A lawn for sunbathing in the centre of Lünen

Former department store converted

A lawn for playing and sunbathing in the centre of the town and on a rooftop too. How is that done? As part of the conversion to a modern residential and commercial property, the former Hertie department store was given a completely new structure. A green roof with an area of 850 m² was created by making a deep cut into what was originally a rectangular cuboid building. The ZinCo system build-up “Roof Garden” with Aquatec® was perfect here thanks to its low height, and it was professionally installed by the roof gardeners GRÜN+DACH. The town of Lünen is on the northern edge of the Ruhr, near the city of Dortmund. Hertie department store on Willy-Brandt-Platz dominated the shopping life of the town for 40 years until it closed for good in 2009. Many years later, the housing association Bauverein zu Lünen, purchased the plain, cuboid building once the project developers had managed to convince those with responsibility for the building of the daring plan for its conversion for residential and commercial use. The conversion required parts of the building to be demolished and others to be rebuilt. The idea was that it should look as if the centre piece had been literally sliced out of this huge cuboid building. The ground floor remained in place as did the two-storey south block to the left and the three-storey north block to the right. The large roof area between the two blocks is interrupted in the centre by a glass pyramid, which covers the lower-lying courtyard. The planned new use ranges from shops on the ground floor to a medical centre, offices, various accessible apartments in the upper floors and also penthouse apartments.

Low-level system build-up sought

A usable lawn was to be installed on the approx. 850 m² of roof area over the ground floor, and it was to include pathways for accessing the apartments in the south block and the terraces of the north block. The connection heights were dictated by the doors and windows and no more than 15 cm to 18 cm were possible for the terraces and pathways or the lawn. In addition, everything had to be accessible, where possible. Therefore, the particular challenge lay in the already limited build-up height on site. The ZinCo system build-up “Roof Garden” with Aquatec® proved to be a suitable solution. This system requires a build-up depth of only 15 cm given the intelligent under-floor irrigation system.

The roof gardeners began by installing the Filter Sheet PV onto the root-resistant roof waterproof membrane, followed by the water distribution, storage and drainage element Aquatec®AT 45. The troughs in the 45 mm deep elements have a water storage capacity of approx. 17 l/m² and are fitted with dripperlines. The initial schedule contains the location of supply lines, pipe branches, valves, main and flushing pipes. An irrigation manager controls the underfloor irrigation and feeds water into the system in the absence of natural precipitation.

Finally, Wicking Mat DV 40 is installed to complete the build-up. The wicks soak up the stored water and pass it upwards into the substrate layer by capillary action, so that it is available to the plants directly in the root area. This system results in considerable savings in terms of water consumption as the usual loss of water due to evaporation when using a sprinkler does not apply. Given this efficient underfloor irrigation system, only 10 cm ZinCo system substrate is required for the rest of the build-up to allow the lawn turf to thrive, which is the final step in the installation process.

Two trees for the roof

Structural capacity was not relevant to the planning of the first floor or the former sales area of Hertie department store. Nonetheless, the extreme light weight of this system build-up should be mentioned. At only 165 kg/m² in a saturated condition, it allows for a green lawn even on roof areas that have low load-bearing reserves. Given that weight is not an issue with this roof, two trees have been added to the lawn areas. The two pyramid-shaped Hornbeams found a home in the 4 × 4 m and 60 cm high raised beds made of hot-galvanised steel. The root balls were anchored in a pre-installed and stable reinforced steel mesh, so that there were no distracting fixing elements visible above ground.

Constructing the terraces and pathways

For the roof area, the pathways and terraces were installed onto the classic ZinCo system build-up “Roof Garden” with Floradrain® FD 40. The pressure-resistant, 40 mm high Floradrain® FD 40 elements were installed onto a separation and protective layer with the openings for evaporation face down so that the stormwater can run off immediately. This is the correct type of construction beneath slabs and paved areas as there is no need to store water for plants.

The troughs in the drainage elements were filled with Lava, granulate 2-8 mm, and covered over with the same material to a height of 7 cm. Concrete paving slabs, 40 × 40 × 5 cm, are then laid onto this base. The Eaves Profiles DP 120 create a clear separation with the lawn areas. Facade and terrace drainage channels were installed in front of the connections to the apartment and terrace doors, meaning that any rainwater occurring will run off directly into the drainage level. When building for accessibility, this ensures that water cannot infiltrate the apartments. Strips of gravel are used for the connection areas in all other rising components. Inspection shafts were placed over all roof outlets.

Green living in an urban setting

The Hertie building in Lünen is a good example of how, with a little bit of creativity and innovative ideas on the part of planners and others involved in the build, a completely new structure can emerge from an old and no longer contemporary property, without the need to completely demolish the old building. The play and sunbathing lawn, which can be used by all residents in the new “Hertie House”, is only one such example. In any case, we have quality of life here, right in the middle of the town. There is already a huge range of technical solutions available for implementing an innovative green vision for green roofing.