The ideal situation is where cars are hidden away in underground car parks and flowers and trees are allowed to grow on the exact same spot, and benches, pergolas or playgrounds beckon people to linger a while. These green areas provide a valuable recreation space and a wonderful setting against the grey urban backdrop, as they are generally visible from the surrounding buildings. The question remains: how do you build it? ZinCo provides tailor-made system build-ups specifically for this type of construction.

Lush vegetation, pathways and terraces, sport and playground areas and also fire service entrances or helipads can be installed just as easily on the roof of an underground car park or a courtyard with a cellar, both in commercial and residential complexes, provided their structural load reserve is sufficient. Impermeable concrete slabs that do not require a waterproof membrane are frequently used when building underground car parks. They also do not require root protection as the slabs are root-resistant in themselves. Other types of construction, for example, a courtyard over a cellar requires a root-resistant and waterproof membrane, which must be protected.  This is a top priority as the underground car park roofs and courtyards that are often at ground level and therefore well accessible are frequently used for wheel loaders and for storing materials. Apart from the necessary protection function, depending on the type of building, system build-ups for underground car parks must also provide a drainage function. Substrate depths of only 40 to 60 cm or less are typical, but ponding is never good for plants.  It should be at least 20 cm in the case of a lawn. Small trees require at least 60 cm – the purely mineral Zincolit-Plus is inserted as a sub-substrate (for structural stability/ventilation) beneath the required ZinCo system substrate with organic elements (upper substrate for water storage and nutrient supply).

Drainage is influenced by the slope

Careful consideration should be given to the slope of the drainage layer, we usually recommend a 2% slope. In contrast, a typical water retention irrigation system for intensive green roofs requires a 0° slope. ZinCo has a suitable system for every requirement. The important thing is always that the drainage elements are installed across the entire area in order to ensure permanent drainage beneath both the greened and non-greened areas.  If a number of different paving types is planned for the roof area, for example, sealed concrete or asphalt paving, then surface drainage is a major factor given that there will be peak run-off levels during intense rainfall. A slope of 2% is also required here to allow the rainwater to flow quickly to the drainage outlets. Where possible, surfaces that allow for seepage are preferable, for example, greened paving joints or greened parking spaces and fire service access with grass pavers. A 0° surface gradient is only possible with raised surfaces where drainage is through the open joints, for example, as is the case with ZinCo Elefeet pedestals. Incidentally, water-bound pathway surfaces and the elastic surfaces of running tracks, street courts and basketball courts are also open-pore.

Loads perfectly distributed

If heavy-duty traffic such as delivery, fire service or refuse vehicles are to be used on underground car park roofs, a load-distributing crushed stone base course with a build-up depth of 30 to 35 cm is required, while a depth of 15 cm is sufficient for use with cars. In addition to pressure loads that arise when vehicles are stationary, horizontal loads arise from braking, steering and accelerating. The loads are to be transferred to the building structure by means of sliding layers, rising building components and stable edgings with rear supports. The stone thickness of paving surfaces is appropriately dimensioned (8 cm for pedestrian, 10 cm for car and 14 cm for truck traffic) and installation is preferably diagonal or as interlocking paving, in order to avoid the pavers rotating out of place.

Entirely building-specific

The classic drainage elements for underground parking decks are Protectodrain PD 250 and Elastodrain EL 202. The latter is of benefit where only a low build-up depth is available and maximum building protection or mainly paved areas are required. Protectodrain PD 250 is the right choice for primarily green areas with deeper build-up depths. The attached rubber protection mat provides a high level of mechanical protection. In both cases it is important that the slope on site is sufficient. If the roof has a slope of 0°, Stabilodrain SD 30 is the best solution, as it bridges ponding water due to its height. It is also intended for use with inverted roofs thanks to its permeability and underlying channels.
Drainage elements for underground parking decks are basically either very stable in themselves – as in the examples mentioned above – or they have to be stabilised by filling them with hard crushed stone or by concreting and reinforcing them. This is the case with Floradrain FD 60 neo and has the advantage that even long drainage distances can be bridged beneath a driving surface. The 60 mm high elements provide the largest drainage cross-section and at the same time the greatest water storage capacity for intensive planting.

Down to the last detail

Generally speaking, all types of edging and foundations for equipment and features such as sandstone walls, pergolas and playground equipment are to be placed above the drainage layer that covers the entire area (lost formwork), in order to ensure that the flow of water is not obstructed at any point. Roof penetration is totally surplus to requirements. Larger trees are firmly secured using special tree anchors. As regards railing fixings, ZinCo supplies railing base elements, and also all accessories, from the height-adjustable terrace grating to façade drains to a drainage shaft over which a truck can drive. Even pools, ponds and other water features can be easily installed on roofs by using a separate pond waterproofing layer above the drainage layer. This will ensure that the pond water will be drawn off to the roof outlet in the event of a leak.

Other hidden benefits

Given the correct design, any number of combinations of green, paving, recreation, sport or water areas can be used on the roof of an underground car park.  There may even be another benefit: Underground car park roofs are easily planned and built with a level of structural load reserve that ensures that large water volumes can be stored there during heavy rain events and released at a time delay. With the ZinCo retention spacers RS 60 and RSX 65, at high loads almost 60 l/m² of water can be stored beneath the green roof build up, and depending on the design, perhaps even more. In this way, the underground car park makes a significant contribution to alleviating the effects of heavy rain events.

Author: Roland Appl, ZinCo GmbH