Substrate vs Substrate Free
Extensive Roof Planting. Substrate and Substrate Free Based Options.
Extensive type planting should be characterised by the following:
- Sustainable. Components should have minimal impact on the environment. Once installed the planted element should provide significant environmental benefit particularly with regards mediation of storm water runoff and improvement in building performance.
- Biodiverse. The roof covering should complement, support and provide habitat for a range of species. This could include plant, invertebrate and bird species.
- Low maintenance. The aim should be the instigation and development of a self-supporting ecosystem on the roof surface. This ecosystem should be naturalistic. Typically, only 1-2 visits per year should be needed to include inspection and maintenance.
- Self-sustaining. Once established the planted element should not require any irrigation, particularly under UK climatic conditions.
Blackdown Greenroofs design extensive roof planting schemes that work within the recognized industry guidelines. Our systems are based upon a formulated substrate as support layer for the plants.
Systems are available in the UK that do not meet the FLL minimum requirement for Extensive type planting. We would term these as substrate free systems. This type of system does have certain advantages:
- Very lightweight. Typical saturated weights of less than 50kg per m² are not uncommon. This can allow significant structural savings.
- Easy to install. Often they are simply rolled out onto the finished waterproofed surface.
- Instant cover. Invariably because of its thinness this type of system is supplied pre-grown with 100% plant cover at point of completion.
However, these types of system often have the following disadvantages:
- Poor long term performance. The build up is insufficient to support the installed plant layer. Rather than flourishing and evolving the plant layer simply persists. Over time there can be a steady reduction in species diversity. Ultimately as the minimal growing medium component is eroded the plant cover density also reduces. The times scales involved for this process can be relatively short. We have seen significant reductions in plant density after only 2-3 years. This poor long term performance can be particularly marked in the drier areas of the UK.
- Minimal species diversity. Often only the hardiest of Sedum species can survive long term on this type of system. Research has also shown that thin layer systems are not attractive as habitat for local invertebrate and bird species. System characteristics can therefore be a diminishing diversity of the installed plant material coupled with little encouragement of either the displaced or surrounding species to use the roof space as habitat. This is significant if encouragement and support of biodiversity is part of the projects directive.
- Irrigation system specified. Adequately designed and constructed extensive green roofs should not require any form of irrigation once established. In our experience irrigation systems are only of use for plant layers installed at high pitch (+30°). Inclusion of any form of irrigation system in the UK is indicative of inadequacies in system design and build up. It is also an unsustainable approach to roof planting.
We would suggest that the short term benefits of thin layer systems should be carefully balanced against the possible long term disadvantages.
We would be pleased to discuss any questions that you may have regarding the above information.