Sustainability: Green roofs

01 Introduction

Green roofs are flat or gently pitched roofs overlaid with a growing medium and plants, creating a habitat on what would otherwise be a plain surface. They are sometimes called living roofs or vegetated roof structures.

Green roofs are one of the most tangible elements of a sustainability strategy, being visible, natural and distinctive. On many projects, green roofs can be used as a symbol of "greenness" and, as a result, their contribution to sustainability and biodiversity can be overlooked.

In addition to providing an attractive, roof-level habitat, living roofs can also contribute to the management of surface water run-off, the mitigation of local air pollutants and the extension of the useful life of the roof membrane. These benefits are secured in part by the building owner and, if there is enough green roof to make a difference, by the wider community.

02 Planning background and demands

Although green roofs are often a feature of low-impact green development, the planning context for green roofs is increasingly the redevelopment of urban brownfield sites. This is because these sites represent the greatest opportunities to use the potential benefits of green roofs.

Key issues that can be addressed in part by the specification of green roofs on urban brownfield development include:

• Control of storm water run-off in response to constraints on peak flow capacity in the drains and loss of porous land surface through redevelopment
• Negative effects of increased development density, including poor quality urban microclimates
• Loss of valuable habitats for flora and fauna that have increasingly colonised derelict urban land
• The need for green space in dense developments.

Green roofs are therefore part of designers' armoury of solutions to combat some of the implications of PPG3-driven in-town development.

However, living roofs have no specific place in the urban design policy framework and therefore can be treated as another item on the "nice to have" list. With energy efficiency and on-site renewables dominating the agenda, they could end up being way down the list of options.

By contrast, in Germany and Switzerland, green roofs have been promoted as a key element of flood mitigation strategies and there is a flourishing market for them.

03 Green roof technologies

Current green roof technology has been around for 30 years and is well established. Irrespective of the green roof type being specified, it will have the following key components:

• Root barrier - Sits above the waterproof membrane to prevent root penetration.

• Insulation - On inverted roofs, tapered insulation is required to create a drainage fall. The extra weight of growing medium is set off against the weight of pavours or ballast required for conventional roofs.

• Drainage layer - Formed in gravel, granulated clay or preformed plastics

• Filter mat - A geotextile designed to retain fine soil particles within the growing medium.

• Growing medium - Selection and overall depth is determined by the planting strategy and wider sustainability objectives.

• Planting - Planting can range from "ready to use" cultivated sedum blankets, through the setting of individual plant plugs or shrubs, to self-seeding strategies.

There are three broad families of living roofs, all using related technologies:

• Extensive roofs - These have a shallow growing medium and support a narrow range of hardy plants with a short flowering season, such as sedum or some grasses. Fully established sedum blankets can be planted, giving an instant green roof effect. However, because of limited water retention, some extensive roofs can suffer from loss of plants during prolonged dry periods. Extensive roofs have the advantages of light weight and require only limited maintenance. However, the biodiversity contribution of this kind of roof is restricted. Roof systems based on sedum blankets, no more than 30 mm thick, are the most common specified in the UK.

• Semi-extensive roofs - These have a deeper growing medium of up to 150 mm that enables a wider range of planting and potentially the creation of a more diverse habitat. Semi-extensive roofs also have significant capacity for water retention and can be an important element of a sustainable urban drainage system. Planting on semi-extensive roofs tends to rely on individual plant plugs. This means the range of species can be more diverse, but that they take two years or more to mature. Deeper soils also mean a wider range of invasive species can take root and more weeding and maintenance is required.

An evolution of the semi-extensive roof is the "brown roof", designed to replicate the original ecology of a site through the use of recycled site material and self seeding - these roofs have an important role in preserving valuable derelict land habitats, but could be quite sensitive in terms of visual planning.

• Intensive roofs - These are the traditional solution for roof gardens. The growing medium may be up to 1 m deep in places to allow for the planting of shrubs or trees.

Roof gardens are typically planned as an amenity and the species used are often less hardy. Some form of irrigation may be necessary as a result. Roof gardens also require high levels of maintenance. Clearly the design of an economical structure to deal with substantial dead loads will be a key determinant of scope and costs of an intensive roof garden.

