A green roof also referred to as a living roof, live roof, or vegetated roof, is a layer of substrate and plants on a roof.
A vegetated roof brings many benefits such as the reduction of urban heat islands, a decrease in water volumes that need to be treated by sewage treatment plants, increased urban biodiversity, and improve air quality.
A new generation green roofs also provide detention and can replace tanks and cisterns for stormwater management, but more about that later.
This page is for green roof novices. If you are new to vegetated roofs, welcome! We’re glad to have you here! Perhaps, you are here because you have heard of the environmental benefits a green roof can bring or simply appreciate the aesthetics. Whatever the reason, on this page you will find the basics of vegetated roofs, with links to further reading.
So, what is a green roof?
Green roofs can generally be split into three categories: intensive , extensive , and semi-intensive . Those odd names pertain to the amount of maintenance required.
An intensive green roof requires a more intensive maintenance effort because an intensive green roof is a deep system, more than 30cm (1 foot) deep, and grows tall plants such as trees and shrubs. This is sometimes called a roof garden or "landscape on structure."
The other vegetated roof category is extensive, which is thin (less than 15cm or 6 inches) and requires less maintenance. When we talk about green roofs at purple-roof.com, we usually refer to extensive.
Extensive vegetated roofs do not use topsoil or potting soil. Topsoil contains clay, which is excellent on the ground, but clay particles are tiny and can impede drainage. A vegetated roof needs to drain.
Potting soil is far too light and has a too high organic content, so it would blow away or "deflate" as the organics decompose.
Instead, extensive green roofs use an engineered soil media, which is mostly rock, with carefully screened particle sizes, and with precise amounts of organics.
This is basically a man-made soil, designed to work in thin layers on a roof.
Extensive vegetated roofs are thin and lightweight. Here we see Sedum plants of an extensive green roof peeking up from the roof of bus stop shelter in Utrecht, Netherlands. This vegetated roof is probably about 5cm (2 inches) total thickness and probably weighs around 73 kg/m2 (15 lbs/sf).
What about between 15 and 30 cm (6-12 inches)? Those are called semi-intensive. Note that there is broad, though not uniform, agreement on the delineation of extensive, semi-intensive, and intensive categories. In the end, these categories are just for general reference.
Living roofs are often incorporated into rooftop amenity spaces. This condominium rooftop includes paved seating areas, surrounded by a combination of extensive green roofs (low, like a carpet around the edges) and intensive green roofs (in this case within raised planters).
The modern extensive vegetated roof was invented in Germany and is something of a miraculous accomplishment that many of us now take for granted.
What is so remarkable about extensive green roofs? For starters, it was no small feat to get plants to perform reliably in such a thin substrate. If you just spread 5-10cm (2-4 inches) of soil on your roof, what do you expect would happen? Would it blow away? Wash off? Grow weeds?
Extensive green roofs should support a beautiful carpet of perennial plants, not weeds, not blow off, not wash off.
Are green roofs a new phenomenon? Or an old phenomenon? Both! The village of Mikladur, Denmark, shown here, features many buildings with traditional Scandinavian sod roofs, which have been part of the vernacular architecture for at least 1,000 years. But these sod roofs are somewhat thick, and relatively heavy, and have little in common with the modern extensive green roof. The modern extensive green roof is thinner, lighter, can be adapted to almost any climate zone, and uses different materials.
But the evolution of vegetated roofs did not stop with the development of the modern extensive living roof. Since that concept, referred to as the Traditional Green Roof concept, was proven in the 1980s, other innovations have come along.
In the late 2000s, mineral wool became more common in vegetated roofs to hold more water with a lower weight, which we call the Sponge Roof concept. Around the same time, vegetated roofs were sometimes installed over ponding water, often referred to as a Blue Roof , and either allowed to drain slowly or wicked up into the green roof soil, which is known as the Blue-Green Roof concept.
The Purple-Roof concept is a hybrid between those two, with the critical difference being that the Purple-Roof concept slows water down using friction.
