Green Roof

A green roof (also called a vegetated, living or eco-roof) is a roof covered wholly or partly with plants growing in a layer of substrate placed over waterproofing. A typical build-up includes a waterproof membrane, a root barrier, a drainage layer, a filter fabric, growing medium and vegetation. Modern green roof technology was developed in Germany in the 1960s and spread as cities sought ways to manage stormwater.

Types

• Extensive: shallow substrate, roughly 2-13 cm (1-5 in) deep, planted with hardy species such as sedums and grasses. Saturated weight is roughly 50-120 kg/m² (10-25 lb/ft²). They need little maintenance and are the most common type on existing buildings.
• Intensive: deeper substrate of about 13 cm (5 in) or more, supporting shrubs and small trees, with weights of roughly 390-730 kg/m² (80-150 lb/ft²). They often function as accessible roof gardens and need regular maintenance and stronger structures.
• Semi-intensive: an intermediate category used in some guidelines.

Stormwater

Green roofs retain part of the rain that falls on them and slow the rest. The US EPA reports that extensive and intensive green roofs can reduce runoff by about 60 percent and up to 100 percent respectively, with best performance in small storms. A US General Services Administration review found reductions in peak runoff rates of up to 65 percent and delays in runoff of up to three hours; extensive roofs typically hold the first 13-19 mm (0.5-0.75 in) of rain.

Temperature and energy

According to the EPA, green roof surfaces can be up to 31 °C (56 °F) cooler than conventional roofs, and they can reduce heat flow through the roof substantially. They also help reduce urban heat islands through evapotranspiration.

Membrane life

Because plants and substrate shield the waterproofing from UV and temperature extremes, the GSA review estimated a life expectancy of about 40 years for a green roof membrane versus about 17 years for a conventional roof.

Biodiversity

Green roofs can host insects, birds, bees and butterflies, even on tall buildings. Habitat value depends mainly on plant type, substrate depth and variation in structure; some cities, notably Basel, require native soils and plant mixes to support local species.

Costs and considerations

Green roofs cost more to install than conventional roofs and require structural capacity, careful waterproofing and leak detection, and ongoing maintenance such as weeding and, in some climates, irrigation. Savings from stormwater fees, energy and longer membrane life can offset part of the cost. Policies in cities such as Toronto, Copenhagen, Basel, Paris and Singapore have encouraged or required them.