THE ROLE OF GREEN ROOFS IN COST-EFFECTIVE CITY GREENING
Rune Kongshaug, Vikram Bhatt
McGill University, School of Architecture, Minimum Cost Housing Group (MCHG)1
Abstract
The role of green roofs is here considered within the framework of how to optimize land-use as
a response to rapid urbanization in both Northern and Southern cities. Comparing high-cost,
green roofs with alternative low-cost greening solutions—both at and above grade—the authors
suggest how green roofs could benefit from a minimum cost perspective, which seeks to
maximize city greening and target ordinary residents in the most cost-effective manner,
employing participatory, community-based design methods. Community gardens, rooftop
container gardens, and urban agriculture illustrate how green roofs could be low-cost,
participatory, and community-based and act as engines for urban renewal, strengthening social
networks and residents’ self-reliance, promoting equitable access to land and economic growth.
Our paper is a critical review of the benefits of green roofs as compared to other ecological and
community-friendly uses of under-utilized urban spaces. The topic of green roof design and
implementation needs to be understood in a wider framework how to best prioritize resources
for the greening of cities. To what end and for whom do we practice urban planning and design?
In this paper we equate urban design and architecture as a means to provide design solutions
for people. Unfortunately, the general design practice of planners, urban developers and
architects has primarily looked at and served wealthy clients and produced expensive solutions
for which demand and application is limited. The challenge is to produce rooftop garden
solutions that ordinary people can afford. Participatory, community-oriented and low-cost
approaches to city greening are in great demand and are required because they contribute to
efficient land-use, and help redress the balance between urban spaces for living (shelter) and
spaces for growing (ornamental gardens and urban agriculture), which is an essential
component of raising the quality of living in most urban sites. This view has been expressed by
1
Acknowledgements: For the Edible Landscapes seminar on Montreal community gardens (2002) and
publication (2004), a special recognition goes to all participants of the research seminar: Ms. Yingwei Cui,
Ms. Yingzhou Du, Mr. Qiang Fu, Ms. Xiao Tong He, Mr. Rune Kongshaug, Mr. Sachin Narkar, and Ms Li
Xiao. For assistance on research of Montreal green roof projects, a special thanks to Mr. Elikem Ayitey,
MUP.
-1-
national governments´ requests written down in the Harare Declaration (1). The sponsoring by
granting agencies such as the UN-HABITAT and the IDRC of community-based, participatory
projects seeking to leverage phenomena such as urban agriculture as a means for building or
upgrade worldwide city-sites, further validates this point.2 The authors use examples of city
greening—which includes hands-on experience of building rooftop gardens, student fieldwork
on government-sponsored community gardening—to propose a minimum cost perspective on
the role of green roofs in city greening.
Community-based design for greening and building of sustainable cities
The role of green roofs are considered alongside other greening solutions at grade, within the
minimum cost perspective of how to optimize land utilization as a response to rapid urbanization
in both the Northern and Southern hemispheres. To this end, we compare problems and
benefits associated with retrofit and new green roof projects, with rooftop container gardening,
community gardening at grade, as well as urban renewal and housing projects seeking to
integrate urban agriculture. We look at how each solution contributes to overall city-greening,
according to: (a) ability to create value for the building’s owner or its residents; (b) ability to
involve the community through the design, planning and building stages, training and
community outreach; (c) permanent community impacts in terms of increased health and socioeconomic benefits, such as job creation, overall social interaction and food security; and (d)
monetary cost.
Greening Solutions
Rooftops
At grade
Urban Upgrading
Retrofit: Green roofs and
Container rooftop gardening
New urban development
New green roofs
Vacant lots, parking, streets
City and housing design
Community gardening
Urban Agriculture (UA)
Table 1: Comparing greening solutions according to urban upgrading or new developments3
2
The Minimum Cost Housing Group has just recently received funding from the International Research and
Development Center in Ottawa, to conduct a three-year research and design to upgrade and/or build three urban sites
incorporating Urban Agriculture: One site will be in each of Africa, Asia and South America.
3
See the Appendix for a table summarizing challenges and benefits of greening solution at and above grade.
-2-
The minimum cost design perspective
The minimum-cost approach to architecture, and in particular housing design, is concerned with
serving the largest possible number of residents, which (often) means to service the urban poor.
