Daniel Bosworth, Design & Technical Manager at SIG Design and Technology, discusses best practice design for green and blue roofs with Architecture Today’s Technical Editor John Ramshaw.

Green roofs have gained considerable traction in the UK in recent years, aided by both legislative changes and the growing climate emergency. But that is only part of the story; few, if any other building technology provides such a wide range of environmental benefits, including increased biodiversity, carbon capture, flood and urban heat island mitigation, as well as improved air quality and waterproofing durability.

From being a ‘niche’ product 20 years ago, green roofs are now an established part of the roofing industry and have been joined more recently by increasingly sophisticated blue roof solutions. For architects this has not only meant more choice, but also more complexity when it comes to greening the roof space. So how should specifiers approach the design of green and blue roofs? What types of system are available? And how should they be maintained post-installation? Daniel Bosworth, Design & Technical Manager at SIG Design and Technology, in conversation with Architecture Today’s Technical Editor John Ramshaw answers these questions and more.

A 930-square-metre wildflower roof crowns Peppa Pig World in the New Forest National Park, Hampshire, designed by HPW Architects (photo used with permission of HPW Architects)

How do green and blue roofs differ?
Green roofs are designed to replicate the natural growing environment for plants, and as such comprise a layer of vegetation installed over a waterproofing system on a flat or gently sloping roof. They naturally store water in reservoir pockets, making it available to plants through the process of evaporation.

Blue roofs control storm water attenuation from within the building footprint. They extend the storm water buffer of a standard green roof in a measurable way and are therefore highly suited to urban areas where ground space is limited. They can be installed above the waterproofing membrane on warm roofs, above the water flow reducing layer (WFRL) on inverted roofs (EGR EcoVoid), or above the inverted roof and separated by way of a secondary tank membrane (EGR EcoVoid Cloud).

The green roof build-up at Peppa Pig World comprises a warm roof construction laid over a single-ply membrane. A structural retainer was installed at the overhanging eaves to take the majority of the weight during construction (photo used with permission of HPW Architects).

What systems are available and how do they compare?
SIG Design and Technology, in partnership with Wallbarn and Eco Green Roofs (EGR) respectively, provides both modular and built-up green roofs, as well as blue roof systems. There are three distinct types of built-up green roof available: extensive, intensive and biodiverse. Extensive systems are the most cost-effective, employing sedums, grasses and herbaceous plants, which require little maintenance. Biodiverse or semi-extensive roofs as they are sometimes known, are also low maintenance, but are designed to provide richer habitats for flora and fauna. Intensive green roofs utilise deep substrates facilitating the design of roof gardens and high-quality recreational spaces. They are more expensive and can require significant upkeep.

A more recent development is the M-Tray green roof system from Wallbarn. This comprises a series of modular trays containing high-quality engineered substrate and established flowering sedum. They are easy to carry and simply click together on site. Advantages include simplicity of transportation and installation, particularly on hard to access areas, optimal 100mm cavities, and longevity of plant life.

Blue roofs tend to be more complex than green roofs, as each one will have its own unique requirements relating to both the building in which it is installed and the rainwater attenuation/storm event criteria. There are two principal types: active and passive, which refer to the methods of controlling water drainage from the roof. Blue roofs can also be incorporated into green roof designs (blue-green roofs), comprising additional water storage capacity beneath the planted top layer.

The Fielding Meadow development in Ratby, Leicestershire, comprises three rural employment centres and 36 homes. Green roofs are part of the environmental strategy, along with air source heat pumps, biomass heating and a photovoltaic system (photo supplied by Cawrey Homes / Modern Roofs).

How should architects approach the design of green and blue roofs?
For both roof types, specifiers should seek early engagement with SIG and its specialist green and blue roof partners. This will help to ensure that the right product is selected for the right application, thereby avoiding costly mistakes and revisions later on.

There are a number of key criteria for green roof specification, including vegetation type, and substrate design (both of which must complement each other), drainage layer and roof membrane. With regards to the latter, IKO’s Permatec hot melt waterproofing system is generally the preferred solution due to its outstanding durability and long-term performance.

In terms of plant choice, architects should consider both internal and external factors, such as the structural capacity of the building and air pollution levels. The planting method will also need to be specified, either pre-grown mat, plug planting or self-vegetated.

The green roofs at Fielding Meadow comprise a highly insulated, ventilated cold roof system. The flat roof surface employs a Rhenofol PVC membrane under gravel ballast supporting the sedum (photo supplied by Cawrey Homes / Modern Roofs).

Geographical location is one of the key considerations for specifiers approaching blue roof design. Knowledge of local climate patterns and future storm events are critical in establishing the required level of performance for the roof. Specifiers should also consider the four key aspects of SuDS best practice: attenuation capacity, water treatment and filtration, roof space function, and biodiversity potential.

A one-stop-shop approach to the design, manufacture and installation of blue roofs is preferable due to their inherent complexity. This will ensure that the waterproofing layer and blue roof system – among other components – are compatible, avoiding future failures and split liabilities. It will also safeguard onsite programming, ensuring that the blue roof is installed directly after the waterproofing, protecting the latter from potential onsite damage.

EGR will design the blue roof and provide a check and verification report that complements the IKO Waterproofing system and ensures damage limitation risks are upheld throughout the installation process. In terms of waterproofing selection, hot-melt bituminous systems, such as IKO Permatec, are SIG Design & Technology’s preferred system over that of a warm roof membrane system for their superior puncture resistance and durability.

Eaves detail at Fielding Meadow (photo supplied by Cawrey Homes / Modern Roofs)

What guarantees are available for green and blue roof systems?
SIG in partnership with Eco Green Roofs provides a comprehensive design and delivery service for its green and blue roofs. For the latter this includes post installation integrity tests for the waterproofing, as well as a guarantee of up to 25 years for the roof waterproofing system – depending on the roof specification and maintenance schedule agreed. For green/blue roofs, rolling guarantees are available that can match the duration of the waterproof guarantee.

How should green and blue roofs be maintained after installation?
Regular maintenance will maximise the long-term performance of the roof and ensure that the guarantee remains valid. Essential elements to monitor on green roofs include drainage provision, quantity of nutrients, and plant health. EGR’s maintenance recommendation varies depending on the type of roof and is typically twice per annum for sedum and biodiverse, and six times for wildflower. On blue roofs, periodic inspection and maintenance of the filters and restrictors is essential.

Contact Details
For more information, please visit the SIG Design and Technology website.