Roof gardens near historic downtown Utrecht

Construction is currently underway on the final block (of four) of the new Zijdebalen residential development in Utrecht. Located against the edge of the historic city center, the development contains nearly 500 homes. There is a wide variety of housing types, to the design of Venster Architekten BV and Mulleners+Mulleners Architects. Semi-underground parking garages have been installed under the residential blocks, on top of which roof gardens will be laid out.

In cooperation with Van Doorn Dakspecialist B.V., BTL Realisatie B.V., Utrecht office, has realized the roof garden with roofing construction of Block 3. Block 4 will be completed this spring. This was all commissioned by construction combination Hurks-Van Wijnen Zijdebalen v.o.f..

The roof garden of Block 3, designed by Mulleners+Mulleners Architects, was constructed last summer. This enclosed courtyard garden of 1,450 m² consists of both a communal area and several spacious private gardens. The garden consists basically of a tiled surface and planters with wire mesh and climbing plants that divide the private gardens. The communal area includes a pergola construction of over 50 meters in length, creating height and security. Furthermore, several large planting areas have been "cut" into the tiled surface, in which mainly evergreen perennials have been planted. To sit together with the new neighbors, three pieces of picnic benches were placed accompanied by several tree boxes with multi-stemmed shrubs.

Technical aspects
In terms of materialization, yellow- and anthracite-colored concrete tiles with a natural stone layer from Zoontjens Beton have been used. These tiles are placed on adjustable tile supports so that the paving is completely 'level'. The design makes extensive use of Corten steel. The planters and retaining elements, to create separation and growing space, are an example of this. The pergola is made entirely of native larch wood. The uprights  were drilled through the tiles by means of threaded studs onto the concrete bases below. Because of this method, the underlying waterproof layer remained completely intact.

The roofing structure consists of a bituminous vapor retarder, a C-EPS insulation sloping mortar, on which a 2-layer fully bonded bituminous finish of the manufacture Derbigum is then applied. For the green roof construction in the planting beds and planters, pressure-resistant and dimensionally stable water buffering ZinCo drainage strips and intensive roof garden substrate tailored to the type of vegetation were used.

Green
Besides all these technical aspects, which are the basis for good use and a long waterproof life, a roof garden is of course also about the greenery itself. After all, without living greenery there is no garden. Thus, the pergola will eventually be completely covered with spring flowering woodland vine (Clematis), Akebia and summer flowering honeysuckle. These are all beautiful and strong climbing plants with abundant blooms. The planters are planted with various partially evergreen perennials such as cobbler (Bergenia), spurge (Euphorbia), purple bell (Heuchera) and aromatic fennel (Foeniculum). In the shaded parts, pitcher (Matteuccia) and needle fern (Polystichum) call the shots. Various waving ornamental grasses ranging from 50 to as tall as 180 cm complete the picture.

The residents of Block 3 have now received their keys and can now design their own private gardens within the established framework. The garden of Block 4, which will be laid out this spring, is considerably larger. This garden will have a park-like appearance with larger trees, height differences, sloping walkways, another pergola and even a large playground equipment. In this way, the limited spaces near the historic heart of Utrecht will be filled in optimally.