The green roof is ecologically valuable, especially in cities. Many endangered animal and plant species have a completely new habitat here. Green roofs also relieve our drainage systems and sewage treatment plants because part of the annual precipitation evaporates from them into the air. They also improve the urban climate because they filter dust and pollutants from the air. In addition, a green roof simply looks very good and also brings you a benefit. Your roof skin gets a thick protective layer and thus lasts longer. The rainwater evaporating on the roof ensures that the rooms below stay cool even in the summer heat. This is how you proceed if you want to do your extensive roof greening yourself.
Table of Contents
Extensive and intensive – difference
The difference between extensive roof greening and intensive roof greening lies primarily in the way in which the green roof is planted. The extensive green roof has a thin layer structure with substrate and drought-tolerant planting. The intensive roof greening, on the other hand, provides for a fully-fledged soil structure that allows for any design, including planting trees. The extensive roof greening is much easier to design and feasible for many roofs.
Structural requirements for green roofs
Before greening your roof, the load-bearing capacity of the roof structure must first be determined. With the roof greening, the roof usually has to absorb considerable additional loads. If you are planning a green roof for a new building, this additional load is already included in the plans during the design phase. If you would like to add greenery to an existing building, a specialist must check what reserves of load-bearing capacity the given roof construction has and whether these can be fully used. Warm roofs are generally generally suitable for green roofs. This is where the details of the construction matter. With cold roofs it becomes more difficult. The outer skin here is usually quite light and has only low load-bearing capacity reserves. With existing cold roofs, only extensive roof greening is usually conceivable.
In any case, the construction has to be right. The structural implementation must be planned in compliance with the following guidelines:
- The most important basis is the FLL guideline for green roofs
- Guideline for the planning, execution and maintenance of green roofs of the research company landscape development landscape construction eV in Bonn
- The guideline is anchored in various DIN standards and therefore has the character of a law
- In addition, specifications from the flat roof guidelines may have to be taken into account
- they form part of the technical regulations of the Central Association of the German Roofing Trade (ZVDH)
These guidelines require attention to all possible construction details for all possible roofs. The most unproblematic green roofs are usually unventilated roofs without thermal insulation (where, however, sub-zero temperatures on the underside of the construction can lead to frost damage) and roof constructions with rigid thermal insulation.
The structure of a green roof
If you want to use a green roof in the long term, you have to protect the roof membrane and build up the vegetation area in a stable and appropriate manner. The green roof is built up in layers. Starting from the roof waterproofing towards the outer roof, these layers can be divided as follows:
- Roof skin protected by a separating layer
- then comes a root protection
- a protective layer
- and the layers of drainage layer, filter layer and vegetation layer required for vegetation
- Material combinations or multi-layer installation systems possible
With regard to the technical layer of vegetation, a two-layer construction is usually sufficient for extensive green roofs. Both types of greening (extensive and intensive) require root penetration protection, which prevents damage to the roof waterproofing from penetrating plant roots, if the existing roof waterproofing is not root-resistant.
1st step
The protective and separating layer is applied directly to the roof skin. The protective and separating layers are mostly made of geotextiles or rubber, sometimes building protection mats are also used. They are used to compensate for material incompatibilities between the roof seals and the root protection membranes. So tolerated z. B. Soft PVC often not with bitumen. In addition, these layers also protect the applied foils from mechanical damage. They are therefore often laid in two layers above and below the foil. There are also so-called protective and storage protection mats, which, in addition to their protective function, also have a certain water storage capacity.
In the case of protective and separating layers made of fleece, the material thickness is usually given in weight. Fleeces with a weight between 0.1 kg and 1 kg per square meter are used. If you use recycled products, you must pay attention to possible interactions between the protective and separating layer and the seal or root barrier. For example, in the case of rubber scrap mats, migration of plasticizers is possible between the seal or foil and the rubber scrap mat.
2nd step
For new builds, root-resistant roof seals can be planned, which are available in various materials, such as bitumen, ECB, EPDM, PVC, polymer bitumen and mastic asphalt. If you use such a root-resistant roof seal, you do not need an extra root protection sheet to protect against root penetration. When purchasing a root-resistant roof seal, make sure that the dealer presents you with the certificate of the FLL test procedure or provides proof of at least 10 years of successful use. That’s the rule.
Protection against root penetration is achieved here either by means of plastic webs (PE, PVC, polyolefins) or by applying a continuous liquid sealant, which is offered on a polyester resin (UP), polyurethane (PUR) or polymethyl methacrylate (PMMA) basis. It is important in this area that the tracks are laid very precisely and very densely. You should also lay at least 20 cm beyond the vegetation layer at the edges. Otherwise, roots can later grow into the protective layer. If you plan a strip of gravel or slabs that is not overgrown on the parts of the roof that need maintenance (e.g. roof drains), you will make subsequent maintenance and care in the roof area much easier.
