put plants on the roof? What may initially sound like a crazy idea makes a lot of sense for a number of reasons. A green roof provides food and habitat for numerous animals, and the green layer also has an insulating effect and keeps the heating heat in the house in winter - and the heat outside in summer. This article explains what you need to consider when planning a green roof and how you ultimately create and maintain the green oasis.

Extensive greening is the more common, cost-effective variant of green roofs

Table of Contents

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  1. the essentials in brief
  2. What are the advantages of a green roof?
  3. Types of green roofs
  4. planning
  5. Greening of flat roof or garage
  6. Suitable plants
  7. Maintain green roof properly
  8. Funding programs and costs
  9. frequently asked Questions
  10. Does a green roof prevent the house from heating up in summer?
  11. Does the green roof also have a heat-insulating effect?
  12. Can I also plant and harvest herbs on the roof?
  13. Why are the plants on the roof suddenly turning red?
  14. Which plants are particularly suitable for bees and butterflies?
  15. How many plants do you have to plan per square meter?
  16. How much soil does a green roof need? And which soil should you take?
  17. Does a green roof also have disadvantages?

    18. the essentials in brief

    • A green roof has many advantages: Among other things, it keeps heat out in summer and heat in in winter, protects the roof from environmental influences and increases humidity.
    • With extensive roof greening, only a 12cm thick layer of soil is applied to the roof and planted with succulents and other undemanding plants.
    • With intensive green roofing, a real garden is built on the flat roof, the substrate layer of which is more than 20cm.
    • In both cases, a structural engineer must be consulted before the green roof is installed, and professional drainage is also extremely important to prevent moisture damage to the roof.

    “Normality is a cobbled street. You can walk on it, but no flowers grow on it.” (Vincent van Gogh)

    What are the advantages of a green roof?

    A green roof has a number of tangible benefits for both you and nature. Green roofs or even a roof garden offer a natural form of insulation that keeps the heat outside in summer and the heating inside and the cold in winter. In this way, costs for both heating and air conditioning can be saved, which in turn has a direct impact on the environment through the raw materials saved. Furthermore, the plants themselves bind the climate-damaging carbon dioxide (CO2) and thus contribute to an improvement in the climate balance.

    A green roof also offers these advantages:

    • increased noise protection, since the green roof absorbs noise
    • Plants as air filters: Removal of fine dust and other air pollutants
    • The roof is protected from the weather and temperature fluctuations
    • Stored water evaporates, providing cooling and a comfortable level of humidity
    • retention of excess rainwater
    • Improvement of the living and working environment
    • Green roofs are often supported by grants

    The best time for greening the roof is spring or autumn.

    Types of green roofs

    In Germany, the undeveloped areas are declining more and more, especially in times of acute housing shortages in the big cities. With a green roof you give nature back a piece of growth space and is therefore a kind of balance. There are basically two types of green roofs.

    extensive green roof

    A so-called extensive roof greening is applied to otherwise unusable roof areas, such as those found on flat roofs such as carports and garages, but also on pitched roofs, on the roofs of factory buildings or other buildings. Typical of this type of greening is the extremely thin substrate layer, which is no more than seven to twelve centimeters on average. Plants growing here must be adaptable, robust and undemanding as well as capable of regeneration. This means that after a longer dry or frosty phase, the plants will sprout again on their own. Also characteristic is the low maintenance effort: After the installation of the green roof, you basically have nothing more to do.

    intensive green roof

    With intensive green roofs, on the other hand, you are basically creating a garden on your roof. Of course, such a roof garden only works on accessible roofs, after all, such a system is quite maintenance-intensive. The substrate layer applied here is more than 20 centimeters, which is why the selection of plants is much larger and you can also fall back on plants that are more demanding in terms of nutrient and water requirements.

    The intensive roof greening is expensive and complex - but creates a real roof garden

    Planning a green roof

    Before you get down to work with a lot of motivation and want to cover your roof with lots of green plants, you should first stop and spend some time planning. In particular, the following points should be considered in advance:

    • statics: An extensive green roof adds an average of 80 to 170 kilograms per square meter to the roof. Not every roof can withstand this, which is why you should consult a structural engineer before starting construction.
    • roof structures: The roof construction itself also has an influence on the type of roof greening. While both extensive and intensive greening are possible on a warm roof, you can only create extensive forms on a so-called cold roof.
    • roof pitch: Creating a green area on a flat roof is not a problem, and plants can also be planted on slopes of up to about 30 degrees. However, from a gradient of 15 degrees, fall protection is mandatory.
    • root protection: The roof also needs waterproofing and root protection to prevent damage.
    • irrigation and drainage: A drainage layer makes sense for two reasons. On the one hand, this directs excess rainwater straight to the drain, on the other hand, it supplies the plants with valuable moisture.