We expect living roofs to continue to evolve to meet the needs of cities around the globe.
This is a massive green roof over the Music City Center in Nashville, Tennessee. The green roof is part of the project stormwater compliance and is also designed to be beautiful when viewed from above. This profile consists of a synthetic drainage layer, a slope stabilization mesh, approximately 10cm (4 inches) of green roof engineered soil media, and pre-grown Sedum mats. This thin assembly weighs approximately 146 kg/m2 (30 lbs/sf).
Tough plants provide a green roof’s reliable perennial plant cover! These are usually Sedums and other succulents, and sometimes grasses. Succulents are ideal because they tolerate hot, dry, cold, wet, and frozen conditions, and they require little maintenance and little nutrient .
Green roofs serve many purposes, especially in the hands of a skilled designer. This beautiful living roof on Howlett Hall at Ohio State University in Columbus, Ohio, features extensive (thin, say around 100mm [4 inches]) and semi-intensive (less thin, say around 200mm [8 inches]) profiles. The deeper semi-intensive areas feature taller plants. Collectively this interesting design provides a significant habitat for bees and butterflies. Of course, a healthy green roof soil provides a habitat for beneficial microorganisms that support such biodiversity.
Several of the authors above have done a great deal of research into increasing the biodiversity benefits of green roofs. A biodiverse roof has a considerable variation of living things such as plants, insects, and microorganisms.
Why is biodiversity important? It is essential for resilience within ecosystems. In a drought, or extreme heat, or flooding, or unusual cold, or unusually warm winter, some normally thriving species might suffer, but others will likely perform better. As living systems, this is important within green roofs, particularly as green roofs are thin ecosystems subject to harsh rooftop conditions.
However, even if you only add a blanket of 15-20 sedum species, a lot of insects, birds, and other organisms will find refuge on this roof. This is a substantial biodiversity increase compared with a bare black roof with no vegetation. Also, it is possible to use sedum mats as soil cover and then interplant other species to further increase biodiversity
This beautiful green roof at the Brooklyn Botanical Gardens in Brooklyn, New York, was designed by one of the authors noted above, Charlie Miller of Roofmeadow. This is an excellent example of a biodiverse roof, incorporating both native and adapted species that withstand drought, provide much-needed nectar for pollinators, serves stormwater management purposes, and is lovely on the eyes!
So why on earth would anyone grow plants on their roof? For aesthetics? Yes, but that is just the starting point.
Green roofs are an essential part of sustainable buildings because they provide many environmental benefits. Principal among these is stormwater retention, but the Purple-Roof and Blue-Green Roof concepts are also adding stormwater detention.
Since green roofs grow plants, and plants require water, of course, green roofs hold on to (or retain) water for the plants to use. This is needed because cities are otherwise covered with hard surfaces that create instant runoff, that overloads sewers and can cause flooding.
The Purple-Roof concept involves taking this one step further and using green roofs to not only capture water but to slow down any water that does become runoff. This prevents flooding and hence environmental and economic damage.
Green roofs are critical for cities to be sustainable environments.
Stormwater detention also provides a clear return-of-investment (ROI) for the vegetated roof in urban settings. You can read more about this in the article Using a Detention Roof to Create a Green Roof ROI.
Other benefits include urban habitat, reducing building cooling costs, mitigation of the urban heat island effect, extending the life of a waterproofing membrane, improving air quality, and - yes - aesthetics.
The video below does a great job of showing examples of living roofs and discussing how living roofs absorb and retain rainwater. This video features Dr. Virginia Stovin, living roof researcher at the University of Sheffield, UK.
Dr. Virginia Stovin discussing the benefits of green roofs and green roof research at the University of Sheffield, UK
Living roofs... you've come a long way, baby!
The examples above show many aesthetic uses of vegetated/living roofs. All vegetated roofs have aesthetic potential. But the main reason green roofs are uses is to solve a fundamental challenge: stormwater management.