Building a Roof Garden

Centuries old, the roof garden concept has been most recently embraced in Europe. Both functional and aesthetically-appealing, this technology improves the structure’s energy efficiency while adding plant life in populated areas.

How Roof Gardens Help

Living roofs are not just good for wildlife:they make a positive contribution to the environment in a number of other ways.

• They help to cool the room below in hot weather. Conversely,in winter a living roof can provide insulation.
• Living roofs act as sponges, retaining water before allowing it to evaporate into the atmosphere.In heavy rainfall,this can reduce the likelihood of local floods and this is one of the main reasons living roofs are now a legal requirement in Germany.
• They protect a roof’s waterproofing from the effects of ultra-violet light and the weather, especially frost.This means that the roof is less likely to leak.

The three types of roof garden

There are three basic types of roof garden and these three types relate to the amount of maintenance they require, the depth of soil and the types of plants the area will support. The three types are:

• Extensive living roofs – Easy to maintain and use shallow soils. Extensive roofs are lightweight so tend to be used on sheds, garages and small extensions. They tend to be quite harsh environments so plants suitable are often ones found on cliffs and other harsh environments. However, whilst these roofs are easy to maintain they can also lack visual appeal.
• Semi-Extensive living roofs – These roofs have deeper soils and can therefore support a greater number and wider variety of plants, making them more decorative. However their depth makes them heavy so they require a strong structure to support them. They can combine the relatively low maintenance of Extensive roofs with a more aesthetic appearance.
• Intensive living roofs – Intensive roofs are big old things capable of supporting full gardens and trees. As such they require large and strong support structures and are not suitable for the majority of domestic buildings. Therefore they won’t be touched on much here.

What roofs are suitable?

Flat roofs lend themselves the best to roof gardens although it is also possible to green other roof types such as pitched, barrelled and domed roofs. Here we look at some of the structures typically used for domestic roof gardens:

• Garage – Garages with flat asphalt roofs should be able to support a lightweight living roof such as a sedum blanket or, if a concrete deck has been used, a deeper substrate-based, extensive living roof that can be used to grow wildflowers.
• Sheds – Sheds are often quite lightweight structures so it is often inadvisable to build any greenergy on them without checking for suitable support first, although it is easy enough to encourage moss to grow. It is however quite easy to install additional support to domestic sheds, thus allowing you greater range in your efforts.
• Extensions – It should be possible to build a roof garden on an extension but the type largely depends on the supporting structure underneath.

How to do it?

Ok, now we get down to the nitty gritty part where we help you with actually building your roof garden. To build a garden fit for flowers requires several layers to be constructed:

• Waterproof layer – The base layer. Added to the existing surface, this will give greater security and peace of mind even if the roof is already soundly waterproof.
• Roof membrane – Waterproofing layers, such as asphalt and bitumen, are very susceptible to damage from plant roots and any root penetration may lead to leaks. A pond liner or butyl lining or 300 micron damp-proof polythene should be laid over the waterproof layer and, wherever possible, in one continuous sheet. Otherwise, the sheets should overlap by at least 20cm.
• Filter Sheet – This sheet allows moisture to drain off of the roof whilst ensuring fine materials don’t escape.
• Moisture Blanket – For extensive living roofs, this blanket will ensure that the growing medium contains enough moisture to support life. Commercial ones can be bought which do not degrade but it is possible to use cardboard or old blankets to achieve the same effect.
• Drainage layer – Like the moisture blanket, this helps to retain moisture while allowing excess water to drain away. Commercial systems store water and are made of plastic or geotextile materials. Sedum mat on the roof of an extension.
• Soils and Substrates – The top layer. The growing medium should be lightweight and freedraining yet of a material that retains moisture. Many people use aggregates mixed with light sub-soils such as crushed porous brick and limestone chippings.
• Seeds and Plants – Sow seeds on the substrate, or put in plug plants (small plants in individual cells) and watch them grow!

What can you grow in your roof garden?

There are various plants you can grow in your roof garden, each with their own characteristics. We try and explain the choices below:

• Mosses and Lichens – The lightest living roofs – and the simplest to create – are those supporting mosses and lichens. Mosses are a group of small green plants that do not flower or fruit but produce spores.They require such small amounts of nutrients that many species are able to live in inhospitable places,clinging to walls and stone and tiles waiting for rain.Lichens are composite,symbiotic organisms made up of fungi (which dominate)and algae or cyanobacteria.Food manufactured by the vegetative element of the organism through photosynthesis is enjoyed by the host fungus.As a result,lichens are able to survive extremes of temperature and drought and can colonise surfaces too sterile for most other organisms (including metal,glass and plastic).
• Sedum Roofs – Sedums are the most widely-used plants for living roofs as they have many advantages in terms of hardiness and drought tolerance.Being succulents,they actively store water in their tissues and have a number of ways of reducing their need for water in dry weather. Under conditions of severe stress many sedums change colour from green through to red,purple and brown.Although they are very tough, you must remember that sedums growing on very thin substrates,or on simple moisture mats,may die back and become patchy during periods of extreme dryness.
• Wildflower RoofsThe conditions on a living roof (free-draining substrates with low fertility) are ideal for the creation of highly diverse and species-rich grassland plant communities.More often than not,these dry grassland ‘roof meadows ’are more successful than those that people try to grow in their gardens.
• Calcareous grassland living roofs – The use of limestone chippings and mixtures of crushed brick and concrete will allow a calcareous meadow to develop.Such meadows are very species-rich and can still be found in the wild on steep slopes and the remnants of unploughed pasture. The soils on these habitats are thin, typically between 50 mm and 100 mm – the same sort of depth as that on an extensive living roof.In the wild,these grasslands can support as many as 30 to 40 species per square metre,so even a small living roof will,potentially,be home to many different plants.

Maintaining your Roof Garden

Most people want a living roof that is low on maintenance.As with all other types of garden and landscape, the amount of work needed will depend on the intended outcome!If a perfect,manicured green space is what ’s required then the area will need a lot of attention.However, extensive living roofs need relatively little maintenance.Semi-extensive areas will need more attention to keep them looking good and to maintain their diversity of species.

• Sedum Roofs – If a pristine green carpet of sedums is the goal then this will mean a fair amount of weeding to control invading plants. This may have to be done two or three times a year. However a less intensive regime will result in the development of more mixed vegetation, as grasses and other plants invade.
• Wildflower Roofs – For extensive living roof types it should be possible to develop a wildflower meadow with little or no intervention. Low fertility substrates will give rise to short vegetation that will not need cutting back each year.

Happy Roofing

Designing and tending a living roof in the garden should be fun.If,over time,things do not develop quite as expected then add a little more substrate where possible or sow a few more seeds,or both.Get up on to the roof and have a look at the bugs and beasts that you have attracted.Above all,let other people know and help spread the word!

Oh, and please remember to seek out and follow all relevant safety advice when accessing roofs and high places.

Picture Credit: http://www.flickr.com/photos/nataliemaynor/2716695967