A Roof Garden or Green Roof is quite simply a space on which a layer of suitable substrate and selected plants are installed. Roof gardens are broken into two broad categories, Intensive Roof Gardens and Extensive Roof Gardens.

Key Components of a Green Roof

1. Anti-root membrane
2. Drainage boards
3. Filter and water retention fleeces
4. Speciality growing media
5. Vegetative blanket (Sedums or Grass & Wild Flowers)

Typical extensive green roof

Green roofs help combat the effect of pollution, and has the benefit of rainwater attenuation - reducing flood risk by absorbing rainfall, promote biodiversity, and reduce the 'urban heat island effect'. Already a common feature in many parts of Europe - Paris , Venice , Berlin and Basel , green roofs could bring the same benefits across Ireland .  New construction techniques allow for many different types of green roofs , ranging from a simple monoculture - sedum roofs, to more complex mixtures - natural meadows and wild flowers that can become important wildlife habitats, even to the construction of roof-top allotments and formal gardens, the variations and opportunities are endless.

Specific benefits of green roofs include Rain Water Attenuation

Green roofs store rainwater in the plants and growing medium and evaporate water into the atmosphere. The amount of water that is stored on a green roof and evaporated back is dependent on the growing medium, its depth and the type of plants used. In Summer - green roofs can retain 70-80% of rainfall and in winter they retain between 25-40%. In Germany , the world leader in green roofs, 25 million m 2 of green roofs were installed between 2000 and 2001. This area is primarily down to legal requirements in certain 'landers' for roofs to be installed for their benefits in alleviating storm water run off. In Portland, Oregon - one of the leading cities in USA for installing green roofs - green roof policies are being driven over concerns of storm water run off and the consequences of it on water quality in rivers, and therefore the continued health of rivers for salmon [a key cultural indicator].

Green roofs also reduce and delay run off during times of heavy and prolonged precipitation. A study in Germany has shown that during a 10mm rainstorm, 200 litres of rainwater fell on an 18m² extensive green roof and only 15 litres actually passed from the roof to the ground. Green roofs, therefore, reduce the impact of run-off on the storm water drainage system, and can reduce the likelihood of local flooding.

Research from around the world indicates that Green Roofs reduce annual run-off from roofs by at least 50%, and more usually by 60-70% - contributing to urban drainage and flood alleviation schemes. Moreover, the rate of release following heavy rainfall is slowed, reducing the problems associated with storm surges. With an increasing need for developments to have limited water run off, the UK Environment Agency now highlight the use of green roofing in their May 2003 publication "Sustainable Drainage Systems (SUDS) - an introduction" and the Agency have been supportive of supplementary planning documents which refer to green roofs - such as the Leeds Biodiversity and Waterfront Development SPD (2005). Given the Irish experience with flooding - we could benefit from the experience of others - and apply their ideas in Irish Planning Policy.

Buildings account for 30% of total energy use and 27% of green house gas emissions. As urban centres continue to grow and develop, the benefits of incorporating Green Roof Systems into new and existing buildings are abundantly clear:

• Insulation benefit of the green roof layer - increased energy efficiency (heating in the winter & cooling in the summer).
• Longer building roof membrane life - protected from the effects of UV radiation. The life span of the roof membrane is at least doubled with the addition of a green roof, thereby reducing resource use in roof replacement and repair.
• Air purification.
• Noise pollution reduction within the building.
• Aesthetic appeal - with the vegetative layer designed to blend with the local environment.
• The building foot-print can become an amenity space - Intensive roof.
• Ambient air temperature reduction - reducing the urban heat island effect.
• Rain water run-off attenuation.
• Green roofs (and other practices such as natural ventilation) reduce the need for air conditioning in the summer and as a result can reduce CO2 emissions.
• New developments lead to a loss of habitats - green roofs can contribute to biodiversity and address local biodiversity action plans. In particular they have been shown to favour many rare invertebrates found on brownfield sites, as well as ground-nesting birds such as skylarks.
• Green roofs contribute to a greener urban environment and quality of life for communities in high density developments.

