What is an Extensive Green Roof?
An extensive green roof is a lightweight, grass and plant covered rooftop with a shallow growing medium that helps insulate buildings, provide habitation and increase the roof’s lifespan.
They differ from intensive green roof counterparts in that they are low maintenance, lightweight and only support a small variety of flora and fauna.
Extensive Green Roof Detail
Most extensive roofs are made up of 5 – 6 layers. We’d always recommend using filter fleeces and root barriers, but in reality some tiny DIY living roof projects don’t need them.
And extensive roofs are usually much more streamlined, because the vegetation doesn’t require much sustenance to grow. Sedums and extensive green roof plants are hardy, drought resistant and – almost – maintenance-free.
In fact, utilsing a feed or substrate too nutrient-dense will cause plants to grow too fast. Potentially damaging your roof’s structural integrity through increased load, increasing the amount of maintenance – weeding, deadheading etc – you’d normally need to take.
Extensive Green Roof Layers
Layer | Thickness | Cost |
|---|---|---|
Waterproof membrane | <20mm | £ – ££ |
Drainage layer | <20mm | £ – ££ |
Filter fleece | <20mm | £ – ££ |
Root barrier | <30-40mm | £ – ££ |
Growing medium | 40mm – 200mm | £ – ££ |
Vegetation | 50mm – 300mm | ££ – ££ |
Extensive Green Roof Benefits
Extensive green roof benefits are, obviously, very similar to typical green roof benefits. But there are some subtle differences between intensive and extensive roofs.
Insulation and Energy Savings
Energy savings are widely associated with green roofs. But intensive roofs typically conserve significantly more energy than extensive roofs. Because the thickness of the growing medium allows for much improved insulation.
This study examining thermal performance of green roofs – undertaken in hot, humid climates – suggests that intensive, semi- intensive, and extensive green roofs provide about 20%–60%, 10%–45%, and 20% of energy-savings, respectively. So your extensive roof’s thermal ceiling is significantly lower. Potentially 200% lower
Accessibility & Applicability
One of the major advantages extensive living roofs have over intensive ones is their lightweight, applicable nature. You can put an extensive roof on almost any structure. Sheds, garages, storage units. Which cannot be said for intensive roofs. They require a significantly sturdier structure.
Stormwater Management
Green roof vegetation and substrate retains a substantial amount of water, mitigating runoff in urban areas. Urban stormwater runoff has serious consequences when it comes to water quality and overloading sewage treatment plants and waterways.
This study on extensive green roof runoff management in Connecticut found that the roof retained 51.4% of all precipitation and reduced overall pollutant runoff into the ecosystem.
That a paved urban area contributes 5 times as much runoff as a woodland area of the same size!
Habitats & Biodiversity
Because extensive green roofs are simple to erect – and are largely designed to be maintenance-free – they make excellent habitats, improving biodiversity in urban areas.
As urban development continues, providing green urban spaces on otherwise unusable areas should be paramount. The success of green roofs providing a habitat for the Black Redstart in London is a good example.
Maintenance
Or rather lack of maintenance. Arguably one of the most wonderful extensive living roof benefits is its fuss-free care. Most extensive roofs are created with hardy, lightweight vegetation – think sedums and grasses – that require almost no feeding, watering or deadheading.
Extensive Green Roof Examples
This flat, sedum roof is a typical example of an extensive green roof.
Lightweight, shallow substrate with low-lying vegetation. A maintenance-free, biodiversity encouraging way to better utilise space.
The Moesgaard Museum in Denmark is an excellent example of a pitched, extensive green roof.
The grassy-design is supposed to fit in seamlessly with the area around it – which happens to be a park. Improving insulation and roof longevity.
Frankfurt International Airport is an excellent example of an extensive green roof used for green initiatives.
Noise and air pollution is, of course, particularly commonplace around airports. Who knew? Whilst it may seem like pissing in the wind, the airport actually has 40,000m2 of green roof coverage. Which goes some way to mitigating pollution, dangerous runoff and habitation damage.
The Role of Extensive Green Roofs in Sustainable Development
Extensive green roofs should be an essential tool in sustainable urban development.
They don’t require the same level of time, maintenance or money as intensive roofs, yet yield much of the same benefits. From encouraging biodiversity to mitigating pollution and dangerous runoff.
As cities continue to grow – more than 50% of the world’s population lives in urban areas – it is vital we make better use of our space. Even for the mental benefits associated with greenery. Something lacking in most cities. And whilst UK green roof policies are lagging behind, there’s opportunity to sustainably develop our urban areas that we can control.
Extensive Green Roof FAQs
Extensive green roof substrate is typically somewhere between 400mm all the way up to 200mm!
Most extensive roof systems will be fine with around 100mm of substrate depth. This shouldn’t place undue stress on the roof’s load. Although for smaller projects, even 50mm could be fine.
An extensive green roof can cost as little as a few £100 pounds. In fact, we have an entire page dedicated to the cost of green roofs. Both intensive and extensive.
The maintenance costs can also vary, but should never be more than £100 – 200 / year.
An extensive roof typically weighs somewhere between 10kg – 50kg / m2. Of course this is particularly dependent on the amount of substrate and vegetation on the roof.
A traditional extensive roof includes – as a minimum – plants / grass, a waterproof layer, drainage, a filter layer and substrate. A minimum of four – five layers which, in periods of heavy rain, can retain water and fluctuate slightly in weight.
In almost every situation you cannot walk on an extensive roof. Extensive roofs are not designed to be walked on. That is more the intensive variety.
Extensive roofs are lightweight structures built on traditionally smaller roofs – like sheds or garages – built to encourage habitation and improve the green effect.
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