Flat roof and pitched roof: Which green roof for which roof construction?

Green roofs on flat and pitched roofs
No standard or guideline defines the roof pitch above which a "flat roof", "pitched roof" or "pitched roof" is to be considered. If the green roof guidelines are used as a guide, a "flat roof" is assumed at 0-5°, a "pitched roof" (sloping roofs) from a roof pitch of 5° (= 9 % roof pitch) and a "steep roof" from a roof pitch of 15° (27 %).

Flat roof with green roof: layer structure for roofs without a slope
As a rule, flat roofs should have a minimum gradient of 2 %. With this gradient, both single-layer and multi-layer green roof structures can be used.

Flat roofs with very little or no slope, so-called "0-degree roofs", are not uncommon in Germany and may also be built if the relevant standards are observed. However, there are a few points worth knowing and considering with regard to the suitability of green roofs. Practice has shown that on a flat roof with no or very little slope, the water does not run off immediately in various places and areas with standing water form. In the case of flat roofs and lightweight roof constructions with large spans and a low gradient, a green roof with its own weight and water storage increases possible deflections. This then leads to further water accumulation and standing water.

The problem with extensive green roofs on flat roofs with standing water is the development of vegetation. As a rule, vegetation will develop differently on roofs without a slope and standing water than on roofs without standing water. The plants do not tolerate a permanent build-up of water and waterlogging of the substrate. If there is too much water standing for too long over a large area, the vegetation is likely to be different from what was originally planned. The problem must be recognized and tackled during the planning phase of the green roof. Depending on the local conditions, everything is possible, from plant failures and stunted growth with moss monocultures instead of the perhaps desired sedum-herb vegetation to grass roofs with the establishment of woody plants. Any predetermined vegetation forms cannot be achieved with standing water despite extensive maintenance.

The aim of all vegetation measures is to create a sufficient distance between standing water on the flat roof and the vegetation support layer so that the plant roots are kept out of the water and the substrate does not become waterlogged. The best proven solution in practice is the use of a classic multi-layer structure with a separate drainage layer. Smaller depressions and unevenness with standing water can still be evened out with bulk drainage. The better solution, especially if the water stands on the roof for longer than one centimeter over a large area, is to use a drainage element. It is laid directly in the water and covered with a filter fleece, thus creating a capillary-breaking distance to the vegetation base layer. The height of the drainage element used depends on the height of the maximum water level and should be around two centimetres higher. Their main functions are water drainage and spacing between water and substrate.

Pitched roof with green roof (up to 15° roof pitch)
The layer structure for pitched roofs is similar to that for flat roofs:

• Protective layer
• Drainage
• Vegetation support layer
• Vegetation

In contrast to flat roofs, there does not need to be a drainage layer over the entire roof surface. Due to the roof pitch, it is assumed that water drains quickly towards the eaves, so that drainage in the eaves area may be sufficient. This is where most of the water accumulates and the drainage and drainage system must be dimensioned and coordinated accordingly. With pitched roofs, greater importance is attached to water storage. The steeper the roof pitch, the faster the water is drained away and must be compensated for by a layered structure with a higher water storage capacity. This can be achieved by using a substrate with good interlocking and positional stability with organic components, which must not be too coarse-grained overall. The characteristics are based on an extensive substrate for multi-layer construction, even if it is usually installed in a single layer for pitched roofs. A multi-layer construction with a drainage layer made of plastic elements is also possible. The drainage elements take on a triple function: drainage on the underside, water storage on the top and anti-slip protection. A separate filter layer in the form of a filter fleece is generally not used so that the vegetation support layer and drainage interlock better. The thickness of the vegetation support layer should be at least 8 centimetres (when seeding and 6 cm when using a pre-cultivated vegetation mat) in order to provide the plants with a minimum of substrate and water. For pitched roofs up to 15°, seeding and planting are possible for vegetation application; above 15°, only pre-cultivated vegetation mats are recommended.

Green roof on pitched roofs (from 15° roof pitch)
If the roof pitch exceeds 15° (27%), structural measures must be taken to prevent the green roof structure from slipping, for example in heavy rain. Practical experience has shown that anti-slip measures are necessary for roofs with a pitch of more than 15°. Green roofs are possible up to a roof pitch of around 45° (100% gradient). However, these are extreme cases; roofs with a pitch of up to around 30° are usually greened. The following points must be taken into particular consideration when planning and installing a green pitched roof:

Layer structure for pitched roofs
The green roof structure for pitched roofs with a roof pitch of more than 15° is basically the same as for slightly pitched roofs. What distinguishes pitched roofs from other slightly pitched roofs is above all the slip and shear protection and the application of vegetation:

• Protective layer
• Drainage (in the eaves area or over the entire surface)
• Slip and shear protection
• Vegetation support layer
• Vegetation
• Slip and shear protection

From a roof pitch of more than 15° (27%), structural measures must be taken to secure against shear. Roofs with a pitch of less than 15° and flow lengths of more than 15 m also require special shear protection measures. It is important to use proven, weather-resistant and structurally sound shear protection systems. Wooden structures are unsuitable as they will weather sooner or later. At the beginning of the planning phase, a decision must be made as to where the load of the green roof structure should be transferred via the shear protection (so-called shear sills).

There are three options:

• Shear transfer over the entire surface of the substructure
• Shear transfer above or at the ridge
• Shear transfer to the eaves

The distance between the shear sills depends on the roof pitch and the shear protection system used. The substrate is usually installed and compacted in such a way that it covers the height of the shear sills or covers them by up to two centimetres. The pre-cultivated vegetation mats laid on top must be laid without cavities so that they can grow evenly over the entire surface.