If you wish to achieve the natural look, using a plant which is hardy, resistant to drought and requires little maintenance, Sedum is the perfect answer. Sedum is a herbaceous plant species which is hardy and drought tolerant making it ideally suited to life on the roof top. Our Sedum carpet consists of a mix of sedum species growing in 25mm - 50mm of substrate rooted through a fibre based mat, which allows the blanket to be rolled up for delivery to site. A Sedum roof will contain a mix of about 10 types of sedum, all of which bloom at different times during the year. This results in a roof surface which is constantly changing in colour as various plants come into bloom. Sedum will grow to only 6 or 8 inches in height, this means no cutting is required. In fact other than a twice yearly visit to remove any wind blown weeds and apply a fertiliser, no maintenance is required.

A Sedum roof may be installed all year round.

• Biodiversity - the variety of plant species in LSL Grasses & Wildflower mix supports a diverse range of bird, insect and animal species. Wildflowers are greatly superior to sedums in this respect.
• Lightweight - it will grow in our specifically developed lightweight substrate of minimum 100mm depth.
• Easy to install - simply roll out like a carpet.
• Low maintenance - Glas Grasses & Wildflower usually requires only one cut per year - ideally in the Autumn.
• Drought tolerant - there should be no need for additional irrigation, except in periods of extreme prolonged drought or where aesthetic value in dry periods is important.
• It can grow on a slope of up to 30 degrees.
• Provides a range of flowers over an extended flowering season resulting in a visually pleasing effect.

• Waterproof membrane

• Drainage layer

The function of the drainage layer is to prevent the pooling of water on a flat roof. In particular extensive green roof vegetation which is drought tolerant does not like prolonged soil saturation. On slopes of five degrees or more, drainage may be effective without the need for a specific drainage layer. A drainage layer may also act as a reservoir in dry times. Drainage layers can be made up of from a large range of materials, varying in weight and cost:-

• Granular materials such as gravel, clinker, lava rock, expanded clay granules.
• Porous drainage panels made from recycled plastics.
• Lightweight plastic drainage modules, some of which also store water.

• Filter mat

• Substrate

The characteristics of a good substrate or growing medium are:

• Ability to absorb and retain water while also having free draining properties.
• Provide anchorage for the vegetative plants.
• Absorb and supply nutrients.
• Lightweight.

Substrate depth for the establishment of a sedum green roof can be as little as 50 - 60mm, while wildflowers need a minimum of 80mm. Low maintenance (slow growing) and drought tolerant grass mixtures require a minimum substrate depth of 100mm. Normal topsoil is unsuitable for use on a roof as it is too heavy and too fertile.

• Vegetative layer

The Glas Grasses & Wildflower mix for Green Roofs is sown as a blend of meadow grasses and wildflowers at the correct density for optimum establishment, supplied ready to roll out directly onto the substrate.

Grasses & Wildflower mixture containing non-aggressive grasses and a range of wildflower species including:

• Cat's Ear Ribwort Plantain
• Cowslip Salad Burnet
• Lady's Bedstraw Self Heal
• Meadow Buttercup Yarrow
• Red Campion Yellow Rattle

Landtech Soils Ltd can provide details of specialist roofing companies who specialise in the supply of waterproof membranes for a green roof installation. We can arrange for the supply of drainage layer materials and produce custom made substrates, suitable for all types of green roofs.
Click here to contact us for details.

Glas Pre-Grown Vegetation blankets   - Other uses

In general, it can be said that Green Roof initiatives within other countries have a high international profile and share one feature in common: although they may be supported by national government policy and strategy, they operate at the regional level and focus on the particular needs of that region. They are also all characterised by a high degree of interaction and partnership between local authorities and governments, local research communities, the green roof industry, and community and regeneration organisations, again delivering solutions to meet regional needs.

To date, there is no such example in the Ireland of such a coherent and integrated initiative to bring forward green roof infrastructure and associated sustainable building techniques. To date, most green roof developments have been on one-off projects rather than as part of overall strategy, often on buildings with an environmental remit. Policies relating to green roofs are advisory , and there is no national standard for green roof implementation or maintenance.