Monday, 16 November 2009

Green Roofs

Aiming to remain eco-aware in my design work I attended a course on green roofs recently, organised by the Brighton Permaculture Trust and Brighton and Hove Building Green. The tutors were Dusty Gedge and John Little, and we learned about the ethos and theory of living roofs, as well as the practicalities of construction. It was gratifying to meet people from many different countries, and many different disciplines, all of whom were interested in this topic.
Here are some photos of a community building and housing project we visited in Brighton.

What are green roofs?

For anyone who hasn’t heard of green roofs and the benefits they can bring, here’s just a very short introduction. The term refers to a flat or gently-pitched roof (usually less than 30 degrees) with a growing medium laid over a root-resistant waterproofing layer, which supports living plants. Whilst this may seem somewhat unusual to us in the UK, their use is increasing, with cities like Sheffield at the forefront and many others, including London & Brighton, adopting them for many of the situations listed below. They are a centuries-old tradition in Scandinavian countries, and they have been used for a long time in other European countries. Many countries, most notably Germany and many states in the USA, promote their use as part of their town & city planning laws – often requiring a percentage of green roof space as part of the conditions for granting development permission.

Why is this? What are the advantages?

Firstly, and most obviously, is the beneficial effect of adding extra “green space” for aesthetic and ecological purposes – especially in large cities where roof space can be over 75% of the geographical area. However - and here’s why they’re important for planners and commercial interests - they have solid economic benefits. Green roofs can increase the thermal insulation of roofs, resulting in lower energy requirements (and hence lower carbon footprints) for winter heating and summer cooling. The presence of the green roof can help to protect the waterproofing of the roof by removing exposure to UV radiation and reducing the temperature extremes encountered - which prolongs the life of the roof. The evapo-transpiration of the vegetation produces a cooling effect that mitigates the higher temperatures encountered in cities (the “urban heat-island” effect), improving the climate, reducing humidity, and lowering energy demand. The vegetation can also help improve air quality by filtering out contaminants and adding oxygen. Green roofs help to reduce both the volume and rate of rainwater run-off which needs to be managed during storms, giving economic benefits in reduced storm-water engineering requirements and costs.

Don’t they damage buildings and become a maintenance chore?

The answer is most definitely NO! Provided that the green roof is designed to be within the load-bearing capacity of the building and the root-protected waterproofing and drainage is properly addressed, an extensive green roof is very low maintenance.

There are several “flavours” of green roof, mainly associated with the depth and fertility of the soil (more properly, “substrate”) and hence the type of planting which will succeed. These are broadly grouped into:

1) Intensive: these have deep, fertile soil and the planting (perhaps even including trees) is very similar to that in a ground-level garden – in fact, they are more usually described as a roof garden. They require a very high level of maintenance, just as would a ground-level garden, and don’t offer much in the way of additional ecological benefit since the planting & habitat is so similar;

2) Extensive: these have relatively thin substrates – typically from 50mm (2”) to 200mm (8”) thickness and are deliberately of low fertility. In this way they provide very different conditions to ground-level gardens and are colonised by very different plants – mainly low-growing, drought-tolerant, nectar-rich flowering species, often & ideally native, which gives food & habitat to a wide range of insects & invertebrates, and the birds which feed on them. They are very low maintenance, and this form is what is normally meant by “green roof”;

3) Semi-intensive: refers to a compromise between these two extremes, where varying conditions are provided across the extent of a (reasonably large) roof space, such that a mix of habitat and planting can be used – even including food crops.

Where would an extensive green roof be used?

Around the home - on free-standing structures such as sheds, garages, summerhouses, garden offices, fuel tanks & bunkers, etc. or on parts of the main building such as flat-roofed houses or extensions to them;

At schools - they could be used on similar building structures, or for cycle shelters and outdoor shelters or classrooms, perhaps associated with school crop or wildlife gardens;

In Streets - they could be used for street furniture like bus stations and shelters, train platform covers, electricity sub-station roofs and so on;

For commercial properties - they could be used over factory units and warehousing, supermarkets and their trolley parks ... the list is endless!

You can find out more by visiting

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