There are three main types of green roofs:
- Intensive: The most expensive, thickest type of living roof that provides the best insulation, drainage management and temperature regulation – they can essentially replicate a natural landscape and can even be used for agricultural purposes
- Extensive: The cheapest, most lightweight green roof on the market. Ideal for flat roofs and primarily built as a way to promote biodiversity
- Semi-intensive: A roof that combines elements of extensive and intensive living roofs. Typically used to harness the environmental benefits associated with an extensive roof, but with a more diverse, structurally sound blend of plants and substrate you’d get with an intensive roof
There are also three other types of green roof to be aware of:
- Biodiverse green roofs: As the name suggests they’re primarily used for ecological benefits and contain a mixture of recycled materials, often designed to recreate or improve upon the habitat that was – or should’ve been – there previously
- Blue-green roofs: A combination of blue and green roofing technologies, designed to store rainwater and mitigate runoff
- Biosolar green roofs: You’ve guessed it – a seamless combination of solar and green roof technologies. The ultimate in sustainable roofing
What Makes a Green Roof a Green Roof?
Whilst all green roofs (more or less) share these elements, the type of roof impacts the quantity, quality and variety of each layer.
An intensive roof will have the thickest layer(s) of root construction, drainage, growing medium, plant level and so on because the quantity and size of the plants demand it.
Whereas an extensive roof will have the thinnest layer of each as the hardy, lightweight planting material just doesn’t require the level of structural integrity and drainage than their intensive counterpart.
Intensive Green Roofs
Intensive living roofs are the densest, most costly type of natural roofing solution available. Essentially an intensive roof can exactly replicate conditions at ground level – you’d be better off thinking of it as a fully landscaped garden that just happens to be on a rooftop.
The quantity and quality of materials means that a fully landscaped rooftop garden is far more suited to highly visible, accessible roofs (for the aesthetic benefits too). Not only do you need to consider construction, but also the regular maintenance – how easy is it to get to, let alone maintain?
If you’re considering an intensive roof you’ll need to make sure your roof is structurally sound, or you’ll have to make significant improvements. Intensive roofs can weigh up to 500kg / m² with a substrate depth of up to 400mm.
Just to be clear, intensive roofs are definitely not a DIY green roof job.
Then there’s the cost to factor in. A professional, intensive living roof on the average home could cost anywhere between £5,000 to upwards of £15,000. But it’s worth checking out our green roofs cost page for more detail.
Intensive Roofs are Perfect For...
Extensive Green Roofs
Extensive green roofs are the cheapest, most-common living roof solution that primarily focus on providing environmental benefits.
Their easy installation, ongoing maintenance and ecological awareness make them ideal for designing into new builds or adding to less accessible properties. Instead of adding substrate and plants separately you can lay sedum mats instead of planting and plugging individually.
They provide a lightweight, almost instant green-effect that, because of the versatile, weather-resistant materials and plants used, require little to no maintenance. Hence their ongoing popularity.
Extensive Roofs are Perfect For...
The best extensive green roof plants – typically sedum in variety – are thick leaved, drought resistant and have shallow soil layers (typically 60 – 200mm in depth). Often referred to as succulents, sedum plants store water in their leaves and can dramatically minimise moisture loss.
If you have no real desire to change the structural integrity of your roof, want to minimise costs and have no plans to spend time on or looking at the roof itself, then an extensive roof will suit you best. Especially considering they don’t weigh more than 150kg / m². 3 – 4 times less than their intensive counterparts.
They’re likely to cost between £4,000 – £7,000 for an average UK roof. And their relative simplicity makes them the DIY-lovers green roof of choice – a significantly cheaper solution!
Semi-Intensive Green Roofs
Unsurprisingly semi-intensive green roofs are a blend of intensive and extensive living roofs.
Typically their cost lies somewhere inbetween and they’re designed to try to get the best of both worlds – they harness the environmental benefits of a wild extensive roof with a diverse, more-manageable garden. An aesthetically pleasing ecosystem that can retain more stormwater than extensive roofs (due to it’s deeper substrate layer) and can host a richer ecology.
