Using recycled secondary materials for effective green roof substrate
Dr. Chloe Molineux (firstname.lastname@example.org)
My PhD research was to look at the properties of different green roof substrates and determine how effective they were as growing media. This involved looking at the key components of the substrates and the effect of varying depth on plant growth and coverage. Green roofs are very different habitats to those at ground level. In order to successfully produce a vegetated area several meters high, a great deal of science must be applied to the system. It is not just as simple as replicating what is on the ground at roof level – many factors must be considered: weight, moisture retention, maintenance, wind speeds, temperature and sun exposure, to name but a few. Green roofs (of any type) require a growing medium for plants to establish in. On the ground this is in the form of ‘soil’ but on a roof pure soil is not suitable for three main reasons.
The first is the weight aspect; soil is just too heavy in many cases. The second is maintenance – soil is generally too nutrient rich for green roofing. Ideally plants need to be kept in low nutrient conditions to stay under control so they do not need to be cared for like they might in a garden. Lastly habitats at ground level are complete, sustainable ecosystems (perfectly balanced so that plants and animals work together). Earthworms for example are a key component in normal soils – they burrow through the soil keeping it well oxygenated. Earthworms have not been found on green roofs and are very unlikely to be able to survive in their harsh conditions (they cannot burrow deep enough to not dry out and die). Thus if just soil were used on a roof there would not be enough oxygen for plant roots and soon the plants would fail in the anaerobic environment.
For these reasons substrate is required. The main components of substrates are lightweight aggregates – generally products such as crushed bricks, expanded clay pellets and crushed aircrete blocks – and a small amount of organics to provide plants with nutrients. The aggregates are all secondary as they are from waste sources adding to the ‘green’ credentials of eco roofs. Currently, defective bricks and aircrete blocks that do not meet quality control standards make up approximately 20 % of those manufactured and are therefore crushed and used for other markets (Molineux et al. 2009). This aggregate component is important as it maintains oxygen in the growing medium and the small amount of soil provides the ideal low-nutrient environment for green roof plants. Red brick Yellow brick Expanded clay pellets The type of aggregate used will also influence the properties of the overall substrate, for this reason my research looked at the internal structure of the aggregates and how this related to how much water they could hold. Drought stress on plants during very dry periods is one of the main factors resulting green roof failure.
The substrates must have good water holding capacities (and this is only increased with increased substrate depth), but likewise they must also allow the rainwater to drain away efficiently to avoid stagnation – leading to plant roots rotting and thus plant death. My results indicated that different aggregates appear to absorb water differently, some being more efficient than others. Using a microscope to look inside the aggregate particles, findings showed that it is the connectivity of the internal pore spaces, rather than the overall quantity of pores, that is the most important factor regarding the ability of a substrate to uptake water. Crushed brick and crushed aircrete were particularly effective at doing this and so substrates that Eco Green Roofs supply include these materials.
As part of my project I also set up an experimental roof site to test plant diversity and abundance in various substrates at two different depths (5.5cm and 8cm). This modular system was located on the roof of the Biological Sciences building at Royal Holloway, University of London. Results showed that increasing depth increased both species richness (number of different plant species) and abundance (total number of plants). These results support the work found by Brennisen (2006) and Kadas (2007) where they claim that depth is one of the most important factors to be considered in green roof design. Many green roof modules have failed in recent years due to the want for cheap, light and easy to install systems. For these reasons the key component of substrate has been limited, often to depths of just 2.5cm, whilst other manufactured systems have neglected to include it at all! As previously explained, very shallow substrates cause severe drought stress to plants (often seen by their bright red appearance) and will eventually die. Systems that do not use substrate at all but place vegetation mats directly onto a moisture retention blanket have been known to produce the opposite effect. In very wet weather the plant roots sit in stagnating water and rotting of the roots will occur causing plant death.