Green roofs

Malmö’s work on adapting buildings for climate change is well advanced. Some of the city’s buildings have been given green roofs to help manage the impact of climate change.

Changes in the climate mean increased risks for towns and cities. Risks facing Swedish towns include higher temperatures, regular heatwaves, higher amounts of rain and snowfall and reduced biodiversity.

As climate change progresses, heatwaves are now occurring with increased frequency and are also longer lasting. This has a major impact on people’s health, which is a particular issue for town and urban planning. The numerous hard surfaces in an urban environment, such as roofs, roads, car parks and paving, absorb the sun’s rays instead of reflecting them back into the atmosphere.

Increased rain and snowfall makes it more difficult to manage surface water in towns. When water falls in places where it cannot filter down into the ground, a drainage system channels the water into a water treatment plant. Extra strain on the system, for example due to extreme levels of rainfall, can lead to flooding and to large volumes of water being released without being treated.

Climate change can also reduce biodiversity. Many species are not able to adapt to their changed circumstances, and this can mean that species disappear from affected areas. Densification and reduced numbers of green spaces in towns also affect the ability of various species to survive.

Green roofs to counter climate change

Green roofs is an umbrella term most often used to describe roofs covered in vegetation. The term covers everything from a thin layer of sedum on a roof to deeper beds of shrubs and trees. Using plants on a roof is nothing new; it has been a recurring trend throughout human history.

Green roofs can roughly be divided into three categories according to maintenance needs, depending on the type of maintenance, type of plant and aims.

Extensive roofs

This type of roof can create a natural landscape effect. Vegetation may include different types of mosses, sedums, succulents, bulbs and hardy wildflowers.

The depth of the substrate is between 30 and 150 mm, and the load can be anything from 50 to 250 kg/m2 depending on the type of plant preferred, arrangement and system chosen.

Semi-intensive roofs

This roof requires greater maintenance than extensive plantings, and allows for a slightly wider choice of plants. It normally needs irrigation and some application of fertiliser. The depth of the substrate varies from 120 to 350 mm and the load may vary between 150 and 500 kg/m2. This sort of roof also has an unplanted area which limits the roof’s load bearing capacity.

Intensive roofs

Roof areas intended to be used for recreation or games generally require a high level of maintenance and are therefore intensive by definition irrespective of the choice of plants. The depth of the beds varies mainly depending on the type of vegetation desired, and may be between 300 and 1,000 mm. The load for this type of roof also varies, from 200 to 2,000 kg/m2. These roofs require regular irrigation, pruning, clipping and fertilising.

Green roofs can have environmental, financial and aesthetic benefits for an urban environment. Environmental benefits might include creating biodiversity and reducing the pressure on the town’s surface water system, as the plants absorb water and then transpire moisture back into the atmosphere. In addition, the roof has a role as natural purifier for both water and air, and also produces a cooling effect. Roof vegetation can protect the surface of the roof from the effects of ageing and from material damage, and can insulate the building from summer heat, thus reducing repair and energy costs. In addition to these advantages, green roofs are aesthetically pleasing and provide new opportunities to achieve recreational value in built-up areas.

Costs

The cost of establishing a green roof varies depending on conditions and what is required. The cost of a thin green roof with extensive planting is estimated at around SEK 300-600/m2 including the planting itself. Costs for other green roofs with deeper beds and semi-intensive or intensive planting span a wide price range depending on various factors such as choice of plant, size of site, irrigation system, accessibility, complexity of the roof, etc. On top of the cost of establishing the site there will be costs for maintenance, which will vary depending on the type of roof.