The poor urban resident is not the typical architectural client, and is often left out of the city
planning and design processes. However, since money is scarce, the minimum cost
perspective seeks, first and foremost, to identify design and greening solutions that minimizes
cost, yet seeks to maximize community participation and social outcomes. The residents are
themselves the main asset in this participatory design exercise, and as design professionals, we
seek to leverage their self-reliance and “entrepreneurial spirit” and their ability to organize a
community-based response to improve their situation. This approach also relies on innovative
funding strategies such as micro lending to individual residents, as part of community-wide
efforts to create jobs, building new or upgrading existing housing. Most recently, as part of an a
world-wide effort to upgrade or build three urban sites incorporating urban agriculture, the
minimum cost approach seeks to leverage municipal land and existing infrastructure and other
“in-kind” contributions that poorer city partners can commit, thus diminishing the reliance on
hard cash. Finally, the minimum cost perspective adopts an attitude of learning from those
poorer residents who, through their informal housing, urban agriculture and gardening inform us
about residents’ true needs and how they organize “bottom-up” through grassroots movements
and social activism. Finally, this approach seeks to define new solutions and inform design
professionals, planners and policy-makers how to green and upgrade existing or build new,
more efficient and equitable cities.
In this perspective, green roofs generally present a maximum-cost and complex endeavor that
may fit large and luxury-end construction. How to channel the current green roof movement to
develop a credible scenario for large-scale greening of cities at minimum cost? In this respect,
we found new commercial green roofs of limited value compared to innovative ways that
exploited underused spaces both at grade and retrofit roofing projects, providing a potential
niche market for green roofs, and a potential new direction for the green roof movement that
would warrant lower-cost, retrofit and do-it-yourself solutions, and that would incorporate
community-based design methods.
Lessons from North and South: city-greening as an engine for urban renewal
Urban growth is inherently accompanied with land-use pressures, which prevent efficient or
equitable land utilization. For example: In Southern cities, the appearance of a large informal
-3-
housing sector (squatter settlements or slums) has traditionally been met with the official
response of displacement of residents to outside the city core, bulldozing of their settlements,
with interdiction of urban agriculture—horticultural practices or raising of livestock in the city—
deemed contrary to sound city planning principles or to represent a health hazard. Land prices
are usually higher near the denser city center, and it is generally assumed that Laissez Faire4—
speculative market forces, municipal authorities and real estate developers—would assure the
most efficient land- and resource-use. In Northern cities, planning new urban development has
since WWII deliberately relied on high levels of individual car ownership, high energy
consumption and low energy costs, accentuating a trend towards a diffusion of the city core—
traditionally diverse in terms of both cultural and socio-economic diversity, commercial and
residential mixed uses—into more monotonous spatial divisions made up of garden suburbs for
living, urban centers paved with asphalt for parking, working or shopping (where trees are
usually described as “amenities”), and tree-less “rural” areas for agro-industry.
Neither in the North or South, does current city planning practice seek to provide equitable
access to land resources, or to leverage greening efforts in such a way as to act as engines for
urban renewal that promotes social inclusion, nor does conventional architecture and urban
design practice have a stated goal to diminish the current gap between the richest and the
poorest or society, which could serve as a roadmap for the design practice. This problem may
be even more pressing in the Southern cities, which are surpassing the more developed North,
both in terms of the overall size of cities and urban growth rates, yet adopting city building
patterns of the North, such as the suburban single-detached bungalow as a “way of life”
worthwhile emulating.
UN-HABITAT estimates that close to half of the world’s population, is now living in cities and this
number will grow to 60 percent by the year 2015 (2). Conventionally, cities have functioned as
centers of commerce and manufacturing, and of course, they do continue to serve as seats of
power and culture. Also, informal, not officially planned, urban phenomena such as urban
agriculture (UA) are increasingly recognized as engines for urban renewal, poverty alleviation,
social empowerment, and healthy living in cities. UA includes horticultural practices and raising
livestock inside the cite for pleasure, commerce or food security, which have traditionally been
4
Here alluding to the liberal economical theories Adam Smith, whereby market forces will assure the greater social
good, from which also derives the expression “greed is good.”
-4-
considered a strictly rural activity, now observed as a spreading urban and peri-urban
phenomena in Northern and Southern cities alike (3).
