Green roofs

As cities become more densified, a number of problems come in: there is a loss of biodiversity and green space; stormwater and the risk of flooding become heightened; and the widespread use of heavy construction materials such as concrete and dark coloured materials such as asphalt for roads contribute to the Urban Heat Island. One solution is to use the roofs of buildings to reintroduce vegetation.

A green roof at the University of the Witwatersrand

Green roofs can either be extensive, as shown in the photo, with a fairly shallow layer of soil over the whole extent of the roof-space, or intensive with much deeper soil to support larger plants such as shrubs or even trees. Intensive roofs are quite heavy, so usually they can only be used where they have been designed for in the original construction of the building. By contrast, extensive roofs are quite lightweight and can be installed on a variety of existing flat roofs without the need for structural modification.

The green roof in the photos was installed mainly to improve the temperature of the spaces directly below: the roof acts as an insulator especially in summer when the transpiration of the plants during the daytime assists with cooling, along with the evaporation from the soil. 

We started the project with a structural analysis to be sure that the weight of the green roof would not cause cracking or other structural problems. We needed to remove the large concrete tiles that covered the roof which weighed approximately the same as the planned green roof, so we were quite sure that we would be OK. The other critically important task, when we had removed the concrete tiles, was to check the waterproofing and do any repairs. We gave the roof a new coat of waterproofing paint as an additional precaution.

Because green roofs offer protection from the direct rays of the sun, they can prolong the life of the waterproofing on a flat roof, but this needs to be in perfect condition before installing the roof, especially around the storm-water outlets. The next step is to install a root barrier, a layer of mesh that gives protection to the waterproofing. This is followed by either loose planter boxes or a cuspated sheeting. The green roof planter boxes and the sheeting have a similar profile that allows water to be held in the cusp for use by the plant for many days after rain or manual watering.

A detail at a rainwater outlet, showing the water trapped in the cusps. This would overflow at the arrows in a big storm preventing waterlogging.

The top layer is the growing medium: well composted soil that is selected to suit the type of vegetation that you have chosen. The diagram shows an important feature of the roof: around the rainwater outlets and around the edges of the roof, there should be an area of gravel or cobbles. This is to encourage drainage and to prevent the plants growing too close to the edge which could cause damage to the waterproofing and potentially create a fire hazard.

For the roof in the photos, we chose entirely indigenous succulents to reduce the maintenance, eliminate the need for manual watering after the plants were established and to encourage a variety of birds and insects. If the roof is intended for outdoor activities, you may want to introduce a grassed area, but one should factor in for much more maintenance. We used small areas of paving to create pathways and small seating areas, which we laid loose on top of the soil. Over time, this paving has tended to shift around, but it is easy to restore or reconfigure, although maybe not ideal if the roof is to take a lot of activity.

The paving after ten years has moved around.

During the ten years that this green roof was in place, it certainly encouraged a wide population of birds and insects. It was also found to be beneficial in regulating the temperature of the computer lab and lecture theatre directly below, especially in summer. In the end, though, this roof was removed, because the maintenance department was concerned about the performance of the roof under extreme climate events. This is an important lesson for any type of green infrastructure - it needs to be resilient under a wide range of conditions, and be adaptable to changing circumstances.