Using BSI PAS 100 compost in combination with other recycled materials

Any trials used to explore the potential for the inclusion of substrate in a green roof need to acknowledge a range of requirements specific to the environment. These include: the ability to perform under a range of planting depths; straightforward and safe maintenance needs; and tolerance to extreme weather conditions.

With these stipulations in mind, a trial was developed to assess the benefits of using green compost in combination with crushed concrete, brick,  tile; or granulated fly ash.

Compost is particularly appealing as a sustainable substrate due to its low cost per unit volume and low bulk density. It also offers good water holding capacity, a wide range of beneficial microbes, and is available in a variety of size grades. Its high cation exchange capacity (CEC) is also useful in a green roof medium, which tends to be subject to vigorous leaching action.

PAS 100 green composts of all grade sizes performed well, complementing the properties of commonly used mineral substrates. Compost of 10 - 25mm provided lower levels of nutrients and water holding capacity than a grade size of less than 10mm. This resulted in the larger grade proving itself suitable for a sedum roof, while the smaller grade was more applicable for a meadow roof.Across all treatment media, pH was seen to decrease, stabilising at a satisfactory level of 7.5 – ideal for the majority of plant species.