Traditional roofing is designed to shed water as quickly as possible from the roof surface. Conversely, green roofing actually holds water on the roof. It may also make maintenance more difficult and can cost up to 50% more to install. So why would you consider building a green roof?
What is ecological roofing
A green roof (GR) provides a base on top of a structure in which various plant life may grow. Recent GR progress has been developed in Germany. About 10% of the buildings in Germany are now green. The covering is built up in layers, each layer fulfilling a different function. The base layer sits on top of the substrate and provides a waterproof seal to the building. This may come in the form of metal tray roofing or a synthetic membrane. A drainage layer is added to allow water to evacuate the roof, followed by a filter and root protection layer. The root protection membrane prevents plant growth from puncturing the waterproof membrane and causing leaks. The final layer is the growing medium in which the plant growth is bedded. Green or “living roofs” can be divided into three main categories. Extensive, Intensive and Bio diverse.
Extensive green roofs
Extensive GR, (also called Eco roofing) is used to promote the growth of mosses, herbs and grasses. The growing medium is relatively shallow with a depth of between 60-200mm. The weight of the plant medium is 60-150kg per square metre. Eco roofing is used where there is little or no human foot traffic and low maintenance roofing is required. This type of roofing is the cheapest to install.
Intensive green roofing
Roof gardens and park gardens are constructed using Intensive green roofing. This category of roofing offers the widest range of planting opportunities. Shrubs, flowers and even trees can be utilised. The depth of the growing medium is between 150-400mm. This pushes the weight per square metre up to 180-500kg and requires a significant structure to support it.
Bio diverse green roofing
Bio diverse roofing is designed to be left alone to reproduce a natural ecosystem. Soil levels on the surface are varied to create a wide range of different habitats for different species. Local plants are grown and wind swept seeds are allowed to take root. Bio diverse roofing is often aimed at providing a particular habitat for a threatened species.
The benefits for the building
Green roofing can turn an unattractive urban roof into an aesthetically pleasing feature. As well as the aesthetic value added, living roofs provide numerous financial benefits. The waterproof membrane on roofs is subjected constantly to Ultra Violet light. These UV rays break down the surface of the roofing causing leaks and the need for early replacement. The growing medium of living roofs provides a protective layer preventing damage from the sun. This can double the life span of the roofing and recoup the initial extra cost of installation. Further, green roofing provides a thermal layer keeping the building cooler in summer and warmer in winter. Planners within local government have now realized the benefits of ecological roofing. Many district councils offer financial incentives to developers using green roofing on new buildings. This trend will continue as the need to reduce carbon emissions increases.
The benefits for the environment
Urban roofs built with traditional roofing materials absorb the suns rays and emit them as heat. This has led to the urban heat island effect. This effect is most noticeable at night with urban areas being warmer than surrounding areas. The air temperature in London is about 6 degrees higher at night due to the urban heat island effect. Green roofing does not absorb the heat radiation so the effect is mitigated. Thus, the need for air conditioning may be reduced in hotter climates.
Mass urbanisation leads to high water run off and flooding. Huge areas of concrete and tarmac prevent the natural drainage of heavy rainfall. Green roofing absorbs between 50-75% of rainfall, gradually releasing it back into the environment. This evaporation process also cools the building reducing energy consumption and carbon emissions even further.
Depending on the growing medium and the species present, green roofing can store up to 239 grams of carbon per square metre. It provides a valuable habitat for plants and insects as well as providing food for bee’s and seed eating birds. In effect ecological roofing replaces the green footprint removed by the building.
Modern roofing materials provide a lighter, easier to maintain option for eco roofing. Lightweight mats and module trays allow localized maintenance without the need to take the whole roof apart. Considering the huge financial and environmental benefits of living roofs, the future for green roofing looks rosy.