A Green Roof

Plants have been used on roofs for thousands of years, from sod roofs in Europe to the hanging gardens of Babylon. But in the last 50 years this practice has evolved into what are now called green roofs, living roofs or eco-roofs. Green roofs are those that have been planted with specific vegetation using a well-researched sustainable design methodology. They are an exciting new development in the sustainable building movement, and are gaining in popularity across the world.

Types of Green Roof

While there is no standard classification for green roofs, they can be divided into two basic types:

• Intensive Living Roofs – these incorporate plants from between 1 to 15 feet high, including shrubs and trees. They require deep levels of soil to support them and a weight-loading roof. They support a high level of plant and wildlife diversity, but require ongoing maintenance and extensive irrigation. They are not suitable for most domestic buildings.
• Extensive Living Roofs – these incorporate low-lying plants from 2 to 6 inches high. They require only a few inches of soil to support them, and only need a low weight-loading roof. They are low maintenance and can be used for any kind of roof, including sheds, garages, houses, balconies, extensions and outhouses, and also commercial buildings.

Both types of green roofs can be used for flat or pitched roof construction. Flat roofs are the most common and the easiest to establish and maintain, but green roofs can have a pitch up to 45 degrees. With sloped roofs, there are design issues affecting drainage and soil loss that need to be carefully considered.

How to Construct a Green Roof

A green roof system consists of layers that mimic natural processes and also protect the building and roof. The basic components are: a waterproof layer, root repellent membrane, filter cloth (to allow water to drain but prevent soil escaping), moisture blanket (to ensure enough water retention for plant life), drainage system (to drain excess water), soil substrate, seeds and plants. The soil is the growing medium and should be lightweight and free draining, but also be able to hold enough moisture for the plants to survive. Recycled aggregates such as crushed porous brick are often used in the soil substrate, with the added benefit of increasing its sustainability index.

Plants suitable for extensive green roofs are low growing, rapid spreading, drought-tolerant, have a fibrous root system (to protect roof membranes), low irrigation and nutrient requirements, low maintenance requirements, use native species, and are allergen-free. Short perennials, wildflowers and succulents such as sedum (stonecrop) are commonly used. To help cut down on planting time, impregnated sedum and wildflower mats are now commercially available. These can be rolled out directly onto the soil.

Living roofs can be designed to grow native plants that might otherwise become endangered, and to encourage a wide range of important wildlife including insect species such as butterflies, bees and beetles, and local birds.

Benefits of Green Roofs

There are a number of social, economic and environmental benefits to green roofs, including:

• Increasing home energy efficiency – cooling in summer, insulation in winter
• Filtering and cleaning toxins from both air and water
• Reducing carbon dioxide emissions
• Retaining rainwater before it evaporates, reducing the likelihood of flooding
• Reducing urban temperatures and associated smog
• Insulating against sound and noise
• Preserving and enhancing biodiversity
• Providing aesthetic appeal and ‘green space’ recreational opportunities
• Using recycled materials like aggregates and plastic sheets

The Movement to Green Roofs

Green roofs are a relatively new sector in the construction industry, but have become a widespread feature across Europe. They are most common in Germany, where over 10% of houses now have green roofs, and the industry is growing at 10-15% each year. Because of their environmental benefits, some European countries like Germany have integrated green roofing into their regulations, and many others provide subsidies and incentives to encourage their development and maintenance.

In the UK there has been a lack of government support and guidance on living roofs, which has hindered their uptake. However, green roofs directly address the UK’s Sustainable Development agenda and this situation is likely to change as new policies and standards are developed to support their design and construction. Green roofs are set to become an increasingly important option for builders and planners, turning dead and dull places into green, living spaces.