The Substrate for Green Roofs – Why Is It So Special?

Extensive green roof – lightweight, cost-effective, and probably the most common type of green roof. However, the properties that allow its low weight expose the plants to extreme survival conditions.

Water retention

In general, most of the water stored in a green roof system is retained in the growing medium. The relatively thin “soil” layer of a green roof significantly limits its water retention capacity, which, combined with other factors, subjects the vegetation to harsh living conditions. This is also reflected in the relatively short list of plants suitable for an extensive green roof system. Extreme environmental conditions are taken into account when designing extensive green roofs, but this is only part of the challenge, with other aspects needing consideration as well.

Weight and weeds

The system’s weight is a particularly important feature of green roofs. In practice, due to structural limitations, this criterion is usually decisive when choosing the extensive model. Harsh environmental conditions limit the spread of unwanted vegetation, which, if it thrived, could overload the roof. A few wind-borne seeds may germinate and grow, but medium or large plants will not survive due to lack of water, and in the long term, the desired extensive vegetation remains. Thus, there is a positive side to the extreme conditions on an extensive green roof. For this reason, it is not recommended to increase water retention with an additional layer of mineral wool or excessive compost in the substrate composition.

From this perspective, it should be emphasized that the structure of an extensive green roof must have the characteristics of an extreme environment system. Otherwise, the desired vegetation will gradually be replaced, and to avoid weight gain of the biomass, costly maintenance interventions will be required.

It is worth noting that in extensive systems with irrigation, this phenomenon occurs more frequently. Excess water from irrigation can disrupt the development of planned vegetation, encouraging the emergence of invasive “weed” species. These (various grasses, annual plants, etc.), normally dominated by sedum species, have a higher chance of survival and over time may replace the desired vegetation.

From the above, it follows that the growing medium, or substrate, is a key element in the design of an extensive green roof. Limiting the amount of water to a level that meets the needs of extensive vegetation is important. How can this be achieved? There are two approaches:
The first is calculating the substrate composition to limit water retention capacity.
The second is limiting the thickness of the growing medium.

Characteristics

As expected, the substrate recipe must meet the minimal needs of drought-tolerant plants that make up the extensive vegetation on these roofs. The properties of this type of substrate are: low weight, high filtration capacity, relatively poor in nutrients, porous, and long-lasting—all characteristics that satisfy the needs of plants adapted to harsh, dry conditions. In practice, these characteristics are found in mineral-rich, low-organic-content substrate compositions, as defined by FLL.

Growing medium depth

The other method to regulate the required water is naturally linked to lightweight structures: reducing the nutrient layer thickness. The appropriate depth is determined case by case, considering local environmental conditions, average precipitation, sun and wind exposure, and the plants that will form the desired vegetation. A substrate thicker than necessary for adequate water retention will result in high maintenance costs due to the need to periodically remove unwanted vegetation.

The type and thickness of the substrate must exactly meet the needs of the project plants.
Of course, more can be said about substrates. Briefly, low bulk density, nutrient composition, physical characteristics, granularity—all are considered to create a sustainable green roof system with minimal maintenance needs throughout its lifespan.

Maintaining green roofs is not an expensive operation, but it is absolutely necessary to carry it out on time and properly by experienced teams.