Putting plants on roofs is historic to many cultures, on all continents, since the dawn of time. The modern green roof systems we see today are part of a long lineage of human innovation. In the northern hemisphere, we have several sources of inspiration. In fact, the first European presence on Turtle Island (North America) was Viking. The sod roofs of medieval Scandinavia were re-created at this place, L’Anse aux Meadows, in Newfoundland. Leif Eriksson pre-dated Columbus by 500 years.
Tar-paper-gravel roofs, which were prevalent on public and private homes, and on public, industrial and commercial buildings in many large European cities in the 1880s, are another source of inspiration for modern green roof systems. Those systems were developed to inhibit the spread of fire.
Modern extensive green roofs emerged in the 1970s Germany. In the spirit of the times, vegetated roofs were seen as opportunities to reduce dependency on oil for heating while reconnecting urban dwellers with nature. Today’s “vegetated roof assemblies” are living, engineered systems installed on roof or plaza decks, consisting of waterproofing membrane, drainage, growing medium, and plants. The different types of green roofs are described below.
Modern green roof systems are basically layers on top of a building’s waterproof membrane. Extensive green roofs comprise at least two layers – vegetation and growing substrate – but usually multiple layers are used (e.g., protection mat, drainage layer). The growing medium is predominantly mineral (80%) with minimal organic matter (20%), and under 150 mm | 6″ deep. The drainage layer is designed to move excess water towards roof drains in order to minimise water logging and hydrostatic load.
When saturated, extensive roofs weigh between 75-200 kg/ m2 | 15 – 40 lb/ ft2. This means they can often be retrofitted in place of gravel ballast (used to prevent wind uplift) without the need for structural adjustments. Their minimal loading capacity means these systems are designed for functional, rather than recreational, purposes.
Intensive green roofs can have deep growing media (150 mm | 6 ” or more) that support a wide array of plant species and sizes, including full size trees. They are often accessible and serve as amenity spaces that can be enjoyed by residents or visitors, like roof gardens. Their name relates to the degree of maintenance required, which is similar to what a garden at ground-level would require.
Semi-Intensive green roofs are a hybrid of sorts, a combination of extensive and intensive green roof assemblies on the same roof, often arranged to take advantage of varying structural loading capacities. For example, a particular roof might only have the structural loading capacity for an extensive green roof, however locations directly above the structural support columns could allow a deeper growing medium and larger plants.
Blue roof systems are an adaptation on green roof and flow control roof drain systems for flood and drought resilience and adaptation. Blue roof systems regulate rooftop runoff by storing and controlling the release of rainwater, thereby reducing peak flow. They allow the stored water to evaporate and/or to be used for non-potable requirements (i.e. irrigation, toilet flushing, car washing).
When vegetation and growing media are included on top of the water storage layers, these systems are called blue-green roofs. The fundamental components of a blue roof are a flat roof, and flow control device(s). Blue roof systems are a popular stormwater management option for flat roof buildings, particularly within the industrial, commercial and institutional (ICI) sector (which make up 23 – 30% of most urbanized areas). Blue roofs are ideal for:
