Colorful instead of grey: Overgrown roofs bring color and life to our cities, they improve the air quality and help save energy. Where a green roof is possible and sensible, find out here.
When the first green roofs were built, there was no talk of air pollution and the increasing scarcity of space for plants and animals. The Scandinavians started centuries ago, to cover their roofs with sods, above all, to create a reasonably comfortable temperature in their homes, without using too much fuel. A warm fur made of earth and grass was the simplest and cheapest solution. And a very effective one, because the plants insulate similarly to the thick fur of polar bears: A stagnant layer of air forms between the stalks or hairs, which has an insulating effect. Today we ensure this effect with synthetic insulating materials.
The benefit -A green lung on the roof
Good insulation with the resulting savings in heating energy is more important today than ever. There are also very modern environmental problems such as bad air or traffic noise.
In both cases, overgrown roofs help, because the plants produce oxygen and absorb carbon dioxide, they filter dust from the air and enrich it with moisture. At the same time, they act as sound insulation, similar to dense hedges. In addition, the layer of soil and vegetation act as a temperature buffer: The roof skin underneath is exposed to lower temperature fluctuations, is therefore less stressed and has a longer service life. Finally, the green roof is also a substitute for disappearing ecological niches, where rare animals and plants can find a new habitat.
The conditions – Which roofs can be greened?
In principle, every house could have an overgrown roof, provided, it is not in the deep permanent shadow of other houses or tall trees, where no plant can grow.
The pitch of the roof is also important. roofs with 45 degrees of inclination could still be greened, albeit with great effort. Least problems cause flat and up to 20 degree pitched roofs, because there is no danger here, that the substrate, the real one, living soil layer, comes to a halt. away 8 Degree of slope is usually a conical cut sod beam (that. 16 x 16 cm) laid above the eaves. It holds the substrate layer of the whole roof. In the case of steeper roofs, shear protection must be installed. They can be found on old Scandinavian houses as loosely laid slatted frames, today squared timber (that. 8×8 cm) screwed to the roof surface at a distance of two meters. The alternative to this is a close-meshed clawing tissue, laid in the substrate.
The site – sun or shade, green or colorful
It depends on the location of the house, what plants can grow on the roof. Full sun locations, where the substrate occasionally dries out, are only for thick-leafed plants such as house and roofleek, stonecrop and the like. They form a dense, wonderfully colorful carpet when in bloom, but do not have as good an insulating effect as a growth of grass. Grasses grow best in partially shaded locations, where the soil is mostly wet. Seed mixtures with drought-resistant varieties and "weeds" are suitable.
The technique – How much load can a roof carry?
Whether the roof truss can support a green roof, is a question, which only the structural engineer can answer. For old buildings, for which there are no longer any construction documents with static calculations, a new calculation is absolutely necessary. The only exception is flat roofs, where the gravel layer can easily be exchanged for plant substrate. With a square meter weight of 16 to 18 kg for gravel and 13 to 15 kg with an expanded clay topsoil mixture, the grass roof becomes a relief in the truest sense of the word. It is different with roofs, whose previous covering is to be replaced by a green roof. Here you can compare the weight of the old material with the square meter weight of the new wooden roof skin, Calculate sealing layers and substrate.
In a professionally installed condition and with full water saturation, one calculates (each for 1 cm layer thickness) for topsoil 16 to 20 kg, for sands 20 to 22 kg, for gravel 16 to 18 kg, for expanded clay 8 to 10 kg, for 1:1 topsoil mixed with expanded clay 13 to 15 kg. Weight savings only result in lighter substrates or. thinner layers.
The structure – A roof membrane made up of many layers
Grass roofs are often built as so-called inverted roofs: The thermal insulation, which lies between the load-bearing ceiling and the roof boarding on a cold roof and under the roof seal on a warm roof, is laid here on the sealing layer. Requirement is, that the insulating material cannot absorb moisture. This installation on an already existing warm roof is doubly effective. Regardless of the question, whether and where insulation is provided, a green roof consists of the roof boarding, a layer of geo fleece on top of that, roof seal, again Geofleece and plant substrate with a layer thickness of 10 to 20 cm. In the case of flat roofs, an expanded clay drainage layer covered with geofleece is also required.