There are three types of green roofs and these often become confused. Intensive green roofs (roof top gardens), intensive which are often bespoke planting designs and extensive green roofs which are to serve a purpose or function at roof level.
We are going to look at extensive green roofs in this news post.
Extensive green roofs are the most common type of green roof and the term is an umbrella for;
- Sedum roofs
- Wildflower Roofs
- 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 and 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 types. Sedum’s are a very suitable plant for green roofs, this is because the 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 the 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 devlop depending on the time of year.
Sedum roofs tend to be the lightest weight and generally a very low maintenance.
Bio-diverse roofs or (brown roofs)are now the up and and coming new roof design because of their important ecological value.
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 similar to that found on post-industrial wasteland sites. This has then be placed on the roof at a minimum overall depth of 80mm-150mm, however deeper 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 become outcompete after only one season by more aggressive plants. 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.
Additional features can also be added to the roof to help create this mosaic habitat effect for wildlife, including: log piles for rare invertebrates (such as beetles) and to provide perches for birds, bare areas (using cobbles/stones or cleaned bricks) for certain species of spiders and sand mounds for bee species that like to burrow. We have also introduced bird boxes etc. Although a seed mix can be applied to the roof, often a biodiversity roof is left to colonise naturally with local flora so that the roof supports vegetation found in the local area.