Lessons from greening at grade
For rooftop gardening to present a broad-based design “solution” to urban problems, we
propose a practical framework to compare and contrast alternative greening solutions, in which
the benefits of greening rooftops can also be appreciated. This presentation contrasts newly
developed, “fancy” and expensive garden roof projects with more “laid-back” minimum-cost yet
highly aesthetic interventions, suggesting that do-it-yourself and community-driven retrofit
projects can produce desired social interactions and positive neighborhood impacts.
This study is based on the authors’ own experiences in the design and construction of two roof
gardens: one a container garden built by a research team, the other a green roof built for
residential tenants, as well as field research conducted by the authors, as published in Rooftop
Wastelands (4) and Edible Landscapes (5). The objective of our work is to optimize uses of
under-utilized land and hence redress, qualitatively and quantitatively, the balance between built
live-work spaces and open spaces dedicated for growing both for pleasure and sustenance.
We will use the following greening and rooftop garden solutions as illustrations:
High Cost
Low Cost
At grade
New roof
Re-fit
Nun’s Island condos,
Montreal (2003)
City of Montreal
Community Gardens
(1974-current)
Place Bonaventure,
Montreal (1967-1997)
Luxury dwelling, New
York (2002-03)
University Settlement
Community Center,
Montreal (1974-76)
Table 2: Sample Greening and Rooftop Gardens Serving as Illustrations.
Stakeholders
Similarly to the evolution of community gardening and urban agriculture, green roofs could also
benefit from a community-based, minimum cost approach. In particular, a community-based
rooftop greening project should bring together several stake-holders and community groups: city
officials, individual owners, community representatives, and seek to: involve residents and
community members during the planning and design stages, compensate individual owners for
expensive up-front costs and community-benefits generated in terms water retention, air quality
-5-
improvement, visual impact, and—why not?—encourage owners to engage with the community
through outreach programs permitting controlled community access and/or training.
Public and private ownership, and grassroots activism
Gardens in cities exist in surprising forms, ranging from potted plants in windowsills, private
gardens, community gardens, vacant and parking lots, container gardens on roofs and
balconies, and green roofs. Different forms of gardening present distinct challenges: land
tenure, security, structural requirement, access, planning, design and ongoing maintenance,
and, of course, the cost. The opportunities are also distinct: community impact, privacy, incomepotential and environmental payback. One solution does not fit all.
Private ownership, an individual owner or a condominium seeking public or non-profit
partnerships, should thus be encouraged in terms of: (a) facility in obtaining permits, city
approvals and insurance, (b) financing in terms of matching public grants and tax credits, and
in-kind financial support, e.g. building material, horticultural supplies and technical expertise.
Public ownership in the form of community centers, schools, etc., or social housing (non-profit
cooperative housing) may be set up similarly to community gardens in that it can: (a) provide
initial public funding for retro-fit green roofing projects, structural improvements and materials for
qualifying community organization that can demonstrate organizational commitment, (b)
leverage existing public infrastructure, such as staff and resources dedicated to serve municipal
public parks for ongoing maintenance, (c) reduce overall cost and maintenance by encouraging
a selective, membership-driven process available to the community at a small yearly
membership fee and subject to rules and regulations.
To this end, the City of Montreal, which facilitates community gardens and urban agriculture by
leveraging existing public infrastructure, is a case in point to be contrasted with most cities. Most
cities (consciously or unknowingly) discourage the use of under-utilized vacant lots or roofs for
gardening and urban agriculture practices. As an owner-builder in both New York City and
Montreal, I have experienced first-hand the lack of incentives for owners wanting to build a
green roof in New York. Another example: while community gardening in Montreal and New
York have achieved similar levels of popularity—76 city-sponsored gardens on the island of
Montreal for 1.8M residents (6), and 189 smaller gardens on the island of Manhattan serving
1.5M residents—the levels of city support and social activism varies greatly. New York gardens
-6-
survive only due to grassroots activism of organizations such as “Green Guerillas” who battle
city authorities and their policy of either selling off land or bulldozing community gardens (7).
A participatory and community-based approach to the development of green roofs may need to
identify, then align itself with and leverage, existing social movements and grassroots activism
of community gardening in order to reach a large numbers of residents, and achieve a city-wide
greening of roofs.
