Another key consideration during green roof specification should always be the location and prevailing climatic conditions as these govern the amount of light, heat, shade and moisture the roof will receive. The influence of surrounding buildings, which can also affect shading levels and exposure to winds, also need to be taken into account. These factors should be assessed collectively to determine the viability of different types of planting.

Of course, no matter how carefully the planting is considered, without proper maintenance, the green roof will not thrive post-installation. Establishing a maintenance plan including associated costs needs to be factored in at the design stage for the green roof’s aesthetics and benefits to remain durable.

Specialist guidance

Green roofs have significant advantages for the environment – climate control, storm water attenuation, noise and pollution reduction and heat insulation. They protect against thermal gain and offer an additional habitat for flora and fauna.

Whatever your requirement, it is advisable to seek guidance from technical specialists to ensure your green roof performs correctly, meeting local climatic conditions as well as aspect and biodiversity requirements.

Case study: Liquid waterproofing benefits

Chapter House is a sustainable contemporary home featuring two green roofs. Located in Surrey, the detached self-build property spans 258 m2 and was designed by an architectural practice headed by former head of RIBA Jane Duncan.

Positioned to make the most of the views, daylight and sunshine, the house has been built using a range of natural materials, including timber cladding and Scandinavian bricks.

Homeowner and former builder Nigel Warnes helped to specify a liquid waterproofing resin for the project. The product was required for a sedum roof above the main living area and another above the new garage.

“The sedum roofs are a key feature of the house as both are visible to anyone approaching the property,” explains Nigel. “As a retired builder, I was keen to be involved in their design, especially the waterproofing to ensure the most effective solution.

“I wanted to use a liquid rather than a single ply membrane as it would fully bond to the substrate offering a more durable solution and greatly eliminating the risk of any leaks.”

The liquid resin was applied by Surrey-based Exterior HomeCare and installed ‘wet on wet’ in a single process using a reinforcement fleece. Once cured, the resin formed a seamless and elastomeric membrane that is inherently root resistant. Prior to laying the green roof, electronic testing was also carried out to ensure the solution was as robust as possible.

The waterproofing membrane, which was a vital element of the green roof specification, has guaranteed a watertight barrier between the roof substrate and the green roof systems, and is also root resistant. It is designed to provide the strength and flexibility that gives the homeowner the assurance that a sedum roof will remain viable for at least 20 years.

Victoria Ramwell is UK marketing manager at Kemper System