Green roofs

Typical design

Green roofs are an alternative to traditional roofs. One goal is to intercept rainfall directly where it lands and both reduce the runoff volume generated and delay the runoff.

A typical green roof is constructed with multiple layers. The main layer is the substrate, which often consists of coarse sand or gravel. Different types of substrate exist, and they may also include materials such as crushed rocks, bricks, biochar, organic matter, and/or roof tiles.

The thickness of this substrate layer can vary from a few centimeters to a few decimeters. A roof with a thin layer of substrate is called an extensive green roof, while a roof with a thick layer of substrate is called an intensive green roof. A thicker layer of substrate provides a larger storage volume and will, thus, be able to hold more water and delay it for longer, but it also adds more weight to the roof construction.

The substrate also acts as a growing media for the vegetation. Many different species of plant can be used. Often, grass, herbs and/or succulent plants (for example sedum) are used. The plants provide several functions to the roof, which include:

• Evapotranspiration of water from previous rains
• Maintenance of the infiltration capacity of the substrate over time
• Contribution to biodiversity
• Aesthetic values

Underneath the substrate there is often (but not always) a special drainage layer to help remove excess water that was not retained in the substrate. This layer can consist of a coarse gravel or of a special plastic layer that provides small drainage channels for instance, as well as some additional storage capacity.

Underneath the drainage layer there are several layers to protect the roof and the building. These provide protection against root intrusion and water proofing.

Aims in stormwater management

Green roofs deal only with the rain that falls directly on the roof itself. The main goal of green roofs is to reduce the runoff volumes and runoff rates. The water they receive will not be very polluted, so quality treatment is not the main goal of green roofs. But commonly green roofs can trap for example atmospheric deposition which will, thus, not be discharged by the stormwater. On the other hand, nutrient export has commonly been observed from organic matter in the substrate or fertilizer. If fertilizers are used to support the plants on the roof these can leach out from the substrate during rain and be a source of pollution, so it is best to use plants that will not require fertilizer.

However, green roofs can help reduce the flow rates to other water quality measures downstream. How efficient the retention is depends on the rain characteristics (mainly rain intensity) and the roof design (mainly roof slope and substrate depth, i.e., storage volume in substrate).

Rains with a low intensity / low depth can often be stored completely in the substrate. Thus, for rains of few mm for instance, no runoff is generated. Both the volume and peak flow reduction is close to 100%.

Rains with higher intensities / depths will generate runoff. But since the substrate provides a storage volume, both volumes and peak flows will be minimised, but not eliminated.

For even more intense rains, the substrate will be saturated. Then runoff will occur. Depending on the rain depth, the volume reduction provided by the green roof will be low. However, often still considerable peak flow reductions are provided since the flow velocity on a green roof is lower compared to a similar traditional roof (for example a tiled or tin roof).

Thus the aims of green roofs are different for small rains and more intense rains:

• Small rains: high volume and peak flow reduction
• Intense rains: volume reduction decreasing while peak flow reduction may still be significant.

Implementation in catchments

Green roofs can be implemented on almost any new roof which is not too steep. For existing roofs, it needs to be checked if the roof’s construction can support the additional weight. Green roofs are most commonly used on flat or lightly sloped roofs. While there are some green roof products for steeper slopes, there is a risk that the steeper slopes will lead to erosion of the substrate and that the substrate dries out quickly which is negative for plant growth.

Additional benefits

• During dry periods, evapotranspiration from the plants will remove water from the substrate and release it into the atmosphere. This provides some evaporative cooling to the surrounding area which can help reduce the urban heat island effect.
• Green roofs provide more aesthetic values than roofs made from for example bitumen.
• The vegetation supports biodiversity.
• Vegetation can also take up particulate matter from the atmosphere, improving air quality.
• Green roofs can provide additional insulation to the building, reducing energy required for heating and cooling.