Design methods and tools
A key design objective for green roofs is that it ought to be a cost-effective greening solution,
thus affordable to the community at large. Design processes that engages the owners,
residents or the community at large, stand to facilitate the development of low-cost design
solutions, if anything because it focuses design goals around common needs and objectives,
instead of around individual wants5, and thus helping to prioritize scarce resources. Here the
design practitioner acts as much as a “facilitator” as the architect in the classical sense, in
generating a community response. It is the methods of observation and ability to create a multistakeholder process, which can inform the various participants—rather than the final blueprint—
that becomes the main deliverable of the minimum cost, community-based design method.
Reducing the scale of observation to human dimensions
A dweller-grower is here coined to designate an urban resident who is also a gardener. The
garden may not be attached to the gardener’s house, but when considering the relationships
between living and growing spaces in the city, matters of distance and scale become important,
first in terms of shared urban spaces and the social interactions they crate, but also in terms of
creating a balance between built, human structures, and growing organic, natural forms. An
example from observing informal building and growing practices for people by people in poor
settlements in India, first alerts us to the “riches” inherent in traditional settlements; However
monetarily poor, observing informal settlements (so-called slums) teaches us important lessons
of human scale. For example, working, cooking and eating activities would take place on the
floor rather than in a seated position, and surprisingly small indoor and outdoor spaces give rise
to different activities, such as bicycle repair, weaving, cooking. In this setting, a tree does not
have ornamental value only, but provides shade, fuel and food with a minimal up-front
5
Gandhi shall have said something like: There is enough for everyone’s needs but not for everyone’s wants.
-7-
investment. The simple lessons of trees, and its various productive uses, are seldom
transferred from the informal settlements to formal city planning and design practice, however:
Trees on the architectural plan are usually for ornamental value, only, not providing detail on
growing cycles, different species or varieties suited for local climate, or local customs (8).
Sharing growing secrets in community garden spaces
From the architect’s perspective, one of the most challenging elements of considering the merits
of low-cost gardening solutions, is to view lettuce and chard on equal footing with brick and
mortar. After all, a garden’s value needs to be considered against other alternatives such as
parking, housing, commercial development, roads and parks for recreational use. The length of
the growing season, size of containers, methods of irrigation and fertilization, ease of physical
access, as well as considerations inter-cropping in tight areas will dictate the choice of plants.
In the example of Montreal’s short growing season and community gardens, simple planning
and design tools help transform a small lot—the size of a parking space (9’ x 18’) —into an
effective food garden producing several harvests for an annual investment of less than CAD $47
(9). Some communities had perfected using supports to grow vertically and inter-cropping to the
point of having 3-4 harvests in the short Montreal planting season. Cultural differences also
exist and give rise to meaningful social exchanges in these tight, community garden: In
Montreal, depending on whether the grower-dweller was of Anglo-, French, Indian or South-East
Asian origin, the skill-levels and methods applied by gardeners varied widely. Sharing growing
“secrets” is part of Community Garden social life, hence the value of community urban
gardening as it reinforces the need for and value of shared urban space, social interaction and
learning (10). The University Settlement community rooftop container garden, extended the
growth season even further, employing re-cycled materials and cold-frame technology with a
similarly small investment, and formalized community outreach and training offering gardening
classes and thus leveraging further the inter-disciplinary links between the university and the
surrounding community (11).
Building topography and population density, revisited
Urban land is scarce in some places, but even the most densely populated urban fabrics can
accommodate greening at grade, in vacant lots, between railroad tracks, etc. And rooftop
gardening has been found to be an attractive solution in lesser dense areas, and ought to be
considered as cost-effective even in low-density residential suburbs for community facilities, and
for commercial strips on traditionally large mall or warehouse “boxes” that may be greened more
-8-
extensively than surrounding parking lots, and provide important aesthetic and environmental
benefits. Also, very densely populated areas do not automatically mean that greening rooftops
provides the best and most efficient greening solution, where it may be more pressing to
address community-driven projects, with equitable access, at grade. Again, the choice to
promote any greening solution over another needs to be based on overall community
involvement, social benefits, and whether or not it presents a cost-effective approach to citygreening. Considering the building stock in Montreal, one finds ample room for private and
community gardening, at grade, before rooftop gardening becomes a necessity. Yet, the
several rooftop community gardens have been developed, including on top of the YMCA in
Cote-des Neige (12), demonstrating the underlying community motivations behind urban
greening, regardless of topography and density: Where there is a community, there is room for
cost-effective greening, even green roofs.
