Green roofs, also known as living roofs, are simply roofs designed to grow plants on top of them. They are not just reserved for the likes of hobbit houses, as the excellent insulating properties of the combined plant and soil layers help retain a building’s heat in cold climates and keep buildings cool in warm climates, so they have been used in vernacular architecture across the world for centuries. And in my mind they should play a significant part in our future buildings too.
Vernacular buildings constructed entirely from natural materials
Benefits of Green Roofs
The benefits of green roofs extend beyond their thermal properties however. Plants on rooftops can create beautiful environments for people to be in or look over from upstairs windows or balconies, with some creating wild and wonderful interruptions to otherwise hard rooftop scenes.
Intensive Green Roof with shrubs and trees. Photo: Bere Architects
The plants on top of green roofs help clean our air and reduce global warming by drawing pollutants and carbon dioxide out of the atmosphere. They can also provide biodiverse habitats, particularly for invertebrates like butterflies, bees, earthworms and snails, as well as for birds.
In a situation where development may take away an area of natural habitat on the ground such as an extension in a back garden, green roofs make an excellent replacement to avoid the complete loss of the habitats and avoid the additional pressure on surface water drainage that built development tends to create. In other words, the
soft, natural, water-permeable landscape that was on the ground is transferred to the roof and any upstairs rooms that previously looked out on to an attractive garden, can continue to do so.
A perhaps less glamorous function of green roofs is this wonderful ability to retain water and they can be designed to be deep enough to retain storm water to take pressure off drains and sewers, if the roof structure beneath allows the extra load. The water will naturally evaporate back into the atmosphere or can be harvested for use elsewhere. On a new extension, it may be desirable to design in an increased depth to protect against flooding or excessive water run off in extreme weather events that will become more frequent as the climate changes. The increased depth would also have insulating properties, perfect for reducing the need for energy for heating inside the building and so reduce fuel costs.
Eaves Level Greening at Exmouth Market, London. Photo: Bere Architects
Urbanisation, or increasing built up areas, has a significant impact on ambient temperatures in our environments. Urban areas tend to have higher temperatures than the countryside because the hard surfaces absorb heat, especially dark roof surfaces. This is known as Urban Heat Island Effect. Green roofs are an excellent way to moderate urban heat island effects by reducing the temperature of the roof surface and surrounding air, particularly in the daytime.
Adding a green roof to an existing flat roof can also protect it from ultra violet radiation damage and thus extend its life. A green roof would have a longer lifespan than conventional roofing materials and the extra layers create acoustic insulation to reduce noise levels as well. It is also possible to combine a green roof with solar panels to make it even more versatile.
Multifunctional Green Roof combined with electricity generating Solar Photovoltaic Panels
These various functions of a green roof naturally greatly influence its design, maintenance needs and build costs, as does how easy it is to access, so all these factors require some thought.
To build up a green roof in the UK, the choice would be between installing an Extensive System or an Intensive System. But how would you choose what is best for your project?
Extensive Green Roofs
An Extensive system is likely to be cheaper to build and easier to maintain as it is much shallower in depth, typically between 50-150mm. It is therefore lightweight, easy to install and simple to design. Use of self-sustaining plants such as sedum means there is no need for irrigation and it is likely to have little or even no maintenance requirements. This makes it a good choice for large areas or when there are budget or structural constraints – or if you just can’t get to the roof for maintenance.
Extensive green roofs can be placed on top of lightweight roof decks or inaccessible roofs. They can occupy flat or sloping roofs up to 40 degrees, but shallower slopes work best. As they retain water, they can be used to reduce surface water run off into the gutters.
The type of plants used in an Extensive System would be sedum, moss, grasses and herbs and these can be used in various combinations. Flowering sedum plants provide opportunities for pollination so it would be a good idea to introduce these to at least part of the roof.
Intensive Green Roofs
An Intensive system has deeper soils so can grow a greater variety of plants. It would be built to a depth of between 150-1500mm and can therefore accommodate a wide range of plants, including trees. This means it would require regular irrigation and maintenance, which should be factored into both the build costs and the ongoing long term costs.
The variety of plants possible in an Intensive green roof means they can be indistinguishable from natural gardens – basically anything goes.
Food growing Green Roof with paths
Hard and soft landscaping can be used such as creation of paths and water features, and they can even be used to play sport on. The extra depth means that thermal and acoustic insulation benefits are significant, as well as the opportunity to retain larger quantities of surface water.
The versatility of the deeper soil means Intensive green roofs can host lawns, shrubs, perennials and grasses, edible plants and small trees, with structures deep enough to make roofs accessible such as for a roof terrace or to even beehives!
Construction of Green Roofs and DIY Options
The construction of a green roof is similar whichever you choose with the key variables being the depth of soil/growing medium and roof structure. The layers would build from the roof construction upwards with a green roofing water proofing membrane placed on top off the structural support, a membrane protection layer on top of that, then a root barrier and drainage layer, followed by thermal insulation, an aeration layer, moisture retention layer, an optional water reservoir layer before finally topping off with a filter fabric and soil with planting, or it can be left to self colonise as soil left undisturbed would get populated and pollinated by insects naturally.
It might sound like a complicated project to take on, and an intensive roof on a new build is likely to require specification and supervision by an architect or green roof specialist. It is possible to create a DIY green roof however, with a number of YouTube videos offering options for how to do this on most roof types including garden sheds so take a look at what others have done. There are also roofing firms specialising in simple green roof systems that may be as little as three layers deep, incorporating a roof protection fleece, substrate and sedum blanket all placed on top of an existing roof and sold by the square metre.
Sedum Roof Construction Layers
There is also the option of creating a modular green roof, by purchasing living roof trays that click together to form a self contained green roof. This is a good option for a fuss-free instant green roof – great for a summer DIY project on that garage, shed or extension roof.
I found the prices of green roofs to vary considerably. Simple systems are around £35 per square metre and living roof trays came in at around £50 per square metre. Sedum roofs start at £50 per square metre and a fully planted intensive roof could be closer to £200 per square metre. Do your research and get multiple quotes.
Any structural reinforcement of the roof itself would be in addition to these costs, and so would any planning permission if required, which is unlikely in the vast majority of cases. It is important to check with the planning department if you have any protections or designations on your property, such as a listed building – but even that would not necessarily be a barrier. It is worth bearing in mind that the initial outlay of costs could be gradually recovered in savings on fuel bills where a building is benefitting from a significant amount of thermal insulation due to the green roof.
Extensive Green Sedum Roof, Photo: Garden Affairs
It is the multifunctional role green roofs play in mitigating and adapting to climate change that makes them particularly appealing to me, including dealing with pollution and global warming, preventing overheating and heat loss, reducing water run off and increasing biodiversity. All whilst creating a pleasant view. I hope they become a more mainstream choice for every roof soon.
Check out the references below for more information:
Photographs are from Adobe Stock, unless credited to Bere Architects who are green building specialists based in London, www.bere.co.uk or Garden Affairs who are Wiltshire based garden room specialists.