How green is your rooftop?

Here we review green roofs as an option for your self-build: why you should consider them, what ranges and varieties are available and the practical issues to be aware of

One of the great joys of building your own home has to be the ability to do something different, such as foregoing traditional slates or tiles and going for a green roof instead.

Of course it is hardly a new idea – green roofs have been around as far back as 2500 BC in the Orkney Isles of Scotland, and the excellent thermal protection they provided from the often harsh weather is just as important today. What is new is the range and variety of green roofs that are now available, and the different systems available that make them relatively straightforward to install.

Why do it?
There are lots of reasons to consider having a green roof. In a country setting it allows buildings to blend in more with their surroundings; in built up areas it injects a breath of green life into the hard architecture around it. Green roofs can enhance thermal performance, clean the air, reduce dust and smog levels, and lessen the urban heat island effect in cities. They create a protective layer which increases the life expectancy of the roof and provides a heat shield in the summer.

They are also very good at retaining storm water. The issue of flood management is particularly important in light of the increasing amount of severe flooding experienced across the country in recent years. Green roofs will not solve the problem of persistent rain, but they can certainly help to reduce the impact.

A green roof will increase the available natural habitat for animals and plants, as well as provide a very effective barrier to noise. Last but not least, it has the potential to make use of what could otherwise be dead space, and provide a garden where otherwise there would not be room for one.

If you are convinced that a green roof is the way to go, the next thing is to consider what kind of green roof would be right for you.

Intensive or extensive?
There are several types of modern green roof: intensive, semi-intensive, extensive, and bio-diverse. The latter carries the least benefit, simply consisting of rubble strewn across the roof to allow local wildlife to continue to thrive in a city setting – a movement inspired by the Black Redstart bird that began to thrive in bombed London during the Second World War.

By contrast, intensive green roofs can be designed as simple gardens or fully landscaped recreational areas with all the features of a green space at ground level. These provide a fully useable outdoor living space, which naturally needs to be maintained, as any cultivated garden does.

By far the most popular choice is an extensive green roof, which is constructed using low maintenance planting such as succulents, grasses and herbs. Sedum is frequently used, as it is hardy, low growing and stays green all year round. This type of roof is not suitable for recreational purposes, but needs very little maintenance, and can provide a pleasing carpet of colour as well as an ideal environment to encourage biodiversity.

Practical issues
One thing it is crucial to bear in mind is that any kind of green roof can dramatically increase the loading on a building, so a primary factor is to make sure that the supporting structure can take the additional weight that a green roof imposes.

The additional loading varies depending on the type of green roof selected. 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, then a drainage layer, which could comprise lightweight aggregates or preformed plastic cellular layers, and 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 while the vegetation establishes itself.

Perhaps the hardest aspect to assess is the vegetation itself, certainly for an intensive green roof which may have trees or larger shrubs that will grow to maturity, so this needs to be taken into account. Any calculations should of course be based on the roof being saturated, as the building could also end up supporting a significant volume and weight of water. These loading issues must be properly calculated and included in the design and specification to avoid any structural problems.

Whatever kind of green roof you opt for, as long as it has been designed and constructed properly, it will enhance the lives of those living under it and around it for many years to come.