Community Garden
Neighborhood
Open
Built
Housing Type
Density
Le Mannais
Pere Marquette
Victoria
Bon Voisin
Rosemont / Petite
Prairie
Cote-des-Neiges
Sud-Ouest
56%
44%
Medium
80%
66%
20%
34%
Attached
(Duplex, Triplex)
Detached
Semi-detached
Low
Medium
Table 3: Topography and Housing Types surrounding Community Gardens in Montreal.
Importance of middle-to-high density urban residential housing
Residential urban and suburban housing is in crisis if one looks at the time spent and distances
traveled commuting, and the disappearance of open land to cities: Between 1982 and 1997, the
total urbanized land in the United States rose by 47%, from 51 to 78 million acres, while the
U.S. population rose by (only) 17% (13). In the minimum cost perspective, the cost of adding
peri-urban space for growing would be far less expensive than the cost of developing it for
housing. Moreover, the tradition of having a small growing plot on the outskirts of the city is very
old and was extensively used in the wartime Europe, but it still exists. It is also important to note
that marginal lands can be used for such a purpose, noting that working such lands would not
require daily commuting. Examples of peri-urban agriculture, includes in New York the green
markets supporting local farmers. In Montreal: organic food coops where people control the
food they eat but need not grow it themselves, or “peri-urban partnerships” between residents
and farmers whereby urban residents volunteer for farm work against a share in the harvest,
thus helping the farmer keep expenses low (14). Again, between the extremes of the singledetached suburban house and the city high-rise, the minimum-cost approach considers working
-9-
with medium density housing, and seeks to implement low cost retrofit solutions rather than
demolition and new construction. As a response to demographic pressures on existing urban
housing stock, and the need for urban upgrading and renewal, one can demonstrate that even
in badly designed housing one can grow in very small pockets of land, making sure that such
growing processes or activities need not be capital-intensive as the rooftop initiatives are.
Residential housing thus constitutes both a potentially important market for rooftop gardening in
terms of already existing roofs. In this respect, Montreal represent a special case in terms of
lower than average (Canadian) vacancy rates at around 0.7% (15). Also low rents and low
home ownership rates, are discouraging construction of new housing, except for luxury
condominiums (16).
Dweller-grower motivations & community participation
Urban gardeners in North American cities seem to engage in gardening activities more for
pleasure and less for sustenance as compared to their counterparts in Southern cities. Most
urban gardeners benefit recreationally from gardening, but one immediately notes that the
garden can also act as an inter-generational bond with a large proportion of the elderly
represented, as well as a (surprisingly maybe) large portion of moderate-income people growing
food as an income subsidy. The economic value is an important factor to note in Northern
cities, as well, because it helps strengthening community’s self-reliance.
Project
Age-groups
Income
Motivations
At grade
City of Montreal
Community Gardens
> 55 yrs: 44% in 5
out of 9 boroughs
> 45 yrs: 63% in 8
out of 9 boroughs
30 – 40 yrs. old.
CAD 29-37K6
Food quality: >50%
Leisure: 40%
Ecology/quality of life 20%
Repeat gardeners: 75%
Private access.
Decided to rent space
because of garden.
Rooftop
Green Roof, Luxury
dwelling, New York
USD 200K7
Table 4: Gardener motivations by project and income
6
7
Per household
Per person
- 10 -
Conclusions: High-end vs. Participatory, community-driven approaches
A minimum cost and community-based design practice seeks to maximize social benefits,
pragmatically selecting between greening solutions at or above grade. The city greening
“toolbox” thus includes a range of solutions from the relatively high-end and complex green
roofs, less costly rooftop container gardens, community gardens, and urban agriculture, and
peri-urban partnership seeking to maximize under-utilized land within and in proximity to the
urban center. A common theme amongst these solutions is the ability to promote a participatory
approach involving public authorities, private and community interests, thus leveraging
community involvement and existing public infrastructure. This approach is guided by the
objective to reach ordinary residents, including servicing and including the urban poor into the
design and planning processes.
