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


Rooftop Resource

By Monica Kuhn
Office of Monica E. Kuhn Architect
(C) Copyright August 1995

Monica Kuhn is a registered architect in Toronto.
She specializes in rooftop gardens and permaculture design.

Rooftops are a city's greatest untapped resource. Sloped or flat, large or small, industrial or residential, the possibilities for urban greening, air cleaning, community building, and food production are limitless.

Cover a roof with plants and you have immediately achieved several things:

Environmentally, by increasing the city's biomass, you have increased Oxygen levels in the air - and decreased the amount of CO2, produced by cars and other fuel burning "technologies"; you have cut down on dust and air-borne particulates, since plants act as natural filters; you have had a hand in altering the local climate, because plants absorb rather than reflect heat; and because roots hold and absorb water, every time it rains, your roof is retaining storm water runoff, thereby decreasing the load on the city's already overflowing storm sewage systems.

Your building - and therefore your pocketbook - will also benefit. Layers of soil and foliage have wonderful insulating qualities, keeping your building warmer in the winter and cooler in the summer - and thereby reducing your energy bills; because the extreme temperature swings, and therefore the expansion and contraction experienced by the roof will be moderated, the life span of your roofing membrane will increase; and since the roofing will be covered, the membrane will be protected from harmful UV rays, and everyday wear and tear.

Would you like to hear more? Grow vegetables and you can feed your family; grow flowers and you can attract butterflies! Create a safe, private, outdoor space in the heart of the city - without having to buy extra land; give the people next door a better view; increase the value of your property; teach your children about the environment; increase urban food production; plant a garden over your apartment; meet your neighbors 10 storeys above the street; trade seeds and gardening ideas; barter or sell your surplus; start a small canning business - and make your city beautiful.

Wow! And the best thing is that this is not a dream - it is reality. In Europe and in Asia rooftops are viewed as much more than inaccessible tar and gravel deserts dotted with drains, vents, and condenser units! in some parts of Germany, new industrial buildings must have green roofs by law; in Swiss cities, regulations now require new construction to relocate the area of greenspace covered up by the building's footprint to the rooftop - and even existing buildings, some hundreds of years old, must convert 20% of their roofspace to pasture! This has spawned a whole new industry which specializes in light-weight growing mediums, filter cloths, roofing membranes, plant stock, and how-to books and kits; nurseries, designers, consultants, and contractors have been forced to relearn and re-adjust in order to compete in the new market, with the result that they now have more and varied work.

Many studies looking into the environmental, financial, and social benefits of roofgardens have been completed, and statistics are readily available - if you can read German! However, North Americans are also slowly starting to realize the potential of this 'growing" movement; last year the Danish company Grodania started importing their lightweight rockwool growing medium to Canada, and in September 1994, the National Roofing Contractors Association in the USA published an article in their magazine "Professional Roofing" titled Designing green roof systems: A growing interest.

In reality, the technology and the know-how required to grow plants and trees on elevated structures has existed in Canada and the USA for a long time - just think of all the underground parking garages that support landscaped courtyards; the difference here is that these gardens are at ground level, mimicking a natural situation, and so we don't notice a difference. As well, these gardens were given structural consideration during the initial design phase - not after the fact - whereas most of the roofgardens that people are interested in installing now will have to be retrofits to existing buildings.

If you are interested in growing a green roof, there are several issues that you should be aware of:


The first is loading. Soil, decking, people, planters - and where they are placed on your roof deck - all have an impact on the existing structural/carrying capacity of the roof, as well as that of the rest of the building. It is important to have a structural engineer confirm the additional weight that the roof can accommodate. One cubic foot of wet "earth" weighs +100 pounds, so you can imagine the additional stresses that a garden can create. However, remember that earth is not soil - you will probably be adding compost, mulch, and other fillers which will decrease the weight; nor do all of your planting beds have to be 12" deep; nor will you be uniformly covering the whole roof surface. Heavy planters can be placed strategically over bearing walls or columns; grasses don't need more than 3 inches of growing medium; some plants will grow in gravel... you have a lot of options available to you.


The second consideration is safety. How do you access the roof; how do you get materials and water up to the roof; who will be using the roof; is there a railing; are you insured? Requirements, solutions, and costs will vary depending on whether the garden is on a private residence, an apartment tower, or a public library! The Ontario Building Code has specific regulations regarding structural, health, and safety issues as they relate to new and existing buildings - a call to your local Building Department or your favorite architect will help to get you started.


Your roofing is also an issue. What kind is it and what condition is it in; can you walk on it or should it be protected; will plant roots penetrate the membrane or should you be elevating your planters; how and where does it drain? If you have to replace or repair it in 5 years, can you do so without disrupting your established garden? Again, not to worry, there are as many solutions as there are restrictions or potential problems!


And then there is the specific micro-climate of the roof itself. Gardening up on a roof is quite different from gardening at grade. It is very sunny, sometimes windy, and the temperatures are often extreme. This will have a direct effect on what will grow well, how often you have to water, and whether your plants can survive through the winter. You can temper the effects of heat, cold, and dryness by using containers that retain moisture i.e. plastic vs. terra-cotta; by insulating your planters; by using mulch; by mixing moisture retaining additives into your soil; by layering or interplanting your plants; or by sticking to plants that thrive in these conditions -it is likely to be an ongoing experiment! You can build trellises and shade structures; you can collect rainwater - I could go on and on!

The main thing to remember is that each roof is as different as the gardener who uses it. Toronto's Field to Table has started a low cost market garden using 3" of soil, covered with straw mulch, on plastic sheeting edged with produce crates, which double as pathways; Brock School in the West end has installed an outdoor classroom and working garden on a portion of its roof, with wooden planters, decking, composters, and coldframes funded and built by the children, parents, and teachers; Homes First has provided each of the 75 tenants at the Mary Lambert-Swale housing project with a 5 x 5 foot roofgarden plot - and the tomatoes and the broccoli are thriving! The next time you walk through your city, look up towards the rooftops - I think you will be surprised at hints of green and growing things you can see!

Reprinted with the permission of Natural Life
R.R.#l, St. George, Ontario,
Canada NOE INO
Phone/Fax: (519) 448-4001
Editor: Wendy Priesnitz

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Revised February 5, 2006

Published by City Farmer
Canada's Office of Urban Agriculture