Green and Turf Roofs

Architects, planners, community groups and individuals are increasingly looking for new ways to green the urban environment, and with open space at a premium, roof-tops are seen as an under utilised resource to be developed.

Green roofs give their buildings a living, breathing skin that not only provides attractive greenery and an urban habitat, but also sound and heat insulation for a building, and can relieve pressure on drainage and water treatment works, absorbing and holding back rainfall and airborne pollutants. The green or living roof concept is now well established with many prestige and humble examples constructed in both the drier east and wetter western parts of the UK.

Many buildings have been transformed in this way, including large industrial complexes, schools and colleges, as well as more modest garden sheds, garages and temporary shipping containers. As these building schemes involve creating an environment from scratch they offer novel and interesting opportunities to create something a bit different as compared with landscaping open ground. Control of the type and depth of growing medium, the arrangement of irrigation and drainage, together with the slope and aspect of the roof mean that a wide range of choices exist as to the type of plants and species that can be grown. In general, roof schemes for wild flowers use a shallow depth of growing medium and are designed to develop ‘naturally’ with a minimum of maintenance. If you do not have a convenient roof to green but still want to try the technique you can achieve the same results by constructing a raised bed or sink garden.

Preparation and Construction

A living roof for wild flowers will usually require a minimum depth of 75 – 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.

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.

Construction

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.

Sowing

Seed of a suitable mixture (such as ER1/ER1F or EM6/EM6F) can be sown directly on to the prepared surface or sown in pots to be added as plants.  Seed is best sown in the autumn unless irrigation is provided when seed can be sown in spring, summer or autumn.

Choice of Species

When choosing species, raw soil forming materials like crushed concrete provide an ideal opportunity to select specialist plants which are adapted to periodic drought and nutrient stress but would not be able to compete when sown on typical topsoil.

Pioneer species known to be good at colonizing poor stony ground are good, as are species that can readily re-establish from seeds after drought. As initial establishment on poor materials can be slow, annual wild flowers can be added to also give flowers in the first season. Plants which are attractive or useful to insects add value to schemes. Tall species prone to wind damage are best avoided on exposed rooftops as are ‘weedy’ species with wind dispersed seeds that could cause problems by spreading to neighbouring properties. We have developed a seed mixture which is suitable for sowing on a wide range of substrates.

Grass or no Grass?

Grasses are key components of many ecological systems and important food plants for a range of invertebrates. Many green roof designers however have a preference for mixtures based on wild flowers, with little or no grass component. These have been found to work well when sown onto poor, essentially sterile, substrates and avoid worries about competitive grasses taking over. However for many schemes it is still early days in terms of the progress of succession. Inevitably grasses will in time invade all sites.

The decision to be taken at the design stage is whether to leave this to chance (accepting that ‘weed’ grasses are likely to be the first arrivals) or to pre-empt this by sowing a small quantity of short, attractive but non invasive grasses like Quaking grass and Crested hairgrass from the outset. The final choice of which grasses to include will be influenced by the nature, depth and history of the substrate. Poor, shallow, droughty materials may take a very long time to become grassy (if ever). Deeper topsoil based roofs in wetter regions will become grassy quite quickly so in these circumstances a turf roof approach is perhaps the more realistic or ecologically pragmatic solution and requires a suitable grass mixture with wild flowers and some aftercare (mowing).

Turf Roofs

To some extent green roofs have a historical precedent. In medieval and earlier times most roofs were covered with thatch or turf. Living turf roofs were the norm for low status dwellings and many thatched roofs were planted or turfed along their ridges. Modern construction methods have enabled a revival of this tradition without its failings. Grasses can be sown on roof schemes in most situations. On shallow ‘soils’ in regions with low rainfall, grasses will burn off in summer and cover will be sparse and open. In wetter regions or with irrigation a closed turf can develop which can be green all year round.

Aftercare of sown green and turf roofs

The requirement for aftercare will vary from scheme to scheme. Some will need minimal maintenance, perhaps only occasional tidying cuts for aesthetic reasons. Living roofs with reasonable growing conditions will produce growth that is likely to need annual maintenance for best results. This gardening will involve weeding and mowing along the lines of grassland aftercare. The practical and safety implications of maintenance work at height obviously need to be considered in advance of construction.