The green roof is becoming a more common occurrence in Australian development as the industry looks for improved sustainability and better amenity solutions. But could a new breed of green roof, gaining popularity in the US over the last couple of years, provide even better outcomes?

In simple terms, a green roof is partially or fully covered with vegetation and a growing medium, planted over a waterproof membrane. Sod roofs have been used in Scandinavia for centuries.

They provide a number of benefits, including thermal and sound insulation, amenity space, biodiversity, reduced water runoff and improved water runoff quality.

There are four key types, depending on soil depth and maintenance options.

An ‘Extensive’ green roof is a self-sustaining ecological landscape; a ‘Biodiverse’ one will have a locally sourced growing medium, naturally vegetated; ‘Semi-intensive’ is a hybrid of extensive and intensive; and ‘Intensive’ is equivalent to a garden or park.

Extensive Green Roof


These require only a shallow substrate of 60 to 200 millimetres.

Vegetation might include moss, herbs, grasses, wild flowers (sedum or similar), which are planted during installation.

They are virtually self-sustaining and require minimal maintenance, with annual weeding and the annual application of slow release fertilizer typically sufficing.

Their low cost and low upkeep are upsides, but they tend to be used for inaccessible roofs and do not provide amenity space.

The weight consideration for structural engineers is 60 to 150 kilograms per square metre.

Biodiverse Green Roof

greenroof3Industrial brownfield sites can be important ecosystems, supporting rare species of plants, animals and invertebrates. The redevelopment of these sites threatens these habitats. Similar to extensive green roofs, these biodiverse options help to mitigate this loss.

The growing medium is typically recycled material sourced from or around the site. Vegetation is then left to grow naturally, replacing lost habitat, and coverage develops much more slowly. Additions such as “insect hotels” can also be incorporated but these require specific expertise and knowledge to design successfully.

Their self-sustaining nature means little to no maintenance is required.

Semi-intensive Green Roof

greenroof4These require a moderate substrate of 120 to 250 millimetres.

Vegetation might include herbs, grasses, wild flowers (Sedum or similar) and shrubs, like the extensive green roof, planted during installation.

As they tend to be used for accessible and highly visible roofs and provide amenity space, they require more maintenance. This includes regular weeding, regular application of fertilizer and shrub pruning

The weight consideration for structural engineers is 120 to 200 kilograms per square metre.

Intensive Green Roof

greenroof5This really is a true roof garden requiring a deep substrate of 150 to 400 millimetres.

Vegetation might include lawn, shrubs and trees but again vegetation can still be planted during installation.

Unsurprisingly, these high quality amenity spaces mean higher costs and greater upkeep requirements.

The significant maintenance includes regular weeding and lawn mowing, regular application of fertilizer and shrub and tree pruning

They are also more of a challenge for structural engineers requiring a weight consideration of 180 to 500 kilograms per square metre.

Blue Roof

Click to enlarge

Click to enlarge

The blue roof is a fairly new source of alternative energy and conservation.

It is explicitly designed to capture and slowly release rainwater in order to slow the rate of runoff and reduce the potential for related flooding.

The stored rainfall can be reused for irrigation purposes, as makeup water for cooling, or in recreational contexts.

Blue roofs are often incorporated with any type of green roof to provide hydration to vegetation, although they can also be provided under paving.

They can be combined with light-colored roofing materials to further mitigate urban heat island effects.

They reduce or eliminate the need for underground storage, which in turn educes excavated material and underground construction cost and time.

They are a viable retrofit for existing buildings, although structural capacity needs to be carefully considered and the maintenance required is equal to that of a conventional roof.

During and after rainfall, weirs at roof drain inlets restrict the flow rate of the captured water, creating temporary ponding before gradually releasing stormwater to the municipal sewer system. Most weirs allow for adjustment of the flow rate.

The ratio of captured runoff is typically designed to closely mimic the pre-construction hydrology of the site. A blue roof acts as a temporary sponge, replacing the prior capacity of undeveloped, pervious ground to absorb rainwater.

Blue roofs are gaining particular popularity in New York, where rooftops have increasingly been repurposed from a utilitarian function into places that can be used for the use of the public and for the benefit of the environment.

Rooftops collectively constitute almost 20 per cent of the entire area of New York City. The city’s policy document, PlaNYC, states that “rooftops represent our last big frontier.” The city provides a Green Roof Tax Abatement from City property taxes of $4.50 per square foot of green roof, up to $100,000 and administrators believe the utilization of blue and green roofs could save New York City $2.4 billion over 20 years; without the roofs, taxpayers could end up spending $6.8 billion repairing constantly flooded treatment plants.

Blue roofs complement long and flat roofing styles, and have wide gutters with a sturdy watertight liner. This design works especially well in highly urbanized areas, like New York City, where less space is available for on-site stormwater detention. As cities around Australia and the globe become increasingly dense, blue roofs are certain to be become more and more common.

  • Blue Roofs – I totally agree with the principal, 20% of NY is scary. But who is providing the warranty and for how long? The integrity of the waterproofing liner is the issue as well as the installation, a plastic liner having tradesmen walking over it whilst installing all of the materials is going to be problematic. I can just see the disaster as water brings down ceilings in the penthouses.

    • There are high end roofing and waterproofing technologies that provide performance, longevity, and warranted installations for blue roofs. Plastic liners are rarely used in NYC.
      A monolithic waterproofing membrane can create a "bath tub" effect and pond water with no problems…

  • Nice article Justin – these truly seem like a great solution for improving the sustainability and efficiency of urban environments.

  • I have worked on several blue roof projects and have presented on the subject. There is a diagram of an inverted roof assembly which appears to have loose laid XPS insulation which is very buoyant in the article. How do you prevent the retained water from floating the entire assembly in such an installation?

  • In the UK blue roof are really beginning to take-off as a concept, especially in urban developments where space is at a premium and 'traditional' techniques for stormwater management (ponds, basins, underground tanks) are simply not feasible.

    The illustration is one of our systems and yes it does use loose laid XPS insulation. The effects of flotation are countered in a number of ways; firstly, over the top of the XPS is a sealed taped slimline membrane. It is there to prevent water entering into the insulation layer, instead keeping it in the geocomposite drainage/attenuation layer. Secondly, should any water get through this membrane the roof is ballasted not only with the growing media but also with a significant volume. Finally, should any water get down to the insulation level the outlet chamber has recesses that allows this water to quickly and efficiently drain away.

    Trust this helps.

  • Great article! This is a great solution for sustainability and making a more eco-friendly environment

  • Are you kidding me? This has been around forever. I am in central Illinois…there have been three of these type homes within 15 miles of me since the seventies. Sure it is a nice idea. Ideas are wonderful! BUT YOU MUST DEVELOP THE TECHNOLOGY. It is not responsible to keep writing these little pieces unless YOU are going to do something about it.