A roof garden is any type of vegetation established on the roof of a building. Apart from the decorative benefit, roof garden serves the purpose of providing architectural enhancement, temperature control, recreational opportunities, habitats for wildlife and food. The method of cultivating food on the rooftop is referred to as rooftop farming. It is usually carried out with the help of container gardens, air-dynaponics systems or aeroponics, hydroponics or green roof. Additional platforms known as “aero-bridges” can also be formed between high-rise buildings in addition to the roof space.
I have been intrigued for years at their use and whether they infact provide an environmental benefit that best befits the cost of creating and running your own “roof-top eden”.
Types of Roof Garden
The following are the three basic types of roof gardens classified based on roof vegetation and maintenance.
Extensive roof gardens
This type of roof garden requires minimum layer of soil substrate and hence it is easy to maintain. These gardens are suitable for storehouses, garages, roofs or other additional buildings around the house. At the same time, it has less aesthetical value as the types of vegetation that grow on this is very limited. Lichen and moss are the best-suited vegetation for this type of garden. Lichen can easily grow on surfaces like plastic, metal and glass. Moss, on the other hand, is a small plant that requires less nutrition for its survival and high soil humidity for its growth. This roof garden is less expensive in terms of vegetation types and construction and ideal for a humid climate.
Semi-intensive roof gardens
This type of roof garden requires a deeper soil layer that increases the diversity of plant species. More stable construction is required for this type due to vegetation, greater burden to soil and water retainment. Vegetation can be done with lower species of the Sedum genus. As these species are succulent, they do not need frequent watering. However, vegetation requires additional watering during prolonged dry periods. Wild flowers that do not require maintenance can be grown easily in such roof gardens.
Intensive roof gardens
Vegetation similar to that of a backyard garden like flowers, shrubs, trees and several park elements can be planted in this type of gardens. However, this roof garden requires large, stable constructions and hence most of the buildings are not suitable for intensive roof gardens. Besides basic maintenance needs, it also demands special attention during irrigation.
Roof Garden Design
The cross-section of a roof garden typically begins with an insulation layer at the bottom, a waterproof membrane for preventing leakages from the building and a root barrier for preventing the penetration of roots via the waterproof membrane. A waterproof membrane that can withstand the effects of acids released from some plant roots must be employed.
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A drainage layer made of lightweight plastic, clay or gravel is the next. Besides maintaining the water level, this layer also aerates the growing media. Drainage points must be accessible from the top as the roof system covers the entire roof. In certain designs, the drainage layer can also store water for later use by the plants. On top of the drainage layer is a geotextile or filter mat that allows water to soak through and also prevents fine soil particles erosion.
Finally, the top layers are composed of a wind blanket, plants and growing media. The growing media is light in weight and helps in drainage while supplying nutrients to the plants. The wind blanket is used to hold the growing media in place until the roots of the plants are formed.
Roof Garden Maintenance
The maintenance of roof garden is perceived to be one of the major hindrances to their installation. However, it is essential to consider the maintenance schedule during the design process itself as all types of roof gardens require maintenance. All commercial buildings with roof garden are required to undergo roof and gutter checks atleast twice a year. The maintenance rather depends on the desired outcome of the client, which may vary from weekly checks during summer in case of an intensive roof garden to quarterly or even twice yearly checks in case of the most extensive roof garden.
Biodiverse roofs and the roofs designed to be low-maintenance will anyhow require maintenance checks once or twice a year to clean drains and gutters and eliminate unwanted debris. Extensive sedum roofs would require a more intensive maintenance system with an application of fertilizer once a year or weeding three times. However, a less intensive regime of sedum roofs will result upon the development of more mixed vegetation. For extensive roof gardens, it is necessary to develop a wildflower meadow. Low fertility substrates will in turn give rise to short vegetation that does not require cutting back yearly.
The following are some of the key factors to be considered during maintenance:
- Efforts should be made to prevent blockage of drains and growth of undesirable plants.
- Application of fertilizer should be kept to a minimum as high level of utilization may result in increased nutrient levels. which may negatively impact water quality.
- Vegetation barriers are important as they prevent the spread of fire.
- It is necessary to have fall-protection systems as maintenance will be carried out within 2m edge of the roof. These systems are required to be maintained once a year.
Benefits of Roof Garden
The key benefits of a rooftop garden are listed below:
- It converts CO2 emissions
- It produces oxygen
- It reduces the heat of buildings and energy costs
- It creates a habitat for wildlife
- It reduces ambient temperature
- It captures and harvests rainwater
- It reduces storm water runoff and discharge
- It creates large catchment areas.