Planning matters

Last month in France a law came into force requiring all new commercial buildings to be equipped with either rooftop vegetation (a green roof) or solar panels. France is the first country to enact such a requirement.

Whilst the benefits of solar panels are widely appreciated there is, in my experience, less understanding amongst architects and developers in the UK about the benefits of green roofs. The concept of a green roof goes back as far as the Hanging Gardens of Babylon, one of the Seven Wonders of the Ancient World, believed to have been a series of intensive green roofs built by Nebuchadnezzar II for his wife who was homesick for the plants of Persia. These days, green roofs are generally one of three different types: intensive – parks and gardens including urban agriculture; semi-intensive – garden green roofs; and extensive – natural low maintenance green roofs. The latter are the most common these days in the UK and can take the form of either sedum or biodiverse roofs.

In the past green roofs were seen primarily as a way of satisfying planning requirements to provide external amenity space in urban areas. However more recently it has become clear that they offer a wide range of other benefits which, if properly understood, could increase their attractiveness to the development industry.

Green roofs help to reduce urban heat islands which result from roads and buildings becoming impermeable and trapping excess heat.  In warm climates, this can result in cities becoming nearly 3 degrees centigrade warmer during the day and up to 12 degrees at night. Green roofs are now known to decrease the heat transfer through the ceiling which improves the energy efficiency of buildings. Recent research by Universidad Politécnica de Madrid[1]   shows that, when vegetation density is high, the incoming heat into the building through the roof is 60% lower than the incoming heat without vegetation. Essentially a green roof with high density vegetation works as a passive cooling system, making it particularly suitable for use in warmer climates.

Green roofs impact the process of water cooling in chillers and improve the efficiency of heating, ventilation and air conditioning systems through reduction of temperature around them in the summer. Research into the effect of green roofs in different climates[2] showed that green roofs are able to reduce the cooling energy demand in summertime up to six percent, whilst at the same time having a negligible impact on energy consumption in cold seasons.   In cooler climates, green roofs also act as a wind shield which leads to reduction of heating energy demand.

Green roofs can also make a valuable contribution to surface water drainage, having the ability to limit storm water runoff by between 50% and 90%. They can also help reduce air pollution by providing a natural filter for pollution. Green roofs can increase the flora and fauna diversity in urban areas and they decrease the rate of carbon-dioxide emissions by converting carbon dioxide into oxygen via photosynthesis.

An important factor to consider when designing any green roof is the dead load capacity (or weight) that the roof exerts onto a building. Whilst retrofitting can be difficult, new buildings can easily be designed to ensure that they can withstand the weight of a green roof, making them a viable option on many sites.

Admittedly green roofs have higher capital costs than their traditional counterparts. This is particularly true in the UK, as they are relatively uncommon at present. Capital costs for extensive green roofs are generally between 150-200% more that traditional ‘black’ roofs. Since they are currently not common in the UK, there are few specialist contractors available and this is probably one of the primary causes of the increased capital costs. Maintenance of extensive roofs however is about the same as a traditional roof, only requiring visual inspections every six months. With the increased lifespan of a green roof (approximately double that of a tradition roof) the number of times the roof has to be repaired or replaced is halved - meaning that the introduction of a green roof can actually reduce maintenance requirements and costs.

In an ideal scenario, a green roof would be complemented by the addition of photovoltaics to generate electricity.  Research shows that the two uses are complementary, since the cooling effect of the planting increases the efficiency of the photovoltaics. The original draft law put before the French Senate proposed solar and green roofs rather than a choice between the two. This would have removed the choice between technologies and promoted ‘biosolar’ roofs that deliver both biodiversity and renewable energy generation. Unfortunately the Senate concluded that such a requirement would result in “a negative impact on the economic vitality and employment”. Nevertheless, unlike the UK, the tariffs for renewables are still favourable in France and in some parts of the country, there are incentives for green roofs - so biosolar roofs may have a future in France after all.

Back in the UK Lichfields is working on a planning application for a residential development in the north east of England which incorporates an extensive sedum green roof across the entire 1,500 square metre roof area, along with 40 square metres of photovoltaic panels. The size of the photovoltaics in this project has been limited not by cost but by the potential generating capacity of the site.  The installation of technologies which are capable of generating more than 12kWp of electrical energy cannot be assumed to be acceptable to the local electricity network operator.  Even though one may not want to export the electricity generated, the operator still has a say. Anything above 12kWp needs the agreement of the network operator; below 12kWp, no agreement is necessary. In preparing the application, a review was undertaken of the local heat map for the area, which details the likelihood of larger scale electrical generating systems being approved. The heat map showed that the installation of a larger scale system in this location is unlikely to be accepted. To limit objections to the application, the client decided to keep the application to panels with a maximum generating capacity of 12kWp.

It is clear that there are many reasons why biosolar roofs are not being developed at a faster rate in the UK and there needs to be more encouragement to deliver such schemes. If such schemes are viable in north east England, then there is no doubt that they could be successfully delivered elsewhere in the UK as well.