roofs or eco-roofs have existed for thousands of years¹. They may be popping up on different buildings in our cities but they are an ancient invention. The Hanging Gardens of Babylon, one of the Seven Wonders of the World, used an elaborate irrigation system to create a terraced garden paradise outside of modern-day Baghdad while Europeans once used traditional sod roofs as a means of insulating houses.
Today, green roofs are making a come-back and in some cases they are already prevalent: in Germany, 14% of all roof area is green².
What is a green roof?
Simply put, a green roof is one that allows for the growth of vegetation. It includes a waterproofing layer, a root barrier, a drainage system and growing medium for the plants¹. They are also relatively easy to construct and can be fitted to most buildings with a relatively flat roof. More specifically, they work best when there is less than 20-30 degrees of slope².
The roof will also need to be able to take the weight of a full garden. This is not only the dry weight of the garden but the additional weight of any rainfall or irrigation activities. Most roofs should be able to withstand this weight, but conducting an expert survey might be advisable.
Installation of a green roof
If you are looking to install a green roof, you may wish to first install a protection board that covers your normal roof. A layer of waterproof membrane is placed on top, followed by a layer of soil and on top of that the preferred vegetation is planted. Some roofs, called extensive roofs, have a thinner layer of soil and are best suited for plants with shallow roots such as mosses and sedum. Other roofs, called intensive roof gardens, have a deeper layer of soil or other growing medium and so can sustain hardier plants including trees with stronger roots.
The waterproof membrane is normally made from a material such as ethylene propylene diene monomer, a rubbery material similar to the lining put down when you build something like a duck pond in the garden. This layer prevents water getting in but also stops roots from the plants growing down into the roof. There is also a possibility of adding a further insulation layer that can be placed either on the roof beams or above the waterproof layer²,³. To effectively maintain and water your garden, a filter layer, a drainage layer and a water retention layer are placed. These various layers also help ensure that the soil is not washed away. On top of that, a mix of porous material and soil is placed which is the growing layer.
It is important that the growing medium is as light as possible but still allows plants to flourish. Installers of green roofs rarely use soil as the growing medium because it is heavy and packs tight after repeated rains, reducing water retention and aeration for plant roots. Instead, manufactured materials such as granulated clay or shale. Organic compost and fertilizer is added as nutrients⁴. Lastly, the selection of plants will depend on whether you opt of an intensive or an extensive green roof.
A green roof maintenance
Maintaining your green roof is no different to maintaining your garden. Extensive green roofs are easier to maintain compared to intensive ones, but they all need weeding and cutting. This is why it is important to ensure that there is safe and easy access to the green roof. What is more, you can choose to cover either the entire roof or just a part of it.
Further to adding green to our cities and other urbanised areas, another type of roof is now also beginning to provide shelter to wildlife such as birds and insects. These type of roofs are called biodiverse or brown roofs and work in a similar way to green roofs but have individual characteristics that attract certain fauna and might include sand, stones and specific plants.