PV on a green roof

Successfully combining a green roof with solar technology

Whereas trapezoidal sheet metal is generally used for pitched roofs, we also differentiate between various types of flat roof. Thanks to their angle of inclination (< 5°), flat roofs are ideal for the installation of solar panels. In addition to typical gravel roofs, so-called “green roofs” are a very special form of flat roof. In this case the surface of the roof is enhanced by planting vegetation. The greening of such flat roofs offers significant benefits for nature and environment.

Extensive roof greening offers ecological and structural advantages

In the case of so-called “extensive roof greening”, which offers a possibility for the additional use of PV, the roof is covered with predominantly low-growing plants such as mosses, sedum or succulents. These plants are characterised by their insensitivity to dry or frosty conditions. They are also inexpensive to purchase and quickly form an all-over carpet of vegetation. However, the greening of roofs has more than just visual attractions. First and foremost, a green roof is chosen for its ecological and structural advantages:

  1. Extensive roof greening ensures the lowest possible surface load.
  2. Greening creates an additional habitat for plants and other living organisms.
  3. At the same time green roofs store rainwater and delay its drainage.
  4. The covering vegetation protects the flat roof from UV radiation and mechanical damage.
  5. Green roofs improve the microclimate by binding dust, and also provide sound insulation.
  6. Under extreme temperatures, green roofs heat up much less than other types of roof.

How is a green roof structured, and what are its effects?

A green roof consists of several layers. In simplified terms, three layers are relevant: one to support the vegetation, one as a filter and one for drainage. A mineral substrate is used to support the vegetation. This forms the basis for the plants, and also stores rain water and nutrients. A fleece filter system retains fine particles from the substrate, and accordingly ensures that the drainage system functions efficiently. The drainage layer ensures that any excess water is removed. Because it acts like a sponge, the green roof also means that less rainwater drains off into the sewers, which reduces the pressure on the sewage system.

Utilising the synergies offered by photovoltaics and green roofs

PV and green roofs are by no means incompatible: the installation of solar panels on a green roof as a special version of the flat roof is entirely possible, and even offers a number of advantages! The only requirement is that the greened flat roof has sufficient load-bearing capacity. The various synergies offered by this type of roof are especially attractive. For example, the rooftop PV system accounts for a considerable part of the load and in this way contributes to protection against wind suction. This is important because wind forces act on every flat roof. In return, the different layers of the green roof securely anchor the PV installation.

Advantages of a green roof

Even at extreme temperatures, green roofs hardly heat up at all compared to conventional flat roofs, and therefore also ensure lower ambient temperatures. This effect in turn makes it possible for the solar panels to heat up less and ultimately increases the yield of the rooftop PV system.

The green roof and PV are a highly practical combination

In order to minimise any shading from the vegetation and to protect the microclimate, on green roofs the PV modules are mounted higher: while as a rule the substructure is mounted in such a way that the solar panels are at a height of 10 to 20 cm, green roofs require a distance of at least 35 to 50 cm from the roof. In addition, when mounted on standard flat roofs, the individual modules are arranged in a conical shape. In the case of green roofs, a v-shaped arrangement is preferred in order to provide both sufficient solar radiation and rainfall for the vegetation.

Today the construction of new buildings is often linked to requirements such as specific compensation measures for the purpose of environmental protection. However, these are no obstacle to the installation of a PV system. When it comes to the construction of solar systems on green roofs and the special features that apply here, the solar experts from the Energy Systems division of STEAG Solar Energy Solutions have all the required know-how. They provide the planning and realisation of rooftop systems on existing green roofs for customers from industry, trade and commerce.

Photos: Christin Lossow