Green, blue and grey roofing solutions

For a long time, the usual way of leading rainwater off a roof has been with gutters and downspouts to the ground. In 2016, in cooperation with companies in Klima 2050, researchers started focusing instead at how to delay runoff while at the same time converting the roof into an attractive terrace.

Traditional stormwater management faces two major challenges today. The first is more precipitation in general and more frequent short periods of torrential rain which result in large quantities of water overloading stormwater and sewage networks. The second is densification of urban areas and fewer natural areas for absorbing water.

– The traditional way of handling stormwater is simply by redirecting the water problems to a new location. We must rather find solutions which make it possible for people to handle rainfall on their own property, says CEO of Storm Aqua, Per Møller-Pedersen.

Three experimental roof plots
Storm Aqua (part of Skjæveland Group) and Leca Norway (part of Saint-Gobain Construction materials) are partners in the new pilot project which tests stormwater solutions on the roof of Høvringen sewage treatment plant in Trondheim. The project manager is Tone Muthanna, Associate Professor at the Department of Hydraulic and Environmental Engineering, NTNU. She is PhD adviser to Vladimir Hamouz, who is writing his thesis on the Høvringen project. The roof at Høvringen is divided into three experimental fields, each measuring 90 square meters. Storm Aqua and Leca started work on the first field in the autumn of 2016.

They have created a roof consisting of a permeable cover which is made of paving stones with wide joints which allow water to penetrate and which is placed on top of a special Leca material. Leca is a building material made of burnt Norwegian clay transformed into small, porous beads with a hard surface. The second field will be made with sedum which is widely used on green roofs. The third field is covered with black roofing membrane so as to make it possible to compare the traditional roofing solution with the new ones.

Picture of the three experimental fields on the rooftop at Høvringen

Picture of the three experimental fields on the rooftop at Høvringen

Weather station
The roof is equipped with a weather station which measures temperature, wind and rain. There are walls along the perimeter to facilitate control of water flow, so the roof itself is an isolated unit A sophisticated system measures the amount of water which flows into the drains of each of the three experimental plots. The system can measure the full range of precipitation from drizzle to torrential downpour.

– We have added a 20 cm layer of finely crushed Leca under paving stones with wide joints which are also filled with Leca so that water can filter through. Leca absorbs water temporarily and then releases it gradually. What we mostly want to control is water resulting from short, intense periods of rain, says Jaran Wood from Leca Norway. He continues: Increased quantities of stormwater constitute a socioeconomic burden because they make it more expensive and less energy efficient to run sewage treatment plants. In addition, stormwater which is out of control can cause traffic and pollution problems as well as damage to property.

Leca cleanes stormwater
Wood also believes that the research findings can be used to develop stormwater solutions for more than just roofs. Leca, which is frost- resistant, can also be used under both parking lots and pavements. The advantage of using an area of roofing for the research is that the amount of rain which falls on the roof and the amount that goes out from it can be measured. – Leca functions as a filter which can remove pollutants and is used in systems for cleaning tap water and in sewage treatment plants. Part of the project is to explore the possibility of cleaning different types of stormwater with Leca, says Wood.

Municipalities need stormwater solutions
Høvringen sewage treatment plant treats wastewater from two thirds of Trondheim local municipality. – It will be exciting to see the results from the Høvringen project. Finding solutions for dealing with stormwater is important for us, says Håkon Pedersen, from Trondheim municipality technical department. The municipality is aware that increasing rainfall is already causing problems and damage to infrastructure. – Trondheim is part of a joint project on climate mitigation with other Nordic cities, which constantly strive to separate wastewater from cleaner stormwater. There are also several on-going projects aimed at re-opening old, closed streams. In addition to other environmental benefits, getting the water up to the surface increases capacity and improves flood security, continues Pedersen.

Developing solutions together
– The Høvringen project is a good example of how Klima 2050 cooperation makes it possible to develop solutions together. The idea of water retention on the roof surfaced at one of the thematic meetings during a conversation with Leca, explains Møller-Pedersen. He sees great potential in utilising roofs. Høvringen shows that permeable cover, which is currently used in industry and parking spaces, can also be used on roofs. There has been a lot of focus on green roofs, but Møller-Pedersen thinks that a roofing solution where grey is combined with green and blue may provide an even better solution and possibly also result in new and attractive outdoor spaces in urban areas.

– It is interesting to observe the significant effect of water retention at Høvringen where the infrastructure is not designed to handle short bursts of heavy rainfall, says Møller-Pedersen. For documentation of the quality and the characteristics of their materials, projects such as the one at Høvringen are important for both Skjæveland Group and Leca.

– After seeing the first results from Høvringen, Skjæveland Group has started trial production of a new type of paving stone which is lighter and better suited for rooftops, says Møller-Pedersen, adding that they have also started developing new fields of research in collaboration with Leca near Stavanger.