Planning for the future: rooftop SuDS
Bauder discusses how flat roof design can offset future changes in weather patterns using green roofs and Sustainable urban Drainage Systems (SuDS) to reduce localised flooding and standing water.
Surface water flooding is becoming increasingly prevalent as existing drainage systems struggle with the higher volumes of rainfall. The UK has faced 116% of average during December 2019 according to the met office, and already three storm events in 2020 including Ciara and Dennis. The Environment Agency’s (EA) preferred policy has moved from flood barriers and large-scale water management, to controlling water as close to where it lands as possible through SuDS.
How a green roof can control rainwater run off
Green roofs have become popular for their documented multitude of benefits with planning authorities favouring their inclusion especially if the design contributes towards achieving sustainability objectives, including Biodiversity Net Gain.
An additional advantage is that green roofs act in exactly the way the EA SuDS policy requires. The roof attenuates large amounts of water delaying its progress into the drainage network - typically rainwater landing on a green roof will take 20 mins to work its way through the different layers.
Furthermore, around half of the rainwater landing on a green roof will be held at roof level and never enter the outlets or gutters, the greater the depth of the green roof’s build up the more water is attenuated.
A typical Bauder green roof system comprises FLL compliant substrate that offers water retention of 35%-45% of the volume of the substrate itself but without risk of compaction to allow excess water to flow freely to outlets, negating any risk to the structure of the building from additional weight.
The drainage layer, available in 20mm, 40mm or 60mm dependant on system requirements, provides continuous drainage within hard and soft landscaping. The boards store water in the cup-like compartments for the vegetation to use and have small holes on the upper surfaces to allow excess water to flow gradually from the rooftop. When used underneath hard landscaping, the drainage layer is filled with gravel to give a higher compressive strength. Water storage capacity varies depending on the depth of the cups and whether the board is filled or empty.
Bauder green roof DSE boards and water storage volumes per m²
| |
Empty |
Mineral filled |
| DSE20 |
7.4 litres |
n/a |
| DSE40 |
13.5 litres |
3-4 litres |
| DSE60 |
n/a |
10-12 litres |
The overall water retention and runoff rates of a green roof system will be dictated by the sum of the system layers, including the landscaping. As a general rule, the deeper the substrate, the higher the average water retention will be and the lower the annual run off rate.
Thus, a green roof can significantly assist in managing rainfall levels, but what about stormwater, especially if the water falling is heavy and over a short period of time? This is where a blue roof plays its part.
How does a blue roof alleviate storm events?
Blue roofs are sustainable drainage solutions designed to manage stormwater on a flat roof. The runoff rate from the roof is carefully controlled to meet SuDS requirements and reduce flood risk. The BauderBLUE Roof System attenuates water from a flat roof over a 24-hour period via a restrictive flow outlet. The egress rates are determined by the local planning authority and are often set at 5-10 litres per second per hectare which is the runoff rate of a greenfield site. Calculations are carried out to establish the exact design of the flow restrictor to meet the necessary rate of water runoff. These calculations are bespoke and take into consideration the rainfall characteristics of the geographical area of the project.
The design of the BauderBLUE roof must consider the required target out flow and the maximum live load that the building can withstand. This will decide the maximum height that the attenuated water can reach. This is the ‘H-max’ value. When the water level reaches the H-max, it will drain via the central overflow pipe positioned within the outlet. These overflow pipes will be set at the same height throughout the roof. Although an unlikely scenario, this provision for a ‘once-in-a-hundred-years’ storm event plus 40%, will ensure that the structural integrity of the building is always preserved.
Product in Practice
Last year, Bauder installed a BauderBLUE roof on the new building for the Department of Civil Engineering at Cambridge University. The Bauder technical team designed a roof system combining a blue roof, a green roof and a solar PV array, believed to be one of the first of its kind in the UK, to meet the sustainability requirements of the client.