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Last edited 07 Dec 2020
Designing green and blue roofs
Whether it's due to a desire to capture surface water, increase biodiversity, provide habitat, reduce the urban heat island effect, produce renewable energy, improve air quality, provide amenities to building occupants or all of the above, there's an increased awareness about the possibilities of sustainable roof infrastructure, but for many, the challenge now comes in the implementation.
Developers, architects and engineers know they should incorporate green roofs, but don’t know the various types, constraints, or details to consider. Incorrect assumptions and treating them as afterthoughts can lead to quick dismissal of options or incorrect design and failed systems.
While not a comprehensive list, here are some key things to consider when deciding to incorporate a sustainable roof.
Vegetated roofs, even low-maintenance, non-accessible, extensive roofs, need to be maintained and irrigated properly for the first two growing seasons after installation. Many roofs fail because a maintenance regime is not properly implemented, especially in the period between installation and project handover. Irrigation systems for the establishment period need proper consideration at design stage. It may be as simple as a tap to connect a hosepipe or, for difficult to access roofs an integrated drip irrigation system.
 Irrigation and maintenance
Appropriately designed extensive roofs may not need long-term irrigation, though they can’t be forgotten entirely, particularly during dry periods. Intensive roofs, such as Jubilee Gardens above Canary Wharf Station, need to be regularly irrigated and maintained like a garden. Blue roofs should be inspected regularly to ensure outlets are not blocked and there is no damage to the waterproofing. It is essential to provide safe access to roofs for regular maintenance and inspection.
It is a common misconception that green and blue roofs cause such an increase in loading that the cost of strengthening the structure to support it makes them unfeasible. Before dismissing a sustainable roof option for this reason, it’s important to understand the range of options available. Green roofs can come with varying depths of substrate and quantities of soil and blue roofs can hold different depths of water, so they can be designed to fit the structure, whether new or retrofitted. Some manufacturers suggest weight can be offset, for example blue roofs can eliminate the need for screed.
This is a key concern, especially for blue roofs, but it comes down to good detailing and installation. Many suppliers provide warranties and installation supervision. If installed correctly, there should not be any more concern than for a typical roof.
This means more than incorporating a diverse set of plants, it also means different soil depths, textures, habitats (like log piles and insect hotels) and coverings. The early input of an ecologist can ensure the roof responds to local wildlife opportunities.
Green and blue roofs can be installed on both warm and inverted roofs. The systems are generally separate from the thermal envelope, so are excluded from thermal calculations. They do still provide cooling benefits to the surrounding area, roof mounted solar panels and top floor units.
The type, depth, source, richness and particle size matters. Different plants require different depths and richness of soil. Variation in depth and type fosters better invertebrate habitat. Using recycled construction waste can be good for the circular economy, but leaching from the reused materials must be considered for water and soil quality, and concrete rubble should be avoided.
The usefulness of green roof substrates for water storage depends on depth and saturation, which is calculated based on season and size of storm. In addition, less compacted soil and presence of voids can help store more surface water. Blue roofs can be placed under all green roof types, as well as under decking and podiums. For blue roofs, the drainage benefit comes from the depth and design of the outlet. Smart technology is being developed to allow blue roofs to discharge in line with upcoming weather patterns. Green and blue roofs mainly reduce discharge rates, though green roofs can have limited volume reduction benefits from evapotranspiration.
 Water quality and reuse
In green roofs, rainwater is filtered as it goes through the soil, providing water a quality benefit for discharge or reuse. Green and blue roofs can also be linked to rainwater harvesting systems, though for green roofs, the residual colour makes the reused water suitable mainly for external use.
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