Indicative overall thickness, associated live loads of fully saturated roofs and rainfall retention capacity are as per the table attached (See table: A - Indicative thicknesses for roof layers).

When selecting a system and completing the design, the following additional design criteria should be taken into account:

• Climate - local temperature and rainfall
• Location, height and orientation of the roof, related to prevailing winds, sun and so on
• Physical constraints such as structural capacity, overall section depth and so on
• Roof membrane specification, presence of roof falls, including risk of ponding
• Drainage specification
• Access provision - for use of the roof as an amenity and for maintenance
• Visibility of the roof - affecting choice of planting and the speed of maturation
• Maintenance strategy.

04 Benefits of green roofs

Living roofs have a wide range of potential benefits. Many of these are tangible but are not necessarily secured by the owner. For example, unless financial incentives are in place, a reduction in storm water run-off will only benefit third parties who are "downstream", rather than the owner. For this reason, it is difficult to establish an economic case for a green roof investment in the UK. By contrast, subsidies and tax incentives in Switzerland and Germany underpin a market of 15 million m² a year.

The key benefits from green roofs are:

• Sustainable drainage - A typical extensive green roof will fully intercept between 50% and 75% of rainwater and will delay all surface run-off, reducing peak storm water flows and the scale of the rainwater installation required. The filtering action of plants in green roofs is also claimed to prevent pollutants such as nitrates, phosphates and particulates from entering water courses.

• Microclimate - A wide number of claims with respect to the positive effect of green roof vegetation on microclimate are made. These are cumulative and depend on the action of a large aggregate area of roof to make a noticeable difference. The beneficial outcomes include:

• Direct absorption of nitrous oxide
• Direct local cooling in the immediate proximity of the roof, through the effects of evapo-transpiration
• Indirect cooling through the reduction of heat radiated into the air by building surfaces or held and released by a building's thermal mass
• Indirect reductions in the formation of photochemical smog due to reduced air temperatures and the absorption of airborne chemical compounds by the roof vegetation.

• Biodiversity - Green roofs provide opportunities to create biodiversity or, in some cases, to recreate environments that could be lost through redevelopment. The extent depends on the variety of planting and the depth of the growing medium. The roof can be designed to provide habitat for nesting birds and invertebrates as well as plant species.

• Building performance - Aspects of building performance that provide direct payback to building owners include:

• Durability of the roofing membrane, where the green roof protects the roof membrane from exposure to UV and from heat ageing caused by thermal expansion and contraction
• Reduced energy costs. Soil acts as an insulant when dry, so may not be effective in providing insulation during wet heating seasons. Plant cover also contributes to reducing heat loss caused by wind. During summer months, diversion of solar heat gain into the thermal mass of the growing medium can reduce the heat energy stored in the building fabric that would otherwise later be released into occupied space, potentially increasing cooling loads. Results from Germany suggest that the energy saving is equivalent to 2 litres of fuel oil/m2 of green roof/year.

• Aesthetics - The appearance of a green roof is likely to be an improvement on an uncovered, flat roof. Overall visual impact will be determined by the following:

• Time available to mature. Pre-grown blankets or scattered cuttings achieve coverage quickly but offer a less diverse range of plants
• Shallow sedum blankets can only support a limited number of plants
• Extensive roofs can die back during extended dry periods
• All roofs need maintenance, but the requirement increases as the depth of the growing medium increases, and as planting becomes more complex.
• The presence of a green roof may also add to the amenity value and the property value of a development.

05 Cost drivers

• Overall area of green roof
• New build vs retrofit
• Planting strategy including requirements for tree and shrub pits
• Depth of growing medium and drainage layers
• Requirements for forming falls
• Requirements for local strengthening to frame and roof deck/slab
• Access requirements
• Maintenance regime and accommodation for maintenance plant
• Irrigation systems

06 Indicative costs

The following costs are for green roofs with areas ranging from 100 to 1000 m2. The rates include the specialist contractor's costs, but exclude allowances for main contractors' preliminaries and overheads and profit. Costs are current in June 2006 based on an average UK location. Rates are based on the surface area of the green roof.