Putting a garden on a roof is nothing new – they have been around for centuries. Roof gardens were discovered amid the ruins of Roman Herculaneum while the Vikings used turf on the tops of their huts. In the nineteenth century, a Home for Working Boys in Bishopsgate, City of London contained a roof garden complete with trees like lime, fir, holly, chestnut and plane. Even up to a few years ago, the Kensington Roof Gardens were a popular catering destination.
In recent years, there has been considerable renewed interest in the installation of roof gardens on all types of buildings. They can be found on top of commercial buildings such as offices and distribution centres, on houses, garages and even sheds, stables, bird houses and kennels. Green roofs can act as public parks, covering shopping centres or road tunnels, museums, tube stations, hotels and public buildings.
Types of green roof
There are a variety of types of green roof available. Sedum roofs are the most common, but other versions can include turf, lawns, beehives, planters and even intensive gardens complete with trees and patios. Such roof gardens can become an integral part of a community. Restaurants use roof gardens to grow produce, hotels and bars find roof gardens provide useful outdoor space while commercial greenhouses have been sited on top of office buildings. In urban areas especially, such green roofs are regarded as an integral part of the greening of a landscape.
Reasons for their increased popularity are not hard to find. Roofs provide considerable unused space within cities and towns. In Greater London, roofs account for an area 28 times the size of Richmond Park. Roof gardens are also environmentally friendly and can help mitigate extreme climatic conditions since they serve more than one function.
Roof Garden benefits
Installing a green roof can provide useful insulation. The presence of vegetation reflects solar heat away from a roof, and reduces the amount of heat leaving the building in winter. During the summer, it provides shade from strong sunlight, helping keep buildings cooler inside thus decreasing the need for air conditioning.
An additional benefit is that the extra layers involved in creating a green roof helps reduce noise pollution due to the natural insulation that is provided. It is also an anti-pollutant measure. The vegetation acts as a filter for polluting chemicals and carbon dioxide, helping improve the air quality within urban areas during the summer months when ozone levels can be high.
The presence of vegetation on a roof, helps biodiversity within an area. Green roofs provide new habitats, encouraging the development of nesting sites and plants suitable for bees. The creation of large scale green roofs are often included in biodiversity action plans within urban areas, or can be required to enable buildings to merge into a locality. In Suffolk, the Admans Brewery Distribution Centre was built in an old quarry. A large green sedum roof formed an essential part of the development since it allowed the building to be almost hidden from sight thus minimising the visual intrusion of the development. Beekeepers are increasingly using green roofs as a valuable source of nectar for their bees.
St Ermin’s Hotel, Westminster houses a kitchen garden complete with greenhouse, herb garden, fruit and vegetables on the rooftop. Among the produce grown on the roof are tomatoes, pumpkins, French beans, potatoes, olives, clementines, apples, plums, raspberries and blackberries. Just below the main rooftop, the third floor roof is home to 350,000+ bees, which visitors can see safely from behind glass. The roof top garden acts as a break-out space for the kitchen staff reflecting how the rooftop can open up useful extra living space. Depending on location, space availability and weight restrictions this can range from a pleasant seating area to a public amenity.
Green roofs can also play an important role in dealing with rainwater. The vegetation and substrate absorbs rainwater. This decreases the amount of rainwater leaving a roof especially during storms and heavy rain. This in turn reduces the risk of flooding as far less water will be entering the drainage system. Many large scale green roofs incorporate drainage systems allowing surplus water to enter holding tanks for use during dry spells.
Main problems
The mere fact that vegetation is being placed on top of the roof of a building, no matter what the size, can instantly create problems.
Poor installation
Badly fitted green roofs can cause damage to buildings, and long term leaks affecting the building interior. Careful attention has to be paid to using the correct type of substrate, waterproof membrane and calculating the weight bearing nature of the roof. A green roof can add significantly to the overall weight, especially after heavy rain.
Poor maintenance
Whatever the type of roof garden chosen, provision has to be made for maintenance. Even low maintenance sedum roofs will need some care and attention throughout the year in order to maintain its appearance and effectiveness. During very dry periods, it may require watering. Consideration should be given to the installation of watering systems from the outset.
Weeds will inevitably appear on the roof, borne by the wind or insects. Access will be essential to remove these weeds otherwise they will overcome the original planting. Deep-rooted weeds like dandelion or even tree seedlings such as sycamore can easily damage the roof construction.
There may be a need to remove autumn leaves, or even to mow turf – which can create further problems depending on the slope of the roof.
Angela Youngman
Green Roofs: A guide to their design and installation Crowood.com
Recycling in the Garden Pen & Sword.co.uk