Thirty years ago, the Austrian artist, architect and environmentalist Friedensreich Hundertwasser predicted that the forest, the green meadow or the lush vegetable garden on the roof would soon be a matter of course.

Plants on the roof cool in summer and insulate in winter

A green roof can double the durability of the roof

Green roofs as roof coverings of the future are actually no longer a romantic daydream, because nature is on its way to return to the city as well. They increase our quality of life, allow rainwater to evaporate better and also have a positive influence on the climatic conditions. Green roofs cool the houses in summer, insulate in winter, and their construction is even financially supported. The Garden Journal has just published a detailed article on the technical and organizational details of green roofs. And yet, most homeowners are still skeptical.

Against all prejudices: Green roofs as a small project

The most common objections raised by real estate owners include fears that a natural green roof could be damaged by the plants and that such projects would be too expensive and too complex in terms of design. Ergo: Just try the extensive greening of the tool shed, the dog kennel or the aviary! And as far as the reduced durability of house roofs is concerned, the Association of German Roof Gardeners e. V. anyway better and analytically verifiable: "The durability of a (bare) flat roof is on average only 15 - 25 years, even with professional execution"*. And: “Numerous green roofs from the 1970s prove that professionally installed green roofs can easily double the lifespan of the waterproofing.”*

Extensive green roofs: not only easy to maintain

Green roofs blend in well with the landscape

This method of the three types of greening for roofs is also optimal in terms of static load capacity, especially for light roof constructions. In addition, extensive green roofs usually do not require additional irrigation. Almost all professionally sealed roof constructions (flat, pitched, pent or barrel roof) are suitable for greening, provided that the structural specifications are taken into account. For roofs without a slope, moisture-binding drainage elements must also be provided so that the plants are not damaged even in heavy rainfall.

Careful planning - the be-all and end-all for green roofs

Careful design planning is not only decisive for the later maintenance effort and the expected costs, but also has visual effects that you have to be aware of before the first wheelbarrows with gravel, substrate and perennials are hoisted onto the house roof. So:

  • Defining framework conditions: type of substrate material, required structure thickness, planning options for tanning;
  • Selection of the planned planting material: taking into account the natural mutual competition or sociability, expected growth forms and the time of flowering, taking species protection into account;
  • Creation of a planting plan: determination of the desired locations with numbers of plants and the selection of the optimal planting method.

If you are unsure yourself, do not be afraid to seek professional advice on any questions or problems that may arise. On the portal of the Deutscher Dachgärtner Verband e. V. you will find current addresses of experienced roof gardeners from your region as well as relevant specialist dealers, manufacturers and associations who support and accompany your green roof project.

Planting options for extensive roof greening

Not all plants are suitable for green roofs

Undemanding and drought-resistant plants that get by with a relatively thin and therefore light substrate layer are particularly suitable for this. Roof pitches between 2 and a maximum of 15 percent are ideal, which can withstand a total weight of around 120 kg per square meter of roof area without damage. Plants with as small and flat root balls as possible or already pre-cultivated plant mats are ideal. In addition, there is also the green roof from the bag. In addition to easy-care rough meadow plants, there are usually different types of Sedum and Sempervivum, which are sufficient for sowing on a three to four square meter roof area. Here are a few more planting ideas from the DDV in a short tabular form:

botanical name German name flower color and flowering time height (cm) notice
Sedum album White Stonecrop white-pink, VI to VIII 5 to 10 forms a loose lawn, different leaf colors (green, red, brown)
China Sedum sedum golden yellow, VI to VII 10 to 15 carpet formers, also penumbra
Sedum hybridum "Periwinkle" Mongolian Sedum golden yellow, VII to VIII 10 to 15 forming dense, evergreen mats, also partial shade
Sedum reflexum trip madam golden yellow, VII 15 to 30 forming loose lawns, old spice plant
Sedum sexangulare Mild stonecrop yellow, VI to VII 5 to 10 Dense mats and cushions
Sedum spurium different varieties Carpet Sedum pink, white, carmine, VII to VIII 5 to 15 Carpet-forming, very vigorous and dense, also partial shade
Dianthus carthusianorum Carthusian pink crimson, VI to IX 20 to 40 loose growth, self-sowing
Dianthus deltoides heath carnation pink, VI to IX 5 to 15 lawn-like, green-brown-green cushions
Dianthus plumarius plume white, pink, red, V to VI 10 to 30 dense grassy cushions, strongly scented
Hieracium pilosella Little hawkweed, mouse ears yellow, V to IX 5 to 10 loose, lawn-like growth
Koeleriaglauca iridescent grass blue green 20 to 30 broad-leaved clumps
Petrorhagia saxifraga stone carnation white-pink, VI to IX 10 to 20 loose, fragrant veils of flowers
Saponaria ocymoides Upholstery Soapwort pink, VI to VII 15 to 20 Undemanding cushion perennial
Sempervivum hybrids roofleek, houseleek various colors, shapes and sizes, VII to IX 5 to 30 Mat and Carpet Former
Thymus serpyllum Creeping Thyme lilac-pink, V to VIII 3 to 5 rampant, fragrant, different varieties

Source: German Roof Gardeners Association e.V.

Finally, pour all or part of it briefly and sit back comfortably, because the work is done. You will hardly have to worry about the new green roof in the future and, above all, hold back on watering. This would only fuel the colonization and spread of unwanted weeds. From time to time it is only necessary to remove the spreading seedlings from the roof, there is not much more for you to do.

*Quote from: DDV practical guide "The 1 x 1 of roof greening" from the German Roof Gardeners Association e. V., Nürtingen, 2016.

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