Treegarden have just completed a new garden shed build in Brighton, featuring a green roof planted with vegetation. Project Manager Chris Hore explains how they did it, and why green roofs are gaining popularity all over the UK.
A client contacted us about dismantling an old concrete shed in the corner of her garden, and rebuilding it in a new location. She also requested that the shed be fitted with a green roof to encourage more wildlife into her garden.
(If you like you can skip the article and go straight to the captioned photo gallery.)
What is a green roof?
A green roof, also known as a living roof, is the roof of any building which has been sealed with a waterproof membrane, then covered with a growing medium and planted with vegetation. The vegetation will bed in and grow, creating a roof-top garden.
Since green roofs are designed to catch lots of rainwater, they naturally become very weighty during periods of heavy rainfall. This is an important consideration when planning a green roof project. To limit weight and discourage water-logging, a drainage layer should be incorporated into the roof’s design, as well as sloping the roof at an angle to allow for water run-off. Guttering, with a water butt below, will ensure the excess water doesn’t go to waste.
Why have a green roof?
Green roofs increase the amount of green space we have in our towns and cities and provide extra habitat for urban wildlife.
Roof gardens can improve quality of life by bringing nature and colour to otherwise drab and uninspiring places. Many gardens in Brighton and other UK towns and cities consist mostly of concrete, brick and paving. The logistical difficulties and costs involved in breaking up and removing concrete prevent many households from being able to plant lawns or shrubbery in their outdoor spaces. The installation of a green roof is in many cases a quicker, cheaper and easier alternative, even providing areas to grow food. Furthermore, the elevated position of a garden on a roof as opposed to at ground level often lifts it out from the shadows of taller objects, allowing vegetation to thrive.
During the winter, a green roof can greatly reduce loss of heat from a room beneath it. In the summer months they provide a cool, sheltered environment. As a sound barrier, the soil or growing medium absorbs low frequencies, whilst the green vegetation itself absorbs higher frequencies. Therefore green roofs situated above recording studios allow less external sound to bleed in or out. A green roof on top of a workshop, on the other hand, helps to contain the loud noises generated by power tools and other equipment. This reduces noise pollution for neighbours and people in close proximity.
Constructing the garden shed
The project started with a disused concrete shed standing in the corner of the client’s garden. The shed had lost its original wooden roof and door due to weathering over a long period. Fortunately though, the concrete walls and supports were still in very good condition. When reassembled they would offer a strong, stable structure on which to create a green roof.
We dismantled the shed and moved the parts to the new location in the garden. In preparation, we had already laid a concrete base at the chosen site. The relocation was no easy task as the concrete slabs of the walls were very, very heavy. Once erected, it wasn’t budging!
Due to the greater weight of a green roof, it was necessary to adapt the shed slightly from its original form. Using 2″ x 4″ timber, we created a cross-beam framework on top of the concrete walls. We designed a slight angle into the framework, to allow for water drainage over an edge. This means that the roof will never fill with too much water and saturate the vegetation. The slope also means that the higher part of the roof will be more arid and the lower part more wet. These different environments allow for a variety of plants to be grown.
With the main body of the shed constructed and reinforced, it was now time to add the green roof!
Creating the green roof
As well as the usual roofing felt and bitumen required for a flat roof, additional layers and materials are needed to create a green roof.
We started by covering the top of the shed with 18 millimeter Marine ply, a high quality hard-wearing plywood. Designed for use where moisture is present, Marine plywood is well suited to outdoor projects such as shed roofs and boat paneling. It also comes in very large boards of around 1.5 meters by 2.5 meters, large enough that we needed only one single board to cover the entire top of the shed. This is preferable as it eliminates the need for joins and results in a stronger, more waterproof roof. We cut the plywood sheet to size and then screwed it securely to the sloped framework on top of the shed walls.
We used a waterproof adhesive to glue strips of standard roofing felt to the Marine ply. Because the end grain (edges) of plywood is porous, we folded the roofing felt over the sides of the roof and pinned it to the timber framework with steel tacks. We sealed the seams between the strips of roofing felt with generous helpings of bitumen.
A green roof requires a further layer of rubbery waterproof liner, with a layer of fleece beneath it to protect the liner from tears. We had previously used Butyl products to line ponds and green roofs, and have found them to be durable, long-lasting and easy to work with. For this project we used Butyl rubber roof liner to waterproof our green roof, with 3000 CBR Butyl Geotextile liner protection underneath it.
We created a fascia around the edge of the green roof with timber boards. This was to hide the layers and give the roof a neater finish, but also to create a shallow box-like structure on the roof to contain the soil and growing medium. The finished box can contain soil and growing medium up to a depth of 8 inches.
Adding the growing medium and vegetation
First we mixed a substrate to be used as the bottom layer, which for a green roof should consist of 30% organic material and 70% inorganic material. We used 30% compost and 70% crushed brick for our bottom layer mix, then filled the rest of the box with our growing medium, a 70/30 mix of vermiculite, a brown clay-like substance, and soil. We planted a variety of dry habitat perennials at the top of the sloped roof, which would be the driest part, and a selection of sedum at the lowest, wettest part. These plants were already beginning to thrive just a few days later, before we’d even had a chance to finish the rest of the project!
Finishing touches for the garden shed
The shed still needed a door. The client had historically acquired a set of old stable doors, and she requested that we use these doors for her new shed. With a bit of restoration and height adjustment they made an amazing and characterful addition to this unusual garden shed.
There were a few final touches still to add. We fitted a gutter along the front edge of the shed, to catch excess rain water as it runs off the roof. The guttering channels the water down a pipe and into a water butt installed below. We then added an exterior waterproof lamp for use at night. Finally, we made a timber bird feeder and fitted it to the side of the shed. The bird feeder even has its own mini green roof!
Garden shed with green roof project gallery
The photo gallery below shows the progress of the garden shed with green roof project from start to finish. If you are in need of assistance with any outdoor tasks, Treegarden offer a full range of landscaping and gardening services. Please get in touch, we’d love to discuss your ideas with you!