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How to Design a Rooftop Garden

Posted by Bridgman on

Rooftop gardens have become increasingly popular, and for good reason too. Above all, rooftop gardens add beauty and leisure space to cities. They attract birds, butterflies, and other insects, making up for some of the wild habitat these critters have lost. Roof gardens even provide space for small-scale food production. Rooftop gardens also have practical benefits. By insulating the roof they keep the building cooler and lower costs for both heating and cooling. The plants improve air quality by filtering out air pollutants. By holding water in the plants and planting medium, the gardens reduce storm water runoff, an important issue in urban areas. A properly designed and maintained roof garden increases the life of a roof system and the value of the property. But rooftop garden design has special considerations; here are a few important points to think about.  

Load Capacity

You must load the load capacity of the roof before you plan your garden. Ideally, a roof garden will be integrated into a new roof system. Otherwise, you must work around the limitations of the existing roof. The first step in designing your rooftop garden is to consult with a structural engineer or architect. The engineer or architect can determine the load capacity of the roof at different spots. The load capacity determines the type of garden you can plant. The weight or load of the garden includes the plants, growing media, containers, structural and design elements, snow loads, HVAC equipment, water storage, and the people who maintain and use the space. A roof garden can weigh between 100 and 725 kilos per square meter. Where load capacity is minimal, a container garden or raised bed garden may be the best choice. Container gardens weigh less than planted gardens, especially if you use lightweight containers and planting medium.  

Other Design Considerations

Include in your rooftop garden design plan:
  • Access to the roof;
  • Emergency exits;
  • Enclosures such as guards and railings;
  • Handicap accessibility, as required or desired;
  • Watering systems (roof storage or automatic system);
  • Windbreaks. Lattices and trellises reduce the flow of the wind without trying to stop it completely.
Zoning and permit requirements vary by location. In addition to checking local codes, you need to check insurance requirements and liability issues.  

The Layered Look

Much like a natural landscape or ground-hugging garden, a rooftop garden is based on layers. Only these layers protect the roof as well as giving the plants a place to grow. A well designed rooftop garden should have the following:
  • Wind barrier. Keeps planting medium in place until the plant roots take hold.
  • Plants. Plants that are drought tolerate and prefer poor soil tend to do best. Sedum plants do well in many locations.
  • Growing medium. The growing medium provides, drainage, nutrients, and a place for roots to take hold. The growing medium should be as deep as possible. Compost and other lightweight materials are good.
  • Geotextile or filter mat. The mat lets water soak through but prevents soil erosion.
  • Drainage layer. A drainage layer keeps the growing medium aerated and holds excess water. It can also be used as water storage. Gravel works well, but Styrofoam peanuts are lighter.
  • Roof barrier. The barrier protects the roof.
  • Waterproof membrane. The membrane should be able to stand up to the chemicals released by some plants.
  • Insulation layer.

Think Before You Plant

You have a lot to think about when you design a rooftop garden. It's fun to plant the trees and flowers, but make sure you set the stage properly before you invite the actors in.     Rooftop Garden Resources: http://www.gardendesign.com/tag/rooftop-garden http://www.designsponge.com/2011/06/rooftop-gardens.html Image Credit: Curbed  


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