Green roofs, also called living roofs or eco-roofs, are roof surfaces that support plant growth rather than presenting a conventional hard covering of tiles, membrane, or metal. They range from simple sedum mats that require minimal maintenance to complex intensive plantings that recreate garden environments at roof level.

Types of Green Roof

Green roofs are broadly divided into two categories based on the depth of the growing medium and the types of plants they support. Extensive green roofs use a shallow substrate of 50 to 150mm and are planted with drought-tolerant, low-growing species such as sedums, mosses, and alpines. They are lightweight, requiring no modification to most standard roof structures, and need minimal irrigation or maintenance after establishment. Intensive green roofs have a deeper growing medium of 150mm or more and can support a wider range of plants, from grass and perennials to shrubs and small trees. They require more structural support, irrigation, and maintenance, but can create genuine garden spaces at roof level.

Benefits of Green Roofs

Green roofs provide a range of environmental, building performance, and amenity benefits. They improve the thermal performance of the building by adding insulation above the roof structure and by moderating the temperature fluctuations experienced by the waterproof membrane. They manage surface water by absorbing and retaining rainfall, reducing peak run-off rates and the load on drainage systems, which is particularly valuable in urban areas prone to sewer overloading during heavy rain.

Green roofs support biodiversity by providing habitat for invertebrates, birds, and plants in environments that are otherwise largely impermeable and sterile. They reduce the urban heat island effect by replacing heat-absorbing hard surfaces with vegetation that cools the air through evapotranspiration. They also extend the life of the waterproof membrane beneath them by protecting it from UV radiation and temperature extremes.

Planning and Building Regulations

Installing a green roof on an existing flat roof is generally permitted development unless the building is listed, in a conservation area where the works would affect its character, or unless the change exceeds the permitted development volume limits. Building Regulations do not specifically require green roofs, but any new flat roof or replacement waterproof membrane on a habitable building must meet current thermal performance standards.

Summary

A green roof is a vegetated roof surface over a waterproof membrane and growing medium. Extensive green roofs with sedum mats are lightweight and low-maintenance; intensive green roofs support more planting but need more structure and care. Benefits include improved insulation, surface water management, biodiversity, and membrane longevity. Root-resistant waterproof membrane is essential under any green roof installation.