What is a green roof?
A green roof is a purpose built roof that contains vegetation on it. Green roofs are fast becoming a standard part of a new build property and can often help to gain planning permission.
There are two types of green roofs and these often become confused:
- Intensive green roofs (roof top gardens), which are often, bespoke planting designs.
- Extensive green roofs, which are to serve a purpose or function at roof level.
Extensive Green Roofs
Extensive green roofs are the most common type of green roof and the term is an umbrella for;
Brown and Bio Diverse roofs
The term extensive green roof looks at the lightweight, low maintenance, high ecological valued roofs.
Extensive green roofs will tend to have a relatively shallow growing medium layer from between 60mm to 150mm in depth. The nutrient level of the growing medium will tend to be relatively low, to keep maintenance at a minimum and discourage evasive plant species out competing the intended vegetation.
Sedum Roofs tend to be the most common green roof type. Sedum’s are a very suitable plant for green roofs, this is because they are a very hardy alpine plant, which are used to living in very tough conditions. Roof level is a very tough environment for a plant because roofs are generally designed to shed water from the roof as quickly as possible. There are a few types of Sedum offerings on the market and they include;
- Sedum blankets, which are, turf Rolls of established plants; these tend to have 90-95% coverage on installation.
- Sedum plug plants, which are small plants usually with a root, ball size of 50-60mm; these are normally planted at 20 per Meter Square.
- Sedum seeds, seeding the roof, this will take 6 months to a year to develop depending on the time of year.
Sedum roofs tend to be the lightest weight and generally a very low maintenance.
Maximising biodiversity of green roofs
Brown roofs/Biodiversity roofs should include a substrate that has larger particle sizes, and a small amount of organic material to produce a low-nutrient environment. This is similar to that found on post-industrial wasteland sites. The substrate is placed on the roof at a minimum overall depth of 80mm-150mm. Undulations/mounding should be created randomly across the roof to ensure a heterogeneous growing environment.
We often use several substrate types to create a mosaic of habitats that will support different plant species; this helps reduce the risk of only a few species becoming dominant over the entire roof.
We feel that annual maintenance is crucial to monitor and maintain the level of species within the system. Although 38 species are being planted and seeded many will be outcompeted after only one season by more aggressive species. By maintaining the roof we can reintroduce more plug plants and also re-seed the annual species to upkeep the 38 species required for the high Breeam Rating.