The reasons why mineral substrates are used for the roof gardens instead of replacements containing soil, topsoil, peat and the like:
1. The first reason is the substrates have optimum drainage properties – the substrate is 36% water absorbent, but at the same time it is made of quarry materials to ensure sufficient permeability of excess water. Composition of brick + lava + compost ensures, besides the retention of water for plants, the retention of a large share of fertilizers in an optimal form as well. The excess water has nowhere to capture and thus it prevents rotting of a plant when soaked for longer.
2. The second reason is the stability of these materials over time-roof substrates based on top soil, peat, bark and similar materials do not retain its volume as a result of biodegradation/rotting of these components. Over the time it is common that if you use e.g. 1 meter high embankment, after 3 – 5 years the volume will drop by 20 – 25%. Subsequently , the substrate needs to be refilled. This problem is not the only one. A more fundamental problem is that by rotting the substrate particles are reduced and gradually the substrate is “closed” and it looses the water permeability, it comes to decay. Mineral roof substrates are permanent in comparison to their replacements and therefore they do not cause the problems mentioned above.
3. The third reason is bulk density. In assessing the volume weight it is necessary to distinguish several levels. The first is the “dry“ bulk density – here is our value m3 about 980 kg, in peat substrates, the more peat and bark they contain the lighter they are, sometimes for example 600 kg/m3. This value, however, is useless to calculate statics. The statics have to calculate the bulk density of the substrate fully saturated with water at full density. When measured after being fully saturated with water and at full compaction our mineral substrate is about 600 kg/m3. With peat substrates these values vary considerably, they depend on the amount of dry and highly absorbent material used in the mixing, e.g. the peat. Therefore, it is always necessary to check this limit, not the dry bulk density marketingly stated.
4. The fourth argument is the porosity of the material and the related retention of soil oxygen for optimum plant rooting. Generally, the higher the oxygen content in the soil, optimally e.g. 9%, the better development of the root system. In that the mineral substrate is possible to compact without expelling any air (free of small, compressible particles, but rather quarry and porous particles) the substrate will even after compaction still contain sufficient soil oxygen. When using peat substrates, as the time proceedes, they break into small and somewhere even gluey particles which by mechanical or natural compaction expel air from the soil.
5. The fifth reason for the use of mineral substrate are its qualities in extreme desiccation on extensive/non-irrigated roofs during the summer. When using topsoil or peat in the substrate the soil becomes “chapped” by dryness which damages the root system of plants. Mineral substrate is stable, only individual particles get dry and there are no cracks. The mineral substrate may be used for these properties at the minimum thickness of 5 cm or more.
If you are interested, we can show you a sample implementation on the roof of a family house in Bratislava – Dúbravka using the extensive and intensive substrates. For more information please do not hesitate to contact us.