Published by City Farmer, Canada's Office of Urban Agriculture


Rooftop Gardens

pole garden

Reasons to Rooftop Garden...


What Kind of Roof Would Be Suitable for a Rooftop Garden?

Theoretically, any roof surface can be greened - even sloped or curved roofs can support a layer of sod or wildflowers. Switzerland has just passed a bylaw which states that new buildings must be designed to relocate the green space covered by the building's footprint to their roofs - even existing buildings -including historical buildings - must now green 20% of their rooftops. This has created an increased demand for research and material/product design, which will soon be available to North American markets.

There is a cable/tent structure over a school in Lyons, France which is covered with grass - the grass provided a refreshing green view from the urban classrooms as well as enough weight to eliminate some of the cables otherwise required to hold down the tent on windy days! The question is how to keep the soil from washing away while you're waiting for the plant roots to grow enough to hold the soil. Plants can grow in a medium of gravel - double duty for a layer of gravel ballast - easy, instant, and no additional weight. Rainwater was sufficient to keep the plants alive. It depends on how the garden is to be used, what you want to grow, and how often you will need to access the roof. Certainly, a flat roof, approximating level ground conditions, would seem to be the easiest to work with and on.


Does the Roof Garden Increase the Risk of Water Leakage or Damage to the Roof?

No. In fact, planting beds/surfaces are often used to protect and insulate roofing systems. However, problems could arise if,

  1. the membrane isn't protected from point-loading, shovels, shoe heels, and dropping equipment. A protective drainage layer between the soil and the membrane under planting beds, and raised surfacing treatment for non-planted areas should avoid damage.

  2. the roof drainage system gets clogged with sand, soil, or vegetation, and water is forced to sit on the roof for long periods of time, allowing it to either break down the membrane or work its way through the seams. Some waterproofing systems are designed for holding water, others are not. Ensure that your roof drains have the proper covers, and that your planters are lined with a layer of filter cloth and gravel to keep the soil from draining out with the water. Often a layer of filter cloth or drainage mat is laid over the whole roof, just on top of the waterproofing, to avoid this problem - especially if there is the likelihood of soil being spilled from planters, or sand removed from sandboxes!

  3. leachate from plants, fertilizers, and/or composters comes in contact with certain roofing membranes, leading to possible membrane breakdown. If, after researching, you find that your existing membrane is not compatible with these chemicals and organic compounds, your containers and planters could be lined with a continuous membrane, or placed on overflow trays; composters should never sit directly on the membrane because of their requirements for aeration and turning/working.

  4. a leak must be located and accessed after the garden is already in place - moveable planters/containers, modular walkways/surfacing treatment, and compartmentalized planting beds could solve this problem.


How can rainwater falling on a site be used most effectively?

Slow it down to enhance absorption by texturing, terracing, planting and ensuring permeability of surfaces, especially on slopes (which should be as gradual as possible).

Direct the flow of runoff to planted areas and detention areas so that as much water as possible can be retained on the site. Do not direct water to inlet drains unless essential due to weight limitations or inadequate waterproofing.

Retain and store rainwater in a covered reservoir/cistern for later distribution to plants (watering by hand or by a "leaky pipe" irrigation system). Ideally, runoff could be directed to a subsurface reservoir to provide water to plants on an "as-needed" basis.


What issues have to be addressed in the design and construction of the rooftop garden?

  1. Occupancy and the size of the garden as they relate to and impact on
    • adjacent or superimposed occupancies
    • occupant load i.e. the number of people allowed to occupy the garden

  2. Occupant load as it relates to and impacts on
    • structural loading requirements
    • exiting requirements

  3. Exiting requirements
    • types of exits allowed and number of exits required
    • distance between exits and travel distance to exits
    • sizes of exits and areas defined as "access to exits"
    • fire separations between exits and the rest of the floor area
    • possible requirements for fire alarms, exit lights, emergency lighting

  4. Handicapped accessibility and Barrier Free Design, either as a Code requirement or as a Client/User requirement.

  5. Requirements for enclosures i.e. guards, railings, parapets, walls around rooftops, terraces, and balconies
    • required heights
    • the placement of elements such as planters adjacent to enclosures which may reduce their effective height
    • climability of enclosures
    • loading and structural stability of guards and railings

  6. Specific requirements for structures/buildings on roofs, relating to
    • effect on overall building height
    • fire rating of structural members
    • exiting

  7. Other applicable issues might include
    • possible modification of window washing anchors on the roof
    • possible upgrading of washroom and service requirements
    • possible upgrading of drainage and water-proofing requirements

Structural Considerations of Rooftop Gardens / Summary

Remember there are two sides to this issue: there is the new loading exerted by the garden (the size and distribution of which can be altered by altering the layout of the garden) and there is the load carrying capacity of the structure (which can be enhanced by increasing the strength of existing load bearing members or by adding new ones). When designing a roof garden consult with a licensed engineer regarding the load carrying capacity of the building you are working with and ensure that the garden design and the structural capacity are compatible.
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Revised August 26, 2003

Published by City Farmer
Canada's Office of Urban Agriculture