Green Roofs

ADVANTAGES OF GREEN ROOFS

Green roofs and roof gardens are becoming more and more important in urban environments, allowing some of the space occupied by the building footprint to return to green surface. Green roofs offer expanded living space, contribute to clean air and also have a considerable rainwater-retaining properties. Green roofs also have an indirect beneficial effect by relieving pressure on the rainwater and wastewater sewerage networks. Inverted roofs are well-proven for both extensive and intensive green roofs and provide a structural solution with long lifespan.

The production process used in the manufacture of extruded polystyrene (XPS), results in a homogeneous, closed-cell material structure with a smooth surface (called the extrusion skin) which provides:

• consistently high levels of thermal insulation
• water and frost resistance
• high strength
• high resistance to vapour diffusion

All of these favourable characteristics make the XPS thermal insulation material suitable for green roofs.

GREEN ROOFS

Extensive green roofs are principally created for their ecological advantages. The entire thickness of the layer is usually between than 6 to 15 cm. Roofs with the thinnest vegetation blankets weigh around 45 to 60 kg/m2, while thicker ones can be 160 to 250 kg/m2 when wet. In well-functioning extensive green roof, the plants used must be well adapted to drought and require little care and maintenance.

Soil mixtures with high water-retention capacity or special mineral substrates that also act as a drain layer can be used as a topsoil layer. In the case of the latter, there is no need for a separate drain layer, and a thickness of min 6 to 10 mm is recommended. The root network of contiguous vegetation also provides for the stability required against wind-load. However, additional ballast is necessary along the green roof edges - 300mm x 50mm deep gravel.

ROOF GARDENS

Intensive roof gardens are roof gardens with a minimum 30-cm thick plantation soil layer, which may even be identical to the garden in the courtyard of the house. The thickness of the vegetation layer is restricted only by the load-bearing capacity of the roof slab, and, if the soil is 1 to 1.5 m deep, even small-sized trees can be planted. Typical forms of roof gardens are gardens for relaxation, children’s playground or parks, but even vegetable gardens are possible. These kinds of roof gardens have the same versatility as a normal garden, so they require regular maintenance, irrigation and appropriate nutrient supply.

The type and composition of the topsoil for intensive roof gardens as well as the crop to be planted should be determined with the involvement of a garden designer. The wind protection of tall plants should be professionally anchored and supported.

INSTALLATION OF GREEN ROOFS

The waterproofing membrane for green roofs should also be resistant to roots (e.g. FLL certification) otherwise the installation of a separate layer for protection against root penetration will be necessary. Ravago's XPS insulation boards do provide additional protection to the waterproofing membrane, but they cannot replace protection against roots.

The vapour-permeable layer laid on top of the thermal insulation prevents fine granules from being washed into the drain layer or between the thermal insulation boards, and provides mechanical protection for the surface of the boards.

Over the thermal insulation, as a drain layer, a 30 to 40 mm thick washed gravel, crushed stone, expanded clay aggregate or various special products (drain carpets, profiled drain plates etc.), light-weight combined systems may also be used to ensure free departure of migrant vapour through the thermal insulation.

In green roofs, irrespective of whether it is an intensive or extensive solution, a topsoil layer with special composition is required, and the vegetation should be carefully selected.