Roof Types

Green, Biosolar & Blue Roofs

More and more impermeable surfaces, such as roads, buildings and paved areas, are being constructed at the expense of permeable surfaces such as fields, meadows and other vegetated landscapes. When rain falls on these hard surfaces, it cannot soak naturally into the ground. Instead, surface water runs straight off into drainage systems and, ultimately, into rivers.

During periods of heavy or prolonged rainfall, existing drainage systems are required to cope with large volumes of surface water. In many cases, these systems were not originally designed to manage such increased levels of run-off. This can lead to drainage systems backing up and causing localised flooding, especially during intense summer storm events.

Sustainable Urban Drainage Systems in new developments

The major flooding experienced in recent years can be partly attributed to the increase in built-up areas and the reduction of vegetated, permeable land. As a result, developers are strongly encouraged by planners to use Sustainable Urban Drainage Systems, commonly known as SuDS, on new developments.

The major flooding seen over recent years can be partly attributed to the increase in built up areas and reduction of vegetated areas. Therefore, developers are ‘strongly encouraged’ by planners to employ Sustainable Urban Drainage Systems (SuDS) on new developments.

SuDS are defined as:

“A sequence of management practices and control structures designed to drain surface water in a ‘more sustainable’ fashion than some conventional techniques” (CIRIA 2000).

The Environment Agency can request that local authorities place conditions on planning permission requiring developers to restrict run-off from a site to greenfield levels for a 100-year storm event.

PPG 25, Development and Flood Risk, recommends that Sustainable Urban Drainage Systems should be considered for new developments and encourages local authorities to include SuDS within their development plans.

Green roofs as source control for SuDS:

Green roofs can play an important role in Sustainable Urban Drainage Systems by helping to manage rainfall at source.
Rainwater falling onto a green roof is held within the pore spaces of the growing medium.
Water is taken up and used by the plants.
Some of the water held within the plants is released back into the atmosphere through evapotranspiration.
Rain droplets are often trapped within the vegetation, where they can then evaporate back into the atmosphere.
Water can also be retained within the drainage layer of the green roof build-up.
The rate of drain water run-off is significantly reduced because rainwater takes time to percolate through the green roof build-up and exit via the drainage outlets. This means that any excess water is released over a longer period, helping terrestrial drainage systems cope more effectively during heavy rainfall events.