Green Roofs: A Growing Rooftop Resource
by Monica E. Kuhn
The next time you walk through your city, look up towards the
rooftops. I think you will be surprised at the hints of green and growing things
you can see!
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, which is 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 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 winter and cooler in summer – 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 from everyday wear and
tear.
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In Europe and Asia, rooftops are viewed as much more than
inaccessible tar and gravel deserts dotted with drains, vents and condenser
units. |
Would you like to know more benefits? 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 ten stories above the street.
Trade seeds and gardening ideas. Barter or sell your surplus. Start a small
canning business. And make your city beautiful.
And the best thing is that this is not a dream. It is reality. In Europe and
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 to the roof the area of green space
covered up by the building's footprint; even existing buildings, some hundreds
of years old, must convert 20 percent of their roof space 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
roof gardens 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 into Canada, and in
September of 1994, the National Roofing Contractors Association in the USA
published an article in its magazine Professional Roofing entitled
“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 roof gardens 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 and plants – 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 weights around
100 pounds, so you can imagine the additional stresses that gardens can create.
However, remember that earth is not soil; you will probably be adding compost,
mulch and other fillers which will decrease the weight. Not all of your planting
beds have to be 12 inches 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 three inches of growing medium, and 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. Building codes have specific regulations regarding structural, health
and safety issues as they relate to new and existing buildings. A call to your
local Building Department or an architect will help to get you started.
Your roofing material itself 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 and should you be elevating your plants? How and
where does it drain? If you have to replace or repair it in five years, can you
do so without disrupting your established garden?
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, by insulating your
planters, by using mulch, by mixing moisture retaining additives into your soil,
by layering or interplanting, by building trellises and shade structures, by
collecting rainwater, or by sticking to plants that thrive in these conditions.
It is likely to be an ongoing experiment!
The main thing to remember is that each roof is as different as the gardener
who uses it. The next time you walk through your city, look up towards the
rooftops. I think you will be surprised at the hints of green and growing things
you can see!
Monica Kuhn is a registered architect in Toronto,
Ontario who specializes in rooftop gardens and Permaculture design. For more
information on rooftop gardens, contact
Green Roofs for Healthy Cities.
This article was published in Natural Life Magazine in 1995.
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