Substrate for Green Roofs
The growing substrate supplies water and nutrients to plant roots, ensures gas exchange at the roots and provides anchorage to support plants.
Growing substrates for green roofs are typically composed of a mix of inorganic (mineral) and organic components. They can include scoria, ash, pumice, sand, coir, pine bark, porous, chemically inertfoams and even recycled materials such as crushed bricks, and roof tiles. Organic matter is usually kept to a low proportion (typically 20 per cent or less) because it has a relatively short lifespan, degrading and slumping over time, and may become water repellent and difficult to re-wet if it dries out. The physical and chemical properties of the substrate mix, together with its depth and total volume, influences what vegetation can be supported on the green roof.
A substrate should:
- have a known saturated weight loading, that forms part of the structural load capacity of the roof; this is referred to as the saturated bulk density
- drain freely, to reduce waterlogging and prevent inundation during heavy rain, but also retain adequate water to sustain plant growth outside of heavy rain events
- be stable over time, usually achieved by using a high proportion of mineral components and a lower proportion of organic components.
Installers of green roof systems will be able to arrange the supply of a suitable substrate mix. See here for more information on substrate characteristics.
Transport and installation of growing substrates requires consideration in the pre-construction planning process. Most can be installed by either lifting bulker bags by crane or ‘blowing’ from a truck-mounted hopper, although each project will have its own specific considerations.
A range of mineral and sheet mulches can be used successfully on green roofs but need to be considered carefully. Organic mulches, particularly fine materials, are generally unsuitable for use on green roofs as they can be easily blown off the roof, degrade rapidly, block drains, or create a fire risk in hot dry conditions. The saturated bulk density of any proposed mulch layer must be included in the weight loading calculations for the green roof build-up.
On large green roof projects, growing substrates can be installed by crane or by ‘blower’. Substrate may be delivered in multiple bulker bags, typically of one cubic metre capacity, although a larger crane lifting bags of greater volume may be possible if site access permits. Care must be exercised in point loading of substrate on to the roof and movement across the roof. ‘Blowing’ a substrate involves the use of a compressor pump and hose to blow the substrate up to the roof. Sometimes substrates that are ‘blown’ onto a roof will have altered properties (due to finer particles aggregating during the process) and may need re-mixing on the roof to ensure success. Growing substrate should be delivered before the day it is to be installed and stored as closely as possible to the crane, with clear access for the forklift or other machinery that will transport it to the crane lifting point. Growing substrates should be installed with minimal handling and be ‘moist’ to reduce the release of fine particles into the air. Appropriate personal protective equipment should be worn: gloves, dust filter masks, safety glasses and hardhat.