Preparation and construction
Growing medium: Soils or Soil substitutes
A living roof for wild flowers will require a minimum depth of 80 - 150mm of growing medium. The ideal growing medium will be able to absorb and hold water, supply basic nutrients and maintain a fairly open structure for root growth. In addition, the material should not be too heavy or contain a high proportion of fine particles that can block filters. These requirements are not usually all found in one material so it is often best to use a blend of materials, preferably from locally sustainable sources. The advantages and disadvantages of the materials commonly employed are reviewed below.
NOTE: care should be taken when selecting the materials for your scheme as some may be deemed unsuitable for particular applications. Our plant based review that follows must not be taken as an endorsement of the suitability of any of the materials for roof construction projects - these issues should be addressed with your designer to make sure that environmental standards are met and manufacturers' guarantees protected.
It is particularly important to remember that this depth of material will be very heavy, especially when wet. It can weigh up to four times the designed load bearing capacity of a tiled roof, so a structural assessment and some reinforcement is essential!
Lime based materials (chalk, concrete etc) are sometimes banned as they may compromise installed drainage and filter systems, and recycled green waste may have water quality issues in its run-off.
Crushed brick and concrete - for wild flowers some of the most interesting results can be achieved using recycled aggregates as a basis for a growing medium ('soil' forming substitute). These materials are generally low in nutrients and often lime rich which makes it possible to grow plants that cannot survive the competition of growth found on ordinary soils. Added to this, in urban situations these materials can usually be sourced locally and there is the satisfaction of physically transforming a piece of concrete jungle directly into green plant habitat!
Crushed brick - is probably the most generally useful of these soil forming recycled materials as it is porous and therefore lighter than other aggregates, can hold water and air within its pores and is often alkaline.
Crushed concrete - is useful as it is lime rich and nutrient poor in character so particularly suited for growing the specialist plants of chalk and limestone 'scree'. Crushed concrete is, however, heavy and will not hold moisture. It is best used in combination with other materials.
Chalk or limestone chippings - where available locally are also excellent materials for plants - unlike concrete they can be quite good at holding water (especially softer chalks).
Sand and gravel based materials - can be used to create urban shingle or dune like communities. Fine sands should be avoided on exposed sites where the material may blow away, or block drainage.
Soil and subsoil - whilst soil is usually a good growing medium it is heavy, often carries a burden of weed seeds and roots, and may be too fertile, so should be chosen and used with care. Soil material with a significant clay or silt content is also best avoided as it can lead to drainage or soil-structural problems on roofs.
Composts/organic matter - raw soil forming materials like crushed aggregates or subsoil will usually need blending with organic matter to supply basic nutrients, hold moisture and reduce root restricting compaction. Well made green waste, domestic or peat-free composts are good but some can be too rich in nutrients so should be used in moderation. Wood fibre or recycled paper fibre are useful alternatives as they decompose and release nutrients very slowly.
Lightweight materials - such as manufactured water-retentive expanded rock granules are frequently incorporated into green roof designs to save weight.
Water retention, Drainage and Waterproofing
A constructed living roof will need to retain enough moisture to support plant growth but drain surplus water so that it does not become waterlogged. It will also need to have a waterproof layer to protect the building below.
Irrigation? -in drier eastern regions rainfall is not consistent enough in an average summer to prevent a shallow 'soil' drying out and the vegetation turning brown with die back. Irrigation is one possible solution especially if recycled water can be used. The better answer is to save water and accept the die back as part of the ecological cycle of your created habitat and select appropriate species that are either drought tolerant or are able to regenerate from seed (eg annuals/biennials). Open sparsely vegetated habitats created in this way are valuable for certain insects and spiders.
A living roof construction will usually contain the following components (from the bottom up):
- Waterproof and rootproof membrane(s) to prevent water and root penetration damaging the building
- Drainage layer - gravel (lightweight) or other material designed to drain excess water
- Filter sheet - geotextile or similar to allow water to drain but retain finer soil material
- Moisture blanket - to hold water (capillary matting or recycled carpet or textiles)
- 'Soil' / Substrate/ Growing medium
To create a diversity of micro habitats, the depth, composition and topography of the 'soil' may be varied. Surface features may include patches of coarse gravel or scattered stones (also help protect surface from heavy rain or wind blow), small boulders or logs.