    Basically, it makes sense to hire a specialist company for larger projects - where more is planned than just applying some potting soil and a few sedum sprouts to the garage roof. This is especially true when planning a roof garden.

    Greening a flat roof or garage - you have these options

    Every green roof consists of several layers:

    • fleece: This bottom layer separates the actual roof from the green roof.
    • protective film: Put a waterproof (root) protective film over it, which has a durability guarantee of at least 20 years.
    • storage mat and drainage layer: Both are designed to both drain and store excess rainwater.
    • fleece: Another fleece serves as a filter, otherwise the drainage would become clogged with fine substrate particles over time.

    Only now do you apply the layer of soil, which usually consists of a substrate specially mixed for green roofs. This contains a high proportion of light materials such as pumice, lava or crushed bricks and only about ten to 15 percent humus. Finally, the planting takes place.

    The following article shows how you can easily green a flat roof - such as that of a shed or garage - yourself with potting soil and sedum sprouts. But be careful: Such a procedure is only recommended for a flat roof on a garage or similar, but not for a residential building!

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    planting opportunities

    Once the roof is watertight and the various layers are in place, you can plant the species you want. When making your selection, make sure that certain plants require a minimum substrate thickness. You have various options for bringing the plants onto the roof. Which one you choose depends, among other things, on how quickly the roof should be greened. As a rule, the growth phase is completed after about three to four weeks, during which the plants generally have a higher water requirement. Only after rooting in can they survive longer periods of drought.

    digression

    When is the best time to plant?

    Therefore, the months between March to the end of June and from the beginning of September to November offer the best growth conditions for the plants, after all, great heat also means a lack of water. Therefore, you should only put up the roof plants during the summer months if you can water them intensively at the same time.

    plant

    Associated with some work, but very efficient - after all, the plants grow fastest this way - is the manual planting of them in the substrate. In this way you also have the concrete design and distribution of the plants in your hands. Plan around 15 to 20 plants per square meter for an extensive green cover of Sedum species. These should have a ball size of about five to six centimeters (so-called "flat ball perennials"). The individually planted plants grow together after some time and finally form a closed area. If there are gaps in this, you can always replant or sow.

    Various types of sedum are usually used for roof planting

    sprout planting

    Of course, it is easier and faster with sedum sprouts, which you simply scatter and which then grow on their own. The disadvantage of this method, however, is that it can take up to a year for the roof to be fully greened. Plan about 80 to 100 grams of sprouts per square meter of planting area. You can purchase these from specialist retailers.

    Sow

    If you're on a tight budget, specialty seed mixes may be the solution for you. However, you need patience for this, because it can take one to two years for the seeds to germinate and the plants to grow - even several years for complete greening. In addition, unwanted grasses can settle on the unvegetated substrate area due to fly insemination, which you have to remove regularly in the first few months.

    vegetation mat

    Green roofs work particularly quickly with a vegetation mat, which is basically a type of rolled mat that has already been planted. The plants have already grown into a special structure made of fleece and substrate and only need to be applied, as with a rolled turf. Vegetation mats are particularly recommended for steep or sloping roofs. Nevertheless, apply a thin layer of substrate about four to six centimeters thick under the mat so that the plants can grow better.

    Suitable plants

    Plants of different heights can be used depending on the type of green roof desired and the thickness of the applied substrate. The most important condition for all species, however, is that they are very robust against weather conditions such as wind, cold and rain - and also tolerate drought well.

    • Low substrate structure (up to five centimeters): Mosses, Sedum species
    • Low substrate construction (five to seven centimeters): low herbs such as chives, wild marjoram, marguerite, Carthusian cloves, dyer's chamomile
    • Higher substrate construction (15 cm): taller flowering plants and grasses

    The following table provides an overview of some of the most important plant species for green roofs.

    kind Latin designation growth height flower color heyday
    dyer's chamomile Anthemis tinctoria 20 to 50 centimeters yellow June to September
    field thyme Thymus pulegioides 5 to 30 centimeters purple June to October
    Rock Carnation Petrorhagia saxifraga 10 to 25 centimeters white-pink June to September
    knapweed Centaura scabiosa 30 to 100 centimeters purple June to September
    Ordinary soapwort Saponaria officinalis 30 to 80 centimeters white-pink June to September
    heather carnation Dianthus deltoides 10 to 30 centimeters Red June to September
    Carthusian carnation Dianthus carthusianorum 15 to 40 centimeters Red June to September
    Small burnet Sanguisorba minor 30 to 60 centimeters reddish May to August
    Little hawkweed Hieracium pilosella 5 to 25 centimeters yellow May until October
    Little Soapwort Saponaria ocymoides 5 to 6 centimeters Red May to June
    Bunch of Bellflowers Campanula glomerata 30 to 60 centimeters blue violet June to September
    Peach-leaved Bellflower Campanula persicifolia 30 to 80 centimeters blue June to August
    Round-leaved bellflower Campanula rotundifolia 10 to 40 centimeters Light Blue June to September
    yarrow Achillea millefolium 15 to 50 centimeters White June to October
    chives Allium schoenoprasum 10 to 40 centimeters pink June to August
    Silver Cinquefoil Potentilla argentea 10 to 40 centimeters yellow June to August
    cranesbill Geranium robertianum 20 to 50 centimeters pink May until October
    wild strawberry Fragaria vesca 5 to 20 centimeters White April to June
    White Stonecrop Sedum album 8 to 20 centimeters White June to July
    meadow daisy Chrysanthemum leucanthemum 20 to 50 centimeters White June to October
    Wild Thyme Thymus serpyllum 5 to 15 centimeters purple June to October
    wild marjoram Origanum vulgare 20 to 60 centimeters light purple July to October

