A planted or green roof can bring about many technical as well as aesthetic benefits to a project. The most apparent feature of a green roof is the appearance – they can vary from ground-hugging Sedums to taller and more wild flowery or grass roofs (or even a vegetable garden!). These different plants bring additional biodiversity to a site, and can offer a much-needed island of life and nature within towns and cities.
There are two main distinctions of types of green roof; Extensive and Intensive. Intensive roofs are thick, heavy and planted in a traditional manner with a thick layer of soil, supporting all types of vegetation (including trees!). Extensive types are more common and are composed of much thinner layers of substrates or soil to support greenery that survives off less nutrient-rich soils. These extensive roofs tend to be a Sedum-type plant commonly as they are very good at surviving in this manner, although local species can be incorporated.
Sedum plants in a slate chippings growing medium
To enable a thin (4” or so) layer of ‘growing medium’ (soil) often an egg-crate layer is added below to hold small pockets of water following rain for shallow roots and where soil is unable to hold so much water. This layer of water retention, and the porous nature of planting and the growing medium, slow down the run-off rates of storm water, which helps keep the drain and sewer network from flooding. By building up these layers over the waterproof roof layer, it also remains protected from the harmful UV rays, which should lengthen it’s lifespan. By having a thick built-up layer on the roof, noise is significantly reduced internally as the roof provides sound insulation. The thermal insulative benefits are more minimal compared to high-end insulation products, but the planting contributes. The main benefit in terms of heat gain and loss, is the ability of the plants to reduce the temperature spikes by providing tiny patches of shade, greater surface area and evaporation to take away the heat.
Additionally, a huge benefit to extensions and densely populated areas is that a green roof is an attractive low-pitch solution. Slates and tiles need quite significant minimum pitches to keep the water out, but a green roof can be laid quite shallow and is much more attractive than felt, and often cheaper than a zinc or lead roof (It is also harder to steal!). Green roofs can be planted, seeded, or if a quick greening is required (or you have a lot of birds) rolled out in pre-grown mats like turf. DIY versions are possible and people have been successful using old carpets over garage roofs etc to provide for water retention layers. Afterall, how many mossy factory roofs have you seen? A little encouragement of nutrients and the right plants is all it needs. Just be sure to put a root-proof layer down first. Green roofs can be very heavy, (the shallow system shown here on this shed is around 100Kg/m2 when saturated) so a strong roof is required.
Intensive deep soil green roof on traditional Swedish building