Green roofs are a term commonly used to describe roofs covered with vegetation. The term includes a wide range of solution, from a thin layer of sedum plants to parks on roofs with bushes and trees.
Using plants on ceilings can be traced far back in human history. The most well-known example is the “Babylon’s Hanging Gardens”, one of the seven wonders of ancient times. In the north, green roofs have a long tradition on many Scandinavian buildings but disappeared from the towns during the 19th century. It was not until the end of the 20th century that green roofs became popular. Now they are widely used as a method for a sustainable and more climate adapted urban development.
Categories of Green Roofs
Establishment of vegetation on roofs can be divided into three management categories. They all refer to how they will be maintained, which plants will be used and whether it will be accessible to people staying on the planted surfaces or not. The different care categories for green roofs are extensive, semi-intensive and intensive.
Extensive Green Roofs
Extensive roof can replicate a natural landscape. The vegetation can vary between different types of mosses, sedum plants, succulents, onion plants and hardened wild flowers. Depth (substrate depth) varies depending on the desired vegetation type and wanted diversity, and usually varies between 30-150 mm. These substrate depths imply a load from the green roof (when fully saturated), which can be between about 50-250 kg / m² depending on depth, the weight on materials in the substrate and other system components. Annual supervision is recommended for all types of extensive roofs.
Semi-intensive Green Roofs
The category semi-intensive roofs require a care that is more extensive than for extensive plantations, but which can still be significantly lower than for a common garden or a park-like environment with lawns and tress. Plants on semi-intensive roofs include a slightly wider selection of plants, such as ornamental grass, woody herbs and some bushes. Irrigation and supply of fertilizers are usually required, but may only be used on the driest period of the summer. The substrate depth for these roofs can range from about 120-350 mm and the load from such roofs (when fully saturated) can vary between 150-500 kg / m². Semi-intensive plantings is a good solution when beautiful plantations are desired next to roof decking for recreational purposes.
Roof surfaces that are intended to be used for recreation and play usually require a high level of maintenance and, by definition, become intensive regardless of plant choice. Different intensive roofs can thus vary a lot in substrate depths, which primarily depends on the desired vegetation type. Lawns or perennial plants can have relatively thin plant beds about 300 mm, or even less. Plant beds over 600 mm allow planting of smaller trees or larger bushes. When establishing park environments on wood floors with larger trees, the substrate depth is usually not less than 1000 mm.
With intense green roofs we get gardens and parks in the sky and the city skyline. The level of maintenance for intensive roofs is comparable to a common garden where regular irrigation, pruning, cutting, etc.
Because the depth of the plant bed on intensive roofs varies considerably, the load does too, from a few hundred kilos to two ton per m2. In addition, the weight of the desired vegetation, such as large trees, and the capacity to carry all people who can stay on the roof at the same time, must be included when the load is calculated. This means that the load on the ceiling is very high compared to extensive and semi-intensive green roofs.
Green Roof Benefits
There are many benefits from green roofs, especially in an dense urban setting. The ecological gains of green roofs are that they create conditions for a richer flora and fauna in the city and thus have a positive impact on biodiversity. It is possible to replicate natural habitats on the roof with plant communities that have not previously existed in the city. Common animals on green roofs are various invertebrates and important bees, such as bumble bees, bees, spiders, butterflies, beetles and hoverflies, as well as some birds.
An important benefit is the reduced run of from green roofs. By planting vegetation on the roof, a lot of the water will be absorbed by the vegetation and substrate. The total reduction is between 40 – 90 % depending on the substrate depth, the slope and size of the roof and what season it is. In addition, the water run of from the roof is always a bit delayed which reduces the pressure on the urban drainage systems.
Green Roofs helps to reduce Urban Heat Islands which occur because dense cities with a lot of concrete and asphalt absorbs a lot of the heat from the sun. Green roofs have better albedo than black or grey roofs and furthermore the evapotranspiration from the plant bed and vegetation keeps the roof cooler.
Green Roofs also expand the life expectancy of the water proofing membrane that is located under the green roof system. This is due to less fluctuations in temperature, decreased surface temperature and UV radiation. The life expectancy is difficult to calculate in an exact figure but can be estimated to between 25 – 50 % depending on water proofing membrane solution and green roof solution.