    tips

    On the other hand, if you are aiming for an intensive green roof with higher substrate layers, you are of course much more free in your choice and can fall back on species that would also grow in a normal garden.

    Proper care of the green roof

    With an extensive green roof you don't have much to do once the plants have taken root. In the first four weeks after planting, however, you should water regularly to promote root growth. Depending on the temperature and the amount of sunshine, you can water the water up to four times a week. Then weed about once or twice a year and take this opportunity to remove above all germinated tree seeds, such as birch trees, as these should not grow on the roof. The roots quickly grow into the drainage layer and can destroy the roof as they grow.

    Funding programs and costs

    A green roof is not cheap, although the costs incurred vary greatly depending on the type of green roof, the size of the area and other factors. A green roof from a specialist company is of course a lot more expensive than one that you have planted yourself. For the latter, plan around EUR 20 to 40 per square meter of planting area, provided it is an extensively planted green roof. Intensive greening costs about twice as much.

    Fortunately, however, you can at least partially recoup the costs, because some municipalities grant subsidies of between EUR 10 and EUR 20 per square meter for such a project or assume a percentage of your costs. In turn, support programs from the Kreditanstalt für Wiederaufbau (KfW) can be used, for example, in the course of building renovation, whereby green roofs are also financed via the “Housing Modernization” program. Another opportunity to save is by recognizing the greening of the roof as an unsealing measure - many municipalities reward this with reduced sewage fees.

    frequently asked Questions

    Does a green roof prevent the house from heating up in summer?

    In fact, a green roof has an insulating effect and keeps the heat out in summer. So inside the house it is cooler.

    Does the green roof also have a heat-insulating effect?

    While the summer heat stays outside, the green roof keeps the heat inside during the cold season. The heat-insulating effect of a green roof can even be calculated based on the building materials, the amount of soil and the mass of plants. However, this is a task for experts such as architects or civil engineers.

    Can I also plant and harvest herbs on the roof?

    Herbs such as oregano, thyme or chives are great for green roofs. They are frost hardy, do well in the sun, are easy to care for and do not need much water.

    Why are the plants on the roof suddenly turning red?

    If the plants on the roof are no longer a nice, lush green, but suddenly turn reddish or brownish, there is probably a nutrient deficiency. In general, nutrient-poor substrate is used for green roofs, so that fertilization is necessary from time to time.

    Which plants are particularly suitable for bees and butterflies?

    Many of the plants suitable for a green roof are valuable insect pastures anyway, so bees, butterflies, bumblebees and other winged creatures can look forward to plenty of food. Of course, it is important in this context that the flowering period of the plants does not only last a few weeks, but if possible extends from spring to late autumn.

    How many plants do you have to plan per square meter?

    If you want to plant perennials and various types of Sedum, you should plan about 15 to 20 plants per square meter. However, the exact number depends on the selected species and varieties, since they have quite different requirements in terms of planting distances. Furthermore, such a roof can also be greened, for example by planting flower seeds on it. There's something about a colorful wildflower meadow on the roof.

    How much soil does a green roof need? And which soil should you take?

    How much soil you need depends primarily on how thick you want the layer to be. For shallow-rooted plants, a substrate layer about six centimeters thick is usually sufficient. Plan around 60 liters of soil per square meter of roof area for this. A nutrient-poor substrate is usually used for this.

    Does a green roof also have disadvantages?

    In principle, such a green roof has no disadvantages, except perhaps the occasional maintenance effort. However, you have to be careful with the building statics, because not every building can withstand the additional load on the roof. Therefore, before starting the construction project, have the statics checked by a specialist and take the opportunity to visit the local building authority: Depending on the project, you may need a building permit for your project.

    tips

    Many low-growing onion flowers are also suitable for greening a roof. Daffodils, hyacinths, tulips, dwarf irises, crocuses and pushkinias bloom in spring, creating a colorful picture.

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Garden design