Specifier Review
green roof

Key considerations for green roof specification

Stuart Hicks from cold-applied liquid waterproofing specialist, Kemper System, discusses the rise of green roof applications and specification best practice.

The green roof market is quite literally growing – in 2017, the annual market growth rate was revealed at 17%, according to ‘The UK Green Roof Market’ report produced by Livingroofs.org.

This year alone, the market baseline for green roof application was £26.2M, compared to the estimated £17.5M. London alone currently installs around 42% of all UK green roofs delivered, with a specific green roof planning policy driving the market. Yet, the report predicts more growth from strategic city and region planning policies across the UK, particularly with the election of the new ‘Metro Mayors’.

This is the evolution of green roofs, from a specialist eco-feature to much more ‘mainstream technology’. Whether the green roof is specified to help adapt towns and cities to climate change, providing habitat for nature, reducing urban heat islands and indoor temperatures, managing storm water and supporting healthy and resilient communities, or simply to gain BREEAM points – it can be a striking addition to a building that matures to enhance the structure’s character over time.

However, the success or failure of a green roof depends on an effective specification that considers not only the structure itself, but the location, orientation, shading and climate of the site. Only by working with knowledgeable waterproofing and green roof specialists that can offer a proven roofing system with the required level of horticultural expertise, can the specifier be confident of a viable long-term result.

Stuart Hicks from cold-applied liquid waterproofing specialist, Kemper System, discusses the rise of green roof applications and specification best practice.

Planning at Design Stage

While it may appear that planting can be decided at a later stage, in fact 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.

Green roofs can also be incorporated into a project as part of a SUDS (Sustainable Drainage System) design. Water stored to irrigate the green roof reduces the potential impact of new and existing developments with respect to surface water drainage discharges.  Again, considering the planting at an early stage means that the specifier is in a much more informed position to understand the level of water that will need to be stored or released from the roof.

A Watertight Solution

The effectiveness of the waterproofing membrane used to create a watertight barrier between the roof substrate and the green roof system is the most vital element of the specification. 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.

The use of Kemperol V210 or Kemperol 2K-PUR however with its additional strength and flexibility, means the waterproofing can be applied either below the insulation or above as in the case of a conventional warm roof.  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 considered the planting specification, without proper maintenance, the green roof will not thrive post-installation. Establishing a maintenance plan and provision for the associated costs needs to be factored in at the design stage for the aesthetics and benefits of the green roof to remain durable.

A Complete Approach

KEMPERGRO® from Kemper System brings together a unique combination of recycled, renewable and sustainable components to provide a complete waterproof roofing and green roof system with a single source specification.

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, liaising with the technical team at Kemper System can provide a solution to meet local climatic conditions, aspect and biodiversity requirements to ensure the green roof performs as per the specification.