A Garden In The Sky

 In Garden Design, News

Roof Gardens

Beautiful gardens can be breath taking but when they are elevated to the skies where they are not supposed to be the experience can become quite magical. Entering a roof garden for the first time can be an unforgettable moment. Living in the smog of a city, suddenly the air on roof garden is fresher, cleaner, the greenery seems lush and you are transported to another world.

Roof gardens are not a new phenomenon. Records show the ancient ziggurats built by of Mesopotamians. Huge pyramidal stone structures built between 4000 – 600 BC with terraces at different levels on landscaped with trees shrubs and plants. The hanging gardens of Babylon are probably the most famous in history. Known as hanging gardens because the vegetation tumbled over the walls of the different terraces.

Credit: History.com

The ancient ziggurats built by the Mesopotamians. Dr Stephanie Dalley of Oxford University Oriental Institute has been researching the mysteries of the Hanging Gardens of Babylon and has found evidence to support their existence

The Romans also created roof gardens. Archaeological excavations of the city of Pompeii give us insight into Roman life. Discoveries show that roof gardens were prevalent in Roman times, often an as an extension of the living room. An outdoor space that was used for dining and socialising much like we use our gardens today. The Villa of Mysteries, unearthed through the excavations, has a U-shaped terraced arcade and plants were grown directly in the soil of the roof.

With increasing development and the need for homes, outdoor space is limited and at a premium. For these reasons the popularity of roof gardens has increased and continues to do so. With the recent pandemic every bit of outdoor space has become even more precious whether it is a small or large terrace or a balcony. A roof garden can create a real impression. One of my clients bought her home purely for the roof space so that she could have a roof garden, which I had the pleasure of designing. However, designing a roof garden can be complex. It is not quite the same as designing a garden at ground level. There are many more considerations involved.

roof gardens
Photo Credit: Richard Bloom. Designer: John Davies Landscapes

A stunning examples of a roof garden with timber benches, bespoke corten planters and lush planting that incorporates multi-stems trees underplanted with a variety of grasses, lavenders, white flowering valerian, purple flowering salvia, veronicastrum and hardy geranium repeated throughout the scheme.

One of the first considerations needs to be the weight loading. Is the structure of the roof sound to be able to take the weight of the garden? The last thing you want to do is create your beautiful garden and then the roof caves in. This could be very costly especially if your roof space is above someone home. My advice is, carry out the proper investigations. Use the expertise of structural surveyors and engineers to be sure that the roof structure will sustain the construction of your garden. Calculations should include the weights of the materials, containers, soil both wet and dry, trees, plants and an approximate number of people that you intend to have on the garden.

The weather is also something that needs to be taken into account. If it’s hot on the ground, it’s a scorcher on the roof. If it’s cold on the ground, it’s even colder on the roof and add to that the wind and the chill factor, the conditions become extreme. The conditions can have detrimental effects on both plants and materials. Choosing materials that will stand the test of time. Hardwood decking such as ipe or balau works well. There is a choice of composite decks on the market but be warned that some of these really heat up. Porcelain tiles have also come a long way and now you can get some that look like stone or wood.

Designer Credit: Emily Erlam. Photo Credit: Richard Bloom

This is a beautifully designed roof garden which has been divided into different rooms through the placement of containerised planting. Movement has been created through the use of plants at different heights whilst repetition in the planting scheme creates rhythm.

Make sure there is drainage. How is surface rainwater going to exit the roof? You do not want water collecting underneath your decking or tile floor to then stagnate and smell. You also don’t want it start causing damp and seeping through the roof to the floor below. It would be a good idea to use a waterproof sealant on the roof surface to prevent any water ingress.

If you are in a built environment, you may want some screening for privacy. A pergola could be included in the design which will not only provide that privacy but will also act as a wind break. Other options are planting pleached trees of an appropriate specie, attaching free standing slatted trellis panels to containers or using some of the beautiful filigree metal panel screens. Choose methods that will allow the wind to travel through and slow it down. Avoid using solid screens as these will only cause more turbulence.

Most roof gardens are of a size where planting will be done in containers. Make sure if you are planting trees that the containers are large enough to keep the root balls stable. I would advise that the root balls be guyed in the container using a dead-man guying system. Be aware of the windsail effect when it gets extremely windy as the tree could rock, fly and cause damage. On a very large roof space planting borders can be created directly on the roof. However, this is where drainage and waterproofing really do matter. Certain plants can penetrate through the roofing felt and cause damage. Root barriers that have no weak points (seams and joins are weak points) should be used. For this type of roof garden use the expertise of professionals.

Design Credit: Adolfo Harrison. Photo Credit: London Resident Magazine

This beautiful roof space feels very cosy and secluded. The verticals of the grape vines and the tall grasses behind the inbuilt seating provide privacy. Water running down the textured wall creates a relaxing sound helping to cut out the sound of the traffic a street level.

When it comes to plants and planting lightened soil should be considered so as not to increase the weight on the roof. This can be used in containers. Make sure there is a sufficient drainage layer under the soil. When planting directly on the roof we have discussed the drainage and waterproofing but be sure to use layers of washed sharp sand and rootzone mixture with a higher percentage of sand. This allows free drainage. Do not use builder’s sand as this holds water.

Photo Credit: Manoj Malde. Design Credit Hay-Joung Hwang

The LG Eco City Garden at the Chelsea Flower Show 2018 represent an immersive and calm roof garden. A tight colour palette of yellow, oranges, silver greys, green and white helps to create cohesion from the indoor space through to the planting and seating area.

On a roof garden, plants need to be really considered due to the conditions. Plants on a roof garden dry out very quickly. This is not just due to the sun. Wind is the killer on roof gardens. It has a drying effect on plants and causes a lot of evaporation. For roof planting I often refer to coastal plants and Mediterranean plants. Plants that grow near the sea or in a Mediterranean climate are able to survive drying conditions. Use plants with smaller leaves, waxy leaves, hairy leaves or greyish leaves. These are all methods of the plant world to reduce water evaporation. I have used mature Olive Trees, Lavenders, Mexican Flea Bane, Pennisetums, Salvias, Rosemary, Perovskia, Panicums,Verbena, Evergreen Jasmine and small Roses very successfully.

Due to the drying effects of both heat and wind on a roof garden, do consider an irrigation system that can be set onto a digital timer. This can be easily adjusted and also stopped through the winter months.

Roof gardens are breath taking and done well they can be so impressive. But wow factors aside I think we all need to be committed to greening up out outdoor spaces and I certainly believe with the amount of construction for future homes that continues to happen at a colossal scale roof gardens as practical useable spaces need to become part of the planning requirements.

Photo Credit: Paul Debois. Design Credit: Manoj Malde

A roof garden designed for a client Hampstead. Bespoke GRP planters are set at different heights. These also act a safety barrier as it was impossible to install any balustrading on the walls of the building. Lush planting is repeated throughout. The colours of the soft furnishings tie back with the floral colours. This roof garden provides a great entertainment space for the client. It has an outdoor dining area, a built-in outdoor kitchen and a chillout area.

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