How to make a flat-top roof garden

Written by samantha belyeu
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How to make a flat-top roof garden
A flat roof is ideal for a rooftop garden. (Digital Vision./Digital Vision/Getty Images)

Roof gardens, or green roofs, are growing in popularity among municipalities and homeowners interested in improving the visual appeal, environmental benefit and economic value of their roofs. Flat-top roofs in particular are well-suited to roof gardens, since they have fewer problems with soil and water runoff than a roof with a steep incline. Consult a structural architect to ascertain the load capacity of any type of roof before designing and installing a green roof.

Skill level:
Moderately Challenging

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Things you need

  • Rigid synthetic board foam or polyurethane board
  • Waterproof membrane such as thermal plastic sheeting
  • Root barrier sheeting
  • Cardboard
  • Expanded clay, volcanic rock or prefabricated drainage system
  • Burlap, jute or old blankets
  • Soil
  • Irrigation system, optional
  • Plant containers
  • Wind breaks or shade structures as necessary
  • Moisture-retaining potting soil
  • Drought-, heat- and/or cold-tolerant plants

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    Planting the Whole Roof

  1. 1

    Decide whether you will plant an extensive green roof garden with small, low maintenance plants and very little roof access or an intensive green roof garden with deeper rooted plants and an area for people to enjoy the garden. A structural architect can tell you whether or not the roof can handle an intensive garden. You must decide if you're willing to take the extra time to water and tend it.

  2. 2

    Design the roof garden in detail. Choose all plants, make any modifications to railing or roof access, include plans for irrigation, exact soil composition and mechanisms for proper drainage. Your municipality may require this information for permits, where applicable.

  3. 3

    Install protective layers. Lay rigid synthetic board foam or polyurethane board with a one-inch overlap as insulation if the building is heated during the winter. Stretch waterproof membrane such as thermal plastic sheeting over the insulation, overlapping the edges by at least eight inches.

  4. 4

    Install specially designed root barrier sheeting to protect your waterproof layer. The waterproof layer alone will not prevent root punctures. Consult a roof garden specialist about how far to overlap this layer depending on the plants you will install, since plants have varying root lengths and can creep between the overlap. Use cardboard to protect this layer from the drainage layer.

  5. 5

    Select a drainage system that will work for your green roof type, intensive or extensive. Expanded clay or volcanic rock work well for any system but a prefabricated plastic drainage system may not handle the weight of an intensive system with heavy soil. This layer guides water to drainage areas from the roof. Lay burlap, jute, or old blankets across this layer to separate it from the soil layer.

  6. 6

    Install the soil, irrigation system, if used, and plants. Water the plants deeply and continue to water on a schedule until they are established or until the irrigation system is programmed to take care of them.

    Container Roof Garden

  1. 1

    Determine the variety of plants you can install based on your roof's load capacity. For example, dwarf trees can be planted in a container but your roof may not be able to handle the load of watered soil required to house the roots of a dwarf tree. Choose plants that are very drought-tolerant, since the combination of sun, wind and the limits of a container dry plants out quickly.

  2. 2

    Purchase or construct containers sized to not only fit the root system of your plants but with extra room to hold moisture and nutrients longer. This gives your plants a buffer against the extremes of the roof environment.

  3. 3

    Install wind breaks or shade structures as space, municipal permits and budget allows to ease environmental stresses on your plants. Arrange your containers, fill them with soil that retains moisture well and plant them. Keep them watered thoroughly by hand with a garden hose or install a drip irrigation system.

Tips and warnings

  • Design your potted roof garden beforehand so you can take into considerations any overhanging trees or sun-blocking buildings that may affect your plant choices. Choose plants with both heat and cold tolerance or have an indoor spot ready to overwinter tender potted plants.
  • Set out chairs and tables on your intensive roof garden or potted garden so that you will enjoy and tend to your garden more often. Plant vegetables and herbs to get nutrition as well as energy savings from your new green roof.
  • Install water catching systems such as rain barrels to supplement water to your roof garden.
  • The waterproof and root barrier membranes must be completely undamaged to be effective. Immediately repair any holes or punctures with an eight-inch overlap of sheeting or nontoxic waterproof sealant.
  • Always check with zoning and other restrictions in your municipality before installing a green roof. Even with the advice of a structural architect, some municipalities require green roofs to be installed by a certified specialist.
  • Ensure that water drains off the roof, no matter what type of roof garden you undertake. Water weight on the roof can cause major structural damage.

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