3rd step
In the case of roof seals that are not root-resistant, sometimes also in addition, the root protection membrane is now laid. These root barriers are usually made of high-polymer plastics. Soft PVC, synthetic rubber (EPDM) or polyethylene (PE) come into consideration here as a material. The root barriers should be at least 0.8 mm to about 1.5 mm thick. This root protection is intended to protect the roof seal against damage from roots in the long term. It is therefore never limited to the vegetation areas, but is laid as a closed and watertight trough over the entire roof.
As with the root-resistant roof seal, you should request proof of root resistance. Either through proof of an FLL test or through proof of at least 10 years of successful use of the product in practical use. With these films, you should also request proof that they do not contain any substances that are harmful to the environment or plants. The root protection consists of individual strips that must be connected in an unbreakable and watertight manner. This can be done by cold welding or hot welding (e.g. with bitumen or PE).
Do not allow yourself to be carried away or persuaded to lay the root protection membrane without welding and just overlapping. Plant roots are looking for water and nutrients. You are pretty sure to find even the tiniest residue of moisture and usable trace elements hidden under the root barrier.
If you plan to plant plants with “real killer roots” such as couch grass, viper’s bugloss or horsetail , you should choose a little stronger root protection right away . If you use fabric-reinforced root protection membranes, you should seal the capillaries with an additional seam seal, otherwise the continuous root protection will not work. Make sure your root protection is UV stabilized. You would have to provide additional protection for root barriers that are not UV-resistant.
If the root protection membrane is mechanically attached somewhere, you would have to take into account beforehand that the foil may change its dimensions with age and temperature changes. Appropriate information should be available in the purchase documents. Then you can attach the film in such a way that you avoid torn wall connections and the like.
Try to develop an eye for possible weak points when applying the root protection. Realize that seeds can be very small and yet have tremendous powers when “attacking your roof skin”.
4th step
If the rooting protection is on, it gets a protective layer that is intended to prevent mechanical damage. As already mentioned, like the first protective and separating layer, it can be made of a geotextile such as fleece. If you assume heavier stress, you should use a building protection mat made of rubber granules or plastic granules. Drainage mats or panels are often used in this layer, which also take on the drainage function. In the case of very high loads, a protective layer of concrete or mastic asphalt is even conceivable.
5th step
The drainage layer is there to absorb and drain excess water from the vegetation layer to prevent “a lake on the roof”. If you choose the material so that your drainage layer can store water, it also serves as a water reservoir. This type of greening is called “greening with water accumulation”.
Drainage layers can be filled with pumice, gravel and lava, recycled rubble (broken bricks), or you can use the drainage board or mat laying systems just mentioned. There are also combined drainage and substrate panels. The material is chosen to match the greening and the structural conditions after the greening has been selected. In the case of extensive green roofs, a drainage layer of around 4 cm is usually applied when loose materials are used. Drainage mats are only applied in layers between 2 and 4 cm thick, depending on the material of the mat.
6th step
Above the drain layer is a filter layer, which prevents fine foreign matter from entering the drain layer from the substrate layer. Fleece is used again here, or fabric, this filter layer is simply laid in strips over the poured drainage layer. Special VV and fabric are used here, through which roots grow, so that the drainage layer becomes the root area in addition to the substrate, which is particularly the case with extensive green roofs with their thin layers. The filter layer is usually incorporated in the drainage mats.
7. Schritt
Roof drains must be planned for the water drainage, which must always be provided with a gravel bed or an inspection shaft. Roof drains and gutters should be designed in such a way that the plants on the roof cannot overgrow them. Pitched green roofs need gravel strips or drainage pipes in the eaves that lead to the roof drain.
Roof gullies or gargoyles in the eaves area and other drainage devices may be required, depending on the pitch of the roof. If you are planning a green roof with dammed water, you should note that the dammed water surface has a similar effect to a vapor barrier. If this is unfavorable due to the roof structure or the use of the rooms below, you could opt for drip irrigation or a sprinkler system.
8th step
If you want to green a roof with a pitch of more than 20 degrees, you need shear protection so that the substrate does not slip off the roof. They can be built from simple wooden slats, but are also made from plastic profiles, plastic panels or tangled fabric and a few other materials.
Fire protection requirements must be met for extensive greening. As a rule, it is considered sufficient if a substrate layer at least 3 cm thick with less than 20% organic content is applied to the roof. A strip of coarse gravel or plate material at least 50 cm wide should be placed around openings in the roof surface (roof vents, chimneys) and wall openings so that no combustible organic material can come near the openings.
That was basically the green roof building instructions. If all of that works, you can start applying the vegetation layer. Then comes the most beautiful thing about installing a green roof: selecting the desired and suitable plants.
Conclusion
This form of green roof is also the most cost-effective form of roof greening. You can probably calculate a few more euros if your local building authority has explained to you the complicated subsidy for green roofs, which in Germany consists of direct subsidies, stipulations in development plans and indirect measures such as splitting the sewage fees.
