Gardening on the Roof
G
reen roofs are now widely viewed by specifiers as a standard design consideration for a growing number of roofing projects.
Here, John Williams of green roof specialists Blackdown Horticultural Consultants in Somerset, discusses how to ensure the success of a sustainable roof and why he thinks that green roof installation has reached the need for a shift in emphasis, from construction to planting strategy.
With an abundance of green roof suppliers
available, it is becoming increasingly important for architects and specifiers to be vigilant in their choice of system and to work with a supplier who not only understands the installation processes involved, but one who also offers expertise in the planting systems that will ultimately determine the long-term performance of the roof. A green roof is not just another building
product, it is a living element. To get the best possible performance from the plants, consideration must be given to site location, local weather conditions and orientation of the roof, combined with planting options, growing medium, nutrient level, depth of system, appropriate drainage layer, mulching and irrigation requirements.
Giving Green Roofs a Fighting Chance
The most popular form of extensive type roof planting is the sedum system. Sedums, commonly known as stonecrops, are the survivors of the plant world. Unlike most other plants they will live in very little growing medium with low nutrient levels. The three methods of planting sedums on a roof are Sedum Matting, Sedum Plugs and Hydroseeding, with each method offering different benefits. The sedum matting grown by a horticultural specialists in Somerset, comprises of a bio-degradable base layer (the coconut bi- product Coir is being used increasingly for this purpose), followed by 20 – 25 mm of growing medium, or substrate. Sedum cuttings are then spread onto the
substrate and left to grow for about a year. When ready for use, the mat is cut into lengths measuring 1 m x 1.5 m, which are then easily transported to site. For applications where an instant result is required, Sedum matting offers low level planting with an instant coverage of 90 – 95%. Whilst the Sedum Plug planting option does
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not provide instant coverage, it does allow the use of a much greater variety of plant species. The plants are grown in trays then taken to site and planted by hand into the substrate at 18 – 20 per square metre. By the time they are sown onto a roof, the plugs already have a mature root system allowing them to establish quickly. Depending on weather conditions and time of planting, this system will achieve 80 – 90% coverage in around 9 – 12 months. If a project presents specific requirements, such as the need for a purely native species or a particular colour, plug planting offers a much greater choice. Furthermore, by using a higher number of species, if any should fail, the remaining plants will act as back-up, maximising the chance of success. Where there are problems with lack of sunlight, or extreme environments such as marine or coastal locations, species need to be selected for their suitability. Sedum lydium for example cannot tolerate the dry conditions found in the East of England, Sedum anglicum is not as hardy, but will thrive in a marine environment, whilst Sedum album can be grown anywhere in the UK. By ensuring the right species are positioned correctly for their preferred weather conditions, the Horticulturalist will be giving the planting its best shot at survival. For projects where time is no object, the third option is Hydroseeding. It represents the most cost effective form of green roof and is an attractive option for large scale roofs. Hydroseeding is applied in the spring to give the system the longest possible gestation period with 80 % coverage expected in around
15 months. Sedum green roofs require minimum
maintenance, however, all living roofs, regardless of the planting system, will require a certain amount of initial nurturing in order to give the best chance of success. Unlike dedicated roofing installers, those suppliers with a Horticultural background can provide advice on the correct care routine required to get the best from a planted roof.
Brown is the New Green
A popular choice for inner-city locations, or
areas where environmental impact is of particular concern, is the bio-diverse, or brown roof, which aims to replicate a building’s footprint by transferring it to the roof. Local soil and materials are sourced for the substrate and the roof is left to self-colonise, thus encouraging resident wildlife into their new habitat. Ecologists are keen to maximise commercial use of bio-diverse brown roofs and work is currently being undertaken in an intensive research and development programme to increase the variety of plant species they use within any given geographical area. So, as the demand for sustainable roofs
increases, it is important that we don’t fail them at the final hurdle by not placing a strong enough emphasis on initial plant specification and its subsequent care. Taking the horticultural route is the only long-term assurance for a green roof that will contribute to the environmental performance of our buildings whilst offering a viable, robust roofing solution that will continue to thrive and grow for years after planting.
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