In the UK, the idea of green roofing has been slow in its development but in Europe, particularly in countries like Germany, it has been part of their green strategy for some time. Green roofing isn’t a new idea, back in ancient times it was a great source of insulation for homes and communities. From the Hanging Gardens of Babylon to the roof top safaris of Sheffield, the history of green roofs is a story of imagination and innovation.
Types of Green Roofs.
Types of Green Roofs While there are an infinite number of potential designs for green roofs, there are two principal types: intensive and extensive. There is another type of green roof, known as ‘semi-intensive’, which lies somewhere between the intensive and extensive varieties. The difference between these types comes down to the depth of the material which is placed on the roof.
Extensive Green Roofs
The substrate depth of extensive green roofs is smaller than that found in intensive roofs, usually around 100-150mm. Given their relative shallowness, extensive green roofs usually serve aesthetic and environment purposes rather than functioning as accessible roof space.
This type of green roof is far easier to install and requires significantly less maintenance than do intensive installations. It is therefore a highly versatile roofing material, suitable for new-builds and retro-fits alike.
This is because the vegetation - usually in the form of mosses or grasses, or a combination of these - is pre-grown and rolled up in mats, which means that it’s easily transportable to the installation site. Extensive green roofs also require little or nothing in the way of structural work. They tend to be much cheaper to buy and install than the intensive variety.
Extensive green roof installations often come ready-made in the form of mats containing 10-20mm of a growing medium and vegetation (known as sedum mats). These mats are usually laid upon another relatively shallow layer of growing medium, which is in turn placed on the filter sheet and protection mat.
In the case of some very lightweight and shallow extensive roofs, it is possible to do without further growing medium, and lay a sedum mat directly upon the filter sheet and protection mat. Irrigation is much less likely to be necessary for this type of installation.
It also possible (though slightly less common) to construct an extensive, substratum based roof, which uses a shallower depth of substrate than that typically found in intensive green roofs. Nonetheless, the range of vegetation that can be grown on extensive green roofs is smaller than that which is possible on intensive ones.
Another less common type of extensive green roof is known as plug planting, whereby seedlings are grown in cell trays containing a growing medium, which are then laid upon a further growing medium, a filter sheet and protection mats.
Extensive green roofs are also specifically engineered to be able to withstand adverse and extreme weather, especially wind, but also extreme high or low temperatures, drought and flooding.
Another possibility with extensive roofs is to install what is known as a ‘biodiverse’ green roof, whereby the habitat of a given animal, bird or insect is reproduced. Biodiverse roofs can serve to recreate the habitats of rare and endangered species. This variety of green roof can include a larger range of different objects (sand, logs, rocks, etc.) being placed on the roof, and not simply uniform mats or plugs.
Intensive Green Roofs
The substrate depth (the thickness of soil in which the vegetation grows) of intensive green roofs is at the very least 120mm, and usually over 200mm, making it much deeper than the extensive variety.
In the United Kingdom intensive style green roofs have been installed on top of skyscrapers and large business premises, such as sites in Canary Wharf in London.
As the substrate depth of intensive roofs is deeper than that of extensive green roofs, they are able to support a greater range of vegetation (flower beds, lawns, trees, shrubs, etc.), as well as water features, benches, gravel paths and so on. Intensive green roofs are often also referred to as roof gardens or parks.
Intensive green roofs tend to be more suited to new builds or large buildings with strong roofs, as the total weight of the installation (including the vegetation, soil, drainage and irrigation systems etc.) is potentially huge. It is, however, possible to make structural improvements to the roof in order to make it stronger. This will require the expertise of a specialist structural engineer.
Moreover, as there is more to intensive green roofs than extensive ones - in many cases they resemble conventional gardens - the maintenance and irrigation requirements are bound to be significant maintenance and the running costs will be higher. By the same token, however, intensive green roofs can have a higher aesthetic impact than extensive ones.
