Green roof: Why use potted plants when you can grow trees on a roof?

Remember the Hanging Gardens of Babylon? It used to be one of the Seven Wonders of the ancient world. Gardens, that too in a desert, may seem like a far-fetched dream, but the Babylonians did it. For them, it was just a statement, but in our times, the lack of green is alarming, and reviving it should be a basic concern.

Dhaka, from a bird's eye view, is nearly all roofs. With a small portion of this mega city allotted to open spaces, trees, and shrubs in scattered chunks; the city is becoming more polluted every day. The lack of connection between the green lands is interrupting the flow of pollination and the diversity of bees, birds, and small animals.

However, room for green cannot be made simply by razing buildings and structures to the ground. What to do, then? Start gardening on the roof. 

Rooftop gardening in the form of landscaping is already popular. However, there are different kinds of rooftop gardening. The focus today is on how to make a green roof. 

Green roofing has come a long way from filling out every corner of the roof with potted plants. A disorganised rooftop kills the space to move around. Although it fulfils the 'green' function of rooftop landscaping, it is not aesthetically pleasing.

The difference between green roof and rooftop landscaping

Although the terms 'green roof' and 'rooftop landscaping' sound similar, architects draw a difference. A green roof is where plants and even trees are planted on the rooftop on a bed of soil, thus giving it a landscape-like form, unlike a random layout of portable pots. 

Soil is placed on specific portions of the roof or the entire surface, depending on the owner's preference, and plants and/or trees are planted onto that soil. 

"The green roof is integrated into the design of a building because there are a lot of factors that need to be considered – like structural load, soil depth, etc," said Sabbir Ahmed Raju, Principal Architect of Geomatrix Architects.

The structural load refers to how much downward pressure a structure can safely endure. Soil depth is how many feet/layers of soil can be placed on the rooftop – as the soil can add a lot of weight – without compromising the structural load limit. 

This form of green roofing can be categorised into two sections: intensive and extensive green roofing. In an extensive green roof, the soil is not that deep, and is only used to plant herbs and grass, and can be turned into a lawn. For an intensive green roof, soil depth varies between six inches to six feet – this method is for planting bigger trees.  

"A combination of both extensive and intensive green roof methods is used in many places. The intensive method can be costly, even though it gives the best landscape experience. It requires a lot of planning and maintenance," said Sabbir.

"Intensive green roofs allow a great variety of plants and trees to be planted on a rooftop. The extensive green roof, however, allows the creation of a natural lawn or a recreational space. It is a great way to turn a roof green and it isn't very costly either," he added. 

The most significant advantage of the intensive method is that actual trees can grow right on the rooftop. However, it is not advisable to plant trees that usually grow very large in nature. No matter how much soil is placed on a rooftop, there cannot be enough room for large trees (such as mango trees) to grow optimally. Their growth will eventually get stunted. It is also strongly advised to plant native plants instead of using foreign species.

Proper drainage is crucial for every green roof. In rooftop gardening, potted plants can be relocated if the floor below dampens. That is not the case for green roofing. Damp-proofing or waterproof treatment has to be undertaken before designing any green roof. 

After waterproofing, a drainage system has to be installed. Additionally, a filtration system also has to be used in order to prevent soil from clogging the drain. 

"Once these are installed, soil can then be placed. All these initial safety layers take up to four to six inches before the soil is spread," said Sabbir.

If green roofing was not integrated into the construction of the building during the planning and design phase, conventional rooftop gardening is advised – using movable tubs and pots of different sizes. 

This is a less expensive and more affordable option and most people can do it without consulting experts. When using pots, the growth and health of the plants are directly related to the size of the pot. This method allows trees to spread roots and grow to considerable heights.

Designing a green roof

If you plant every inch of the lawn or a garden with your favourite flowers, it will look messy.

Both extensive and intensive green roof methods often need to be coupled with walkways and pavements veining through the garden; preferably with lights, sitting arrangement and decorations at regular intervals.

Alongside architects, structural and other engineers, landscape architects are also entering the green roofing scene in Bangladesh. 

"A landscape architect can aesthetically design the landscape of a green roof, keeping in mind the shape of the roof and the clients' needs. Architects give a rough idea about which spots on the roof can be aesthetically pleasing and support heavier loads. Landscape architects design the roof based on that," said Sabbir.

Landscape design on the rooftop will not only give a layout pleasing to view and walk through, but choosing the right plants and trees will attract birds, bees, seasonal butterflies, and on some rare occasions, some migratory birds. This also maintains and somehow recreates the lost biodiversity of nature, which is already being destroyed by rapid urbanisation in Dhaka.

Besides landscaping, rooftop gardens can be a great way to grow your vegetables as well. 

"Seasonal vegetables like eggplants, lettuce, tomato, spinach, etc, can be grown in planter boxes or the intensive/semi-intensive methods. Growing local fruits on the rooftop are also becoming popular among passionate rooftop gardeners," said Alimuzzaman Milton, owner of Smart Garden, a company that provides modern gardening equipment, gardening solutions, and consultancy. 

"Local pomegranate, guava, tangerine, orange, and some breeds of mango are all being planted on rooftops these days."

Tropical or native varieties of Hasnahena, Kamini, Bougainvillea, Jui, and Beli are among the favourites in rooftop landscapes, according to Milton. Ayurvedic or medicative plants are also planted. 

With rooftop gardening using potted plants, these options are limited, but that is not to say rooftop gardening is any less effective than its more pleasant-looking counterparts. 

Even if a green roof was not considered during construction, if a client wants one, there is still hope. 

"We advise clients to use light grower media as an alternative to soil. It is relatively lighter than soil. Using vermi-compost (product of decomposing worms, usually red wigglers, white worms, and other earthworms, to create a mixture of decomposing vegetable or food waste, bedding materials), coco-dust (ground coconut husk) and grain husk, a significantly lighter compost can be used, in place of soil," said Milton. 

"This will nearly halve the weight regular soil exerts. But before using any material or mode of gardening, proper drainage and roof-sloping must be ensured so as not to damage the building," said Sabbir. Milton concurred.