Roofconsult Website Greening up the Rooftops by Andy Patel of Kingspan Insulated Panels
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There are a number of ways we can make our roofs more environmentally sound: improving energy efficiency through optimum insulation and better air tightness for example. But what about the option of literally greening up a roof? A green roof enhances thermal performance, helps to retain storm water (thereby aiding flood management), cleans the air, reduces dust and smog levels and lessens the urban heat island effect in cities. In a country setting it enables buildings to blend in with their surroundings; in a city it breathes green life into the glass and concrete jungle. The benefits are multiple and significant.
Of course the idea is far from new, with evidence of turf roofs on buildings going back as far as 2500 BC in the Orkney Isles of Scotland, providing excellent thermal protection from the often harsh weather, and weather protection could once again be a crucial factor in the rising popularity of this highly environmentally friendly construction genre.
For example, the issue of flood management is particularly important in light of the severe flooding experienced across the country last year. Green roofs will not solve the problem of persistent rain, but they can certainly reduce the impact and should be a consideration in any future developments, particularly in vulnerable areas.
If all of the above benefits are not enough for you, a green roof creates a protective layer which increases the life expectancy of the roof and provides a heat shield in the summer. It visually enhances quality of life for the occupants of the building and those surrounding it, and increases the available natural habitat for animals and plants. It also provides a very effective barrier to noise, greatly enhancing acoustic performance. Last but not least, it utilises what could otherwise be dead space, offering immense recreational and educational possibilities.
So, what is involved in building a green roof? Well, in the first place it is important to understand the various types available, as they all have different attributes, advantages and requirements.
There are several types of modern green roof: Intensive, Semi-Intensive, Extensive, and Biodiverse. The latter carries the least commercial or aesthetic benefit, comprising of rubble strewn across the roof to allow the local wildlife to continue to thrive in a city setting – a movement symbolised by the Black Redstart bird that began to thrive in bombed London during the Second World War.
Extensive green roofs are constructed using low maintenance planting such as succulents, grasses and herbs. This type of roof performs the various vital functions of improving the aesthetics of or masking the building, controlling rainwater run off, and improving thermal performance by helping to keep the building warm in the winter and cool in the summer. It is not suitable for recreational purposes, but requires very little maintenance, and can provide a pleasing carpet of colour as well as an ideal environment to encourage biodiversity. This is generally the most practical and popular choice, offering multiple benefits over time following the initial investment.
Intensive green roofs, on the other hand, can be far more flexible, whether they are designed as simple gardens or fully landscaped recreational areas with all the features of a green space at ground level. Added to the many environmental, ecological and economical benefits then is the advantage of a useable outdoor living space, providing people with the beneficial effects associated with being in contact with a green environment. Naturally, this also carries the commitment of high maintenance with it, as any cultivated garden or green space does.
One potential downside of all of these projects is that a green roof can dramatically increase the loading on a building, so a primary factor is to determine whether a building structure can take the additional weight that the green roof imposes, particularly where an existing structure is being retro-fitted.
Clearly this is less of an issue where the green roof is part of a new design, as the loading will be taken into account when specifying the supporting structure. The challenge then is to identify materials and products that are capable of sustaining the high loading without intruding excessively on the internal space or substantially increasing the capital costs.
The additional loading of a green roof varies depending on the type. Although they all consist of the same basic series of layers, the depth of the growing medium and type of vegetation and features will produce significant differences.
Typically these layers would include a waterproof layer, which must be root proof as well as waterproof. A drainage layer, which could comprise lightweight aggregates or preformed plastic cellular layers, performs the task of removing excess water and preventing the plants from sitting in waterlogged soil.
A filter mat will normally be used between the growing medium and the drainage layer to prevent the latter getting clogged. The growing medium itself would usually be a lightweight ‘soil’ artificially manufactured using material such as vermiculite mixed with organic compost, which may then need an erosion mat to prevent wind erosion in exposed areas whilst the vegetation establishes itself.
Perhaps the hardest aspect to assess is the vegetation itself, certainly for an intensive green roof which may incorporate trees or larger shrubs that will grow to maturity, necessitating a long term projection of weight. Any calculations should of course also be based on the growing medium and substrate being saturated.
So what are the likely impacts of this additional loading on a building? Naturally the primary structure must be designed to withstand the weight, but there would also be an expectation of increased secondary steelwork, which would have cost and possibly space implications. An innovative way round this is to use a structural insulated panel that has the strength to reduce or even eliminate secondary steelwork.
Market leaders Kingspan Insulated Panels have recently launched Kingspan Envirodek™, designed to provide a durable, high performance base for a green roof. The system can provide spans of up to 6 metres and a load bearing capacity of up to 500kg/m2 at 3 metre double spans. Kingspan Envirodek™ is suitable for both flat and curved roof applications and can be installed in an efficient single fix application. Its versatility and strength makes it an excellent choice for green roof constructions.
The potential for such a system to revolutionise the market for green roofs is great, as it simplifies the design process, speeds up construction, lends excellent and instant weather tightness, as well as added quality assurance for projects. The elimination of secondary steelwork also helps to keep costs down whilst maximising space. Even more importantly it reduces the overall loading on the primary structure.
Greening up rooftops is already well underway in other parts of the world: In Germany, for example, it is estimated that 7 percent of all newly constructed flat roofs are green, and over 108 million square feet of green roofs have been planted in recent years. With so much at stake in terms of climate change, flood management and thermal performance, not to mention the aesthetic benefits, it would be good to see a similar step change in our approach to roofing in the UK.
A full, free technical advisory and compliance service is available through Kingspan envirocare Technical Services.
FREEPHONE: 0800 587 0090 Fax: 01352 716111
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