A version of this post originally appeared in the KLH Sustainability blog.
Green, blue, brown roofs…whatever colour, we’ve been getting questions lately about the best ways to use roofs for sustainability. 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, more information is needed to quell the concerns and take the leap.
First, let’s define what some of the various roof options are. There are many names used for similar concepts, but there are key differences that impact the cost, effectiveness and benefits of the roof options.
Green, brown and blue. Green roof tend to be the umbrella term for all different kinds of vegetated roofs, whether coloured green or not. Brown roofs don’t necessary look brown (though sometimes they start out that way), they’re a vegetated approach designed with a primary purpose to enhance biodiversity. Similarly, a blue roof doesn’t look blue, but it’s specifically designed to store water, generally for surface water control purposes. Blue roofs are not themselves vegetated, but are often paired with green roofs on top. Green roofs can also achieve a degree of water storage in the soil without the formal infrastructure of a blue roof.
Extensive and intensive. These are the key types of vegetated roofs, though many roofs fall in the spectrum between the two. Extensive refers to roofs that have a shallower substrate with less soil, have lower-lying vegetation, are often in inaccessible places and have low irrigation and low maintenance requirements once established. They tend to be lightweight and less expensive. Intensive roofs are more like gardens with a variety of vegetation, soil depths and landscape features. They are often accessible amenities, require more maintenance and irrigation, are heavier and more expensive.
Other names. Living roofs are another name for vegetated green roofs. Biodiverse roofs are another, arguably more attractive and specific, name for brown roofs, which are focused on biodiversity and often include log piles & varied substrates to promote habitats. Biosolar roofs are those where the placement of vegetation and solar panels are optimised to take advantage of cross benefits like improved panel efficiency due to cooling from planting and more varied habitats for biodiversity.
Green and blue roofs are widely understood to be beneficial; the challenge now comes in the implementation. We find that 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 (more resources below), here are some key things to consider when deciding to incorporate a sustainable roof, including answers to some of your questions:
Establishment. Vegetated roofs, even low-maintenance 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 & maintenance. Appropriately designed extensive roofs may not need long-term irrigation, though they can’t be forgotten entirely, particularly during dry periods. Intensive roofs 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.
Loading. 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.
Waterproofing. 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 shouldn’t be any more concern than for a typical roof.
Biodiversity. 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.
Thermal performance. 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.
Location. Orientation of the roof, availability of sunlight, amount of wind and visibility will influence the design. Green roofs can be used on shallow pitched roofs and retrofitted onto existing buildings.
Soil. 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 circular economy, but leaching from the reused materials must be considered for water and soil quality, and concrete rubble should be avoided.
Water storage. 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 & reuse. In green roofs, rainwater is filtered as it goes through the soil, providing water 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.
Additional resources for more detail:
- The Green Roof Code: Green Roof Code of Best Practice for the UK 2014, The Green Roof Organisation
- Living Roofs
- Green Roof Guide, Groundwork Sheffield
- SuDS Manual, Chapter 12, CIRIA