Intensive green roofs can be highly expensive to install, as they require the installation of elaborate drainage systems, and perhaps major structural building work. They also require much more gardening work on site. As such, they are in most cases only viable for medium to large scale projects.
Intensive roofs are best if your primary motive for installing a green roof is that of absorbing water and improving the roof’s lifespan - extensive roofs provide less cover from the elements.
Planning Permission for Green Roofs
For most domestic and commercial installations planning permission is not needed to install a green roof but it is worth checking with our local council for any caveats or hurdles that need to be overcome before work begins. New builds often stand a better chance nowadays if they include some kind of green roof in their initial plans.
Financial benefits of green roofs
There are several practical, business-scenario benefits to green roofs. Among other things new rebates and tax deductions crop up that could help ease the financial burden. Green roofs can help community and state costs, a study completed by researchers at the University of Michigan found that greening 10% of Chicago roofs would result in public health benefits of between $29.2 and $111 million, due to cleaner air. Whilst this doesn’t help in individual project costs, the economic benefits of green roofs to our government will most likely only become more apparent as further studies are taken into consideration
The financial benefits to the individual project are as follows:
• Potentially reduced energy bills for both heating and cooling, as green roofs and walls insulate buildings from sunlight and cold air (though the exact amount saved is unknown.)
• Potential increase in property value. Due to the nature of a green roof and their aesthetic qualities.
• It has been suggested that the lifespan of the building’s roof and walls can be increased (in some cases, doubled). This is due to the protection from temperature differentials and the resulting expansion and contraction that normally shortens a roof’s lifespan. They also provide protection from severe weather damage.
• The presence of green roofs and walls in business premises will also present a positive image to customers and boost a company’s green credentials.
• It has also been suggested that in temperate climates green roofs can reduce the risk of fire (though this view has been contested.)
Ecological benefits of green roofs
There are many ecological benefits when creating a green roof. The benefits are usually increased with greater substrate depths - the benefits associated with installing shallower, extensive green roofs are on the whole far more modest than those offered by intensive ones due to the smaller range of vegetation that can be grown, the reduction of water retention and many other factors.
• Lower carbon footprint due to reduced heating and air conditioning demand. This is achieved by adding mass and thermal resistance value.
• As green roofs and walls reflect less solar radiation and absorb less heat than regular roofs and walls, they have the effect of reducing the urban heat island effect. Urban heat island effect decreases air quality and increases the production of pollutants such as ozone. It all also decreases water quality as warmer waters flow into area streams and put tress on their ecosystems.
• Creation of habitats for animals and insects. Green roofs cool and humidify the surrounding air, creating a beneficial microclimate in urban areas. Planted roofs and walls can compensate for ‘green’ areas lost in building development, often making a big difference in planning permission approval, especially in green zones.
• Absorption of carbon dioxide and pollutants. The vegetation in green roofs bind dust and toxic particles helping to filter out smog. Nitrates and other harmful materials are absorbed by the plants and within the substrate filtering out these pollutants and heavy metals. This also helps improve local air quality, which can benefit both humans and animals.
• In the case of intensive green roofs it may also be possible to grow food or crops.
• Water management. Depending on the green roof design, the immediate water run-off can be reduced considerably, by up to 90%. This has been proven to greatly reduce stress on drainage systems and in turn localised flooding. This can help your rainwater management system, greatly reducing construction costs.
• Noise protection. Plants and trees provide natural sound insulation; they can reduce reflective sound by up to 8dB. They have been proven very effective in noisy areas.
• Green roofs can help ensure that new developments are designed to adapt to climate change.
• The ecological and environmental benefits outlined above are usually increased with greater substrate depths - the benefits associated with installing shallower, extensive green roofs are on the whole far more modest than those offered by intensive ones due to the smaller range of vegetation that can be grown.
• Another important benefit offered by the intensive variety of green roof is the possibility of using the roof space (in the case of flat roofs with safe roof access) for leisure activities, such as gardening.