Green roofs

Planted roofs are not a contemporary invention. They already existed in antiquity and during the Renaissance period, were equally “en vogue” just over 100 years ago and, once again, today.

Three “basic types” can be distinguished:
1.) Extensive roof greening
2.) Grass roof
3.) Intensive roof greening

1.) Extensive roof greening:
The term “extensive roof greening” refers to cases where only a few centimetres of “substrate” are or can be made available for the plants (for example due to reasons of weight). As little as 3 to 5 cm of this are sufficient, in which mainly plants from the sedum family (stonecrop etc.) are planted. The load on the roof is around 50 kg per m² and can therefore usually be implemented without additional reinforcements. Once established, they need very little care and do not or only exceptionally require an additional water supply. Only flat roofs or slightly inclined roofs are suitable here. They cannot be stepped on except for care purposes, but are significantly more visually attractive than normal, bare gravel or sheet metal roofs. In ecological terms, they are nevertheless extremely interesting because they can retain up to 70% of the annual precipitation. This can unburden the storm water network significantly. The water quickly evaporates, creating a better microclimate and reduces heating of the roof surface considerably. For this reason, for example there are plans to make the greening of flat roofs above a certain size compulsory in several Austrian cities (e.g. on industrial halls or storehouses, even if they are not immediately visible). Nearly all private, flat garage roofs can also be “greened” at a reasonable cost. Particular attention must be paid to ensure the “root-proofness” of the roof seals and the design of the roof edge. It is also important that only the above-mentioned low substrate layer thicknesses be used because otherwise, grasses in particular soon intrude and rapidly supplant the sedum plants. Such grass, however, then dies during the next dry spell. For this reason, the “sedum/grass” roofs promoted in some books are a nonsense! Today, ready-to-use “sedum mats” are available, which can be laid in a similar way to sod lawn. It is strongly recommended that experienced experts be contracted to perform this work.

2.) Grass roof:
Grass roofs need a soil “substrate” of around 10 cm. Greater roof inclines are also possible in the case of grass. The ecological benefits are similar and even greater than those mentioned above. The main disadvantage of all grass roofs are their care requirements: the grass has to be cut at least once a year, which always represents a significant challenge due to safety reasons (risk of falls) and the use of machines (lawn mower, brush cutter, etc.). Moreover, disposal of the cuttings is complicated and can therefore be expensive. Depending on the climatic conditions and sunlight exposure, a grass roof nearly always requires an irrigation facility, which also poses certain difficulties. The additional load on the roof through the “substrate” is around 150 kg per m² and therefore has a very considerable structural impact on the building. A grass roof is therefore definitely no “amateur project”!

3.) Intensive roof greening:
Intensive roof greening can generally be described by the term “roof garden”. These are virtually always flat roofs, which can also be used accordingly. This can already be achieved with a fairly simple arrangement of plant boxes, which of course require the relevant care. In the case of large plant troughs, an appropriately controlled irrigation system makes sense. Usable lawn areas on roofs tend to be problematic (size, cutting, irrigation, etc.). “Roof gardens” should always be planned and executed professionally, in particular if shrubs and small trees, etc. are planned. Numerous factors must be taken into account, from building structure considerations, wind load, root resistance of the roof seals, as well as the indispensable additional water supply. The smallest of mistakes can result in severe consequential damage (e.g. penetration of water into the premises below). Roof gardens provide great quality-of-life benefits in densely populated areas, but incur high construction and maintenance costs.

VIKING garden expert
Prof. Karl E. Schönthaler