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 shallow soil manner, although local species can be incorporated.
Green roofs are generally on flat roofs or less than 10˚ pitch on modern structures (shallower than the beautiful historic turf roofs of Iceland pictured above). Even for an extensive lightweight green roof a structural engineer will need to consider the additional weight. A standard flat roof is calculated to have occasional maintenance access, snow and wind loads. Green roof systems incorporate layers to hold onto moisture and water for the plants so can add weight quite rapidly. Even the most light systems are around 20-50kg/m2 and heavier intensive systems anything upwards of 100-150kg/m2 or even more with large plants.
There are 3 key benefits to including a green roof in your project.
- 1. It adds biodiversity. (Biodiversity Net Gain is becoming a planning requirement for larger projects). The plants are a source of food for pollinators, provide habitat and also therefore add to the overall food chain balance.
- 2. It keeps the room below cooler in the summer sun. Each plant creates a micro shadow to help keep the space below cool, each plant transpires & evaporates moisture away which in turn pulls heat away. It adds thermal mass which holds off the suns rays for longer.
- 3. It provides acoustic benefits for rain drumming noise and external sources.
Plus, it looks good.