They have a wider, more complex range of plant mixtures than extensive roofs and utilise a deeper, richer substrate and drainage solution, designed with the home’s occupants in mind.
They can utilise hardier sedum plants where appropriate to reduce maintenance and improve installation, whilst adding small shrubs, flowering plants and taller grasses to add to the useable garden effect.
Semi-Intensive Roofs are Perfect For...
Of course this all comes at a price – both structurally and economically – because the taller, more labour intensive plants are heavier (up to 200kg / m²) and need a deeper layer of substrate to bed in (up to 250mm).
And this means they also require a little more maintenance than extensive roofs as these less weather resistant plants tend to require watering, irrigation, fertilisers and pruning.
Biodiverse Green Roofs
Biodiverse green roofs (previously known as brown roofs – although there are some differences) are essentially extensive or semi-intensive living roofs solely created for the ecological benefits. Their careful design and construction can aid planning applications, helping local authorities build a sustainable environment.
Most of the green roof guidelines still apply when considering a biodiverse roof:
- Structural solidity
- Waterproof membrane
- Suitable drainage
What makes a them stand out is the inclusion of features aimed at helping a specific species thrive. They contain a range of recycled materials including rubble, logs and gravel that encourage invertebrate habitation. The vegetation is normally made of of British plugs and seeds, all designed to support a wide variety of plants, birds and animals.
This living roof type is not necessarily a cheap option. Whilst the efficacy of using recycled materials as living roof substrate is often tested, British building regulations don’t allow the transfer of demolition waste to rooftops.
For a biodiverse roof to be genuinely beneficial, it must support an intended animal(s). So moving an array of waste materials isn’t a valid solution as it may not provide the ideal chemical makeup for a certain species to thrive.
If you’re interested in a biodiverse roof, then we recommend checking out the HAP (Habitat Action Plan) for the area in which the roof is built.
We love solar power here at the GRG. With the world’s most efficient panels in production in the UK, 2021 should provide an essential solar surge. As 80% of the European population is likely to live in cities by 2050, encouraging self-sufficient, clean energy systems that support wildlife is more crucial than ever before.
True biosolar roofs integrate solar and green roof technologies seamlessly. Our main concern with rooftop solar panels or tiles was the reduction in available habitats and biodiversity opportunities – not to mention the inability to insulate a roof.
But by properly integrating and installing solar mounts on green roofs, you can:
- Ensure biodiversity
- Generate enough electricity – without a green roof you could make your money back by 2037
- Self-irrigate vegetation through panel runoff
- Improve the power production of the solar PV units due to the green roof’s cooling nature
And whilst solar panels are expensive – around £6,000 for the average household – having the ability to insulate your home, produce your own electricity and create a diverse habitat is a phenomenal opportunity.
Blue - Green Roofs
Blue – green roofs combine both blue and green roofing technologies. Although I’m sure you don’t need us to tell you that.
Blue roof technology aims to increase the volume of water your roof is able to store and control the amount of water released. Particularly valuable in areas prone to flooding or urban areas where the high level of impermeable ground layer can cause runoff issues.
One study in Seoul found that blue roofs reduced runoff by 71% – but a green blue roof combination reduced it by almost 94%.
- Temporarily storing rainfall – potentially for irrigation
- Mitigating runoff
- Reducing flow rates
Blue roofs can makeup a valuable drainage and support layer for green roofs. So a combination of the two encourage biodiversity, reduces energy use, cool cities AND help manage stormwater.
The primary concern most people have with blue – green roof technology is the cost. Blue roofs themselves are relatively cheap – a combination of the two certainly isn’t. But if stormwater management and drainage is essential for your rooftop – and peace of mind – they’re a superb option.
Now you know the types of green roofs available, you should be able to make a more informed sustainable decision.
Based on your budget, goal(s) – environmental, financial or of the aesthetic variety – and rooftop availability, you should now know what living medium you need.
If you aren’t ready to make that final decision, some further reading may be in order.