About Green Roofs

Extensive Green Roof

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. The modern, lightweight systems were inspired by Viking sod houses and by the “Holzzementdach”, or tar-paper-gravel roof that was developed to inhibit the spread of fire. By 1880, tar-paper-gravel roofs were prevalent on public and private homes, and on public, industrial and commercial buildings in many large European cities.

Similar to their predecessors, 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 15 cm deep. The drainage layer is designed to move excess water towards roof drains in order to minimise water logging and hydrostatic load.

Extensive roofs weigh between 60 and 240 kg/m2, which 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.

Blue Roof

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. Their loading capacity ranges from ___ kg/m2, and they generally cost between $ __ and $___. 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:

  • Sites without significant space at ground level for infiltration practices
  • Zero-lot line projects with outdoor amenity requirements

Intensive Green Roof

Intensive green roofs can have deep growing media (8 – 48 inches) 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 Roof

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.