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Green Roof Types Explained

February 26, 2013


I often see tender packages and Q37 documents that describe different roof types. This is a bit of a grey area and this scientific based article taken from Dr Chloe Molineux’s thesis will explain the different types of roof.

Extensive green roofs

Extensive green roofs are the most common and can be categorized into two further sub-types. Firstly, Sedum blankets, are those placed directly onto a moisture blanket or onto a thin substrate layer. These mats, planted with the stonecrop plant genus Sedum (up to 15 species) and like rolls of carpet, are simply laid out over the buildings rooftop. Secondly, substrate-based green roofs can be plug-planted with Sedum species (which are well known for being drought resistant) or can be seeded with wild flower mixes for more natural, species rich meadows. These roofs are constructed with substrates that contain little soil components typically between 5 – 20 % organic matter, and contain lightweight aggregates that make up the bulk of the substrate to between 5 – 10 cm in depth.

Non-Sedum extensive roofs

Substrate-based extensive roofs that are seeded with a wild flower mix or left to colonise naturally have recently been termed brown roofs or biodiverse roofs, as they are comparable to natural Brownfield land. These rapidly decreasing wastelands are mainly found abandoned in urban areas but are increasingly being lost to planners and developers as the pressure for new housing for expanding cities mounts. These wasteland sites generally have a substrate that has the consistency of crushed brick aggregate with little soil nutrition or retention of water, however re-colonisation can lead from bare ground to grassland, scrub and woodland, allowing a wide range of wildlife to become established. Brownfield areas are home to such animals as the brown hare, skylark and lapwing (these birds nest on open areas producing eggs that camouflage with the brick substrates), rare invertebrates (particularly spiders) and a variety of butterflies, reptiles and amphibians. These habitats also provide miti­gation to the now very endangered Black Redstart. Red listed species could therefore be conserved by the green roof alternative that mimics these ever-decreasing natural habitats. The most species-rich site so far identified in the UK is a 27.5 hectares (55 acres) Brownfield site in Essex that has more recorded biodiversity per square foot than anywhere else in the country.

Semi-intensive green roofs

Semi-intensive green roofs are similar to the purely intensive variety but their substrate depths are significantly reduced, usually to between 10 – 20 cm. These types of green roofs are more obvious as being situated on rooftops, although they are also better described as roof gardens due to ornamental vegetation and their ability to sustain large shrubs. Lighter substrates are usually required for semi-intensives as the roof structure has a finite loading capacity; therefore the soil/organic components are often reduced to 20 – 30 % (as this is where the main body of weight is coming from).

Intensive green roofs

Intensive green roofs are those that are found in easily accessible private or public places; often these are parks or attractive gardens. Although many intensive types are not obvious as being on a roof, many have the underground train system running beneath them or are over underground car parks. These green roofs have substrates that are extremely deep (above 20cm), enough in many cases for trees and large shrubs to grow well in. These types of roofs are aesthetically very pleasing but they require heavy maintenance and a complex irrigation system to keep healthy.

Hope this has been an informative read !