How to build a green roof for a shed or store
If you have a small shed, bin or bike store or even a rabbit hutch you could install a green roof to boost biodiversity, soak up rainfall and turn something functional into something beautiful and good for the environment
What are green roofs?
In urban environments where space is at a premium, green roofs present a way not only to maximise growing space, but also to combat challenges to the environment of towns and cities. Green roofs can help keep buildings cool in the summer and warm in the winter. They also provide a wellbeing benefit for people by increasing our access to plants, gardening and nature.
- Biodiversity – Green roofs can attract pollinators and support food networks in areas where available food sources can be scarce.
- Cooling – Plants absorb heat and cool the air via respiration. Green roof temperatures can be 15-10° cooler than conventional roofs.
- Pollution – Plants play an important role in filtering air while also helping to reduce noise pollution.
- Flooding – Green roofs are especially helpful in mitigating the threats posed by flooding in urban areas, by slowing and absorbing rainfall and preventing drains being overwhelmed.
Developers and property owners have various options to mitigate overheating in urban areas. As dark materials absorb rather than reflect heat, a simple and inexpensive choice is a ‘white roof’ – painting or covering a roof in white material. However, not only is this a short-term measure, it also does nothing to help with other issues such as stormwater, air pollution or biodiversity. Green roofs involve more initial investment and maintenance, but can offer far more in terms of long-term savings and increased environmental benefits. Use captured rainfall for cooling and insulation and slowly release water into the sewage system to prevent flooding. While blue roofs have much in common with intensive green roofs, their emphasis is on sustainable water management. Nature often takes over roofs over time without any planning. Lichen and moss can develop on the surface, eventually creating the appropriate environment for more deep-rooted plants. Brown roofs are designed to encourage plants by featuring a substrate such as rubble and soil to encourage plants to settle naturally. Often called roof gardens, intensive green roofs are flat roofs with sufficient depth of substrate, in beds or containers, to grow a range of plants. These roofs require regular maintenance and often include features such as seating and vegetable beds. Extensive green roofs are designed to require minimal maintenance. The plant material covers most or all of the space. These are what are often described as ‘green roofs’ and are what can be created in small areas such as on top of a shed or bin store.
Blue roofs
RBC Blue Water Roof Garden at RHS Chelsea Flower Show 2013
Naturally occurring green roofs
Intensive green roofs
Extensive green roofs
DIY green roofs
If you are starting from scratch, there are lots of kits available to buy for self-assembly which are well within the scope of the average DIYer. However, if you are looking to create something more bespoke, want to save money, or use reclaimed materials then it is not too difficult to make the structure yourself.
1. Draw a design
Even a basic sketch will give you the chance to make sure your measurements add up
2. Materials
Building your own gives you the freedom to choose whatever materials you want
3. Drainage
A rain chain is easy to install, inexpensive and looks great
4. Layers
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Recycled woolen blankets or jumpers make a good water retentive layer
4. Layers
Recycled woolen blankets or jumpers make a good water retentive layer
- Waterproof membrane – to protect the roof
- Root barrier – to prevent roots damaging the waterproof membrane
- Drainage – to prevent water collecting
- Growing medium – a peat-free substrate mix right for your plants
- Plants – dependent on right plant, right place
To this list you can add a water retentive layer (old wool blankets or jumpers can be used) to further slow rain water, a filter layer (a geotextile which can be purchased online), to prevent the substrate clogging drainage and a top dressing (eg decorative shingle) to keep down weeds.
Kits are available which contain all the layers needed in blocks or mats which fit together, complete with pre-sown plants.
5. Plants
Sedum and sempervivum are a popular choice for green roofs as they happily spread in the thinnest of growing medium
About the author – Jenny Laville
Jenny is Shows Manager for the RHS, is RHS level 3 qualified and has had allotments for over 15 years. She currently looks after a plot in Buckinghamshire.