City greening case studies8
The greening solution studied in this paper, which involve participatory, community-based
design tend to outperform high-end, non-participatory projects in terms of their overall social
contributions and low cost, because their participatory nature tends to facilitate deeper human
interactions, create local social capital in terms of social activism, and have a lasting and
positive urban neighborhood impact. As observed in the Edible Landscapes project studying
community gardens within Montreal’s built forms, greening solution will be most impactful
considering existing residential housing stock and retrofit projects. The participation of dwellergrowers in community projects, have the potential to act as a social and economic engine in
low-to-moderate income groups, and help integrate recent immigrants, further easing the
demographic pressures on the rapidly growing city (17).
Leveraging existing infrastructure and social capital
In addition to considering the potential for rooftop gardening in residential and community re-fit
projects, the community gardens in Montreal are one example of how city municipalities may
leverage Park Land zoning to protect garden uses, and leverage capacity from existing
departments, such as “Public Works” and “Parks” to set-up and maintain gardening facilities at
minimum cost. Minimum cost green roofs would need to leverage not only public infrastructure,
but also seek to leverage existing community resources that exist as part of community
gardening and urban agriculture movements.
8
For an overview of each case study, see Appendix 2.
- 11 -
Case Study
Rooftops (New)
The Hilton Hotel
(Montreal)
Condominium
(Nun’s Island, Montreal)
Rooftop (Retrofit)
Luxury loft building,
(New York)
Container garden
(Montreal)
At Grade
Community Gardens
Urban Agriculture
Planting trees
(City programs)
Planting Trees,
(Informal settlements)
Participatory
Social Benefits
Costs
No
High visibility
Niche market
Private and shared
spaces
Private and shared
spaces
For residents only
$4M
$92 / sq. ft.
Some end-user
involvement
$45,000 Design only
(2% of total cost)
No
5 lofts
Controlled access
$63K
$42 / sq. ft.
Very high
(as part of initial goal)
Very important
Outreach
Training
(To be added)
10,000 gardeners
6,000 gardens
76 gardens with
positive
neighborhood
impacts
Upfront Investment:
$2,000 per plot
Maintenance:
$770,000
$128 per plot / year
Initial: Small
Maintenance/ Removal:
Could be high
Tiny
Costs: $287 per
garden/ year
Could be: owner plants
himself and take part of
maintenance
Yes
More pedestrian
traffic, less cars
Shade
Food
Fuel
The case studies suggest that the more projects are participatory, they also tend to be low-cost:
Green Roof, The Hilton Hotel International, Montreal
Condominium green roof, Nun’s Island, Montreal
High-Cost
Less participatory
Skyturf green roof, Loft building, New York
Rooftop container community garden, Montreal
City of Montreal Community Gardens (at grade)
Tree-lined side-walks, by the city municipalities (formally executed)
Tree-planting at grade, by grower-dwellers (informal settlements)
Low-Cost
More participatory
Figure 1: Cost of Project vs. Degree of Individual and Community Participation
- 12 -
Policy choice
In order to bring green spaces into cities, the design professional once again has to ask, for
whom are we building? What is affordability? Observed green roof projects can be divided into
two broad categories: high-end luxury projects addressing a niche audience seeking to
maximize profits, and low-cost, community-driven, participatory projects seeking to leveraging
community input and/or public infrastructure seeking to maximize social benefits. Which way to
go becomes a matter of choice. Municipalities could provide minimum coding and promote the
supply of affordable materials, technical expertise, matching grants and/or tax credits to realize
both modular and permanent green roofing solutions. Incentives and credits for retrofit projects
could later be collected from increased real estate taxes associated with a better maintained
housing stock, down the line.
The minimum cost approach to city greening, thus aimed at serving people, is a pragmatic
decision, because of the interest (demand) from the community and their need is manifest
through their enthusiasm for community gardening, quality of life benefits and social interactions
that these activities generate. Also, cities like Montreal and donor agencies supporting urban
agriculture have understood that in order to succeed, it is also a matter of providing
infrastructure, provide legal frameworks and initial investments (supply). Design professionals
have a duty in this respect, to be engaged in policy-making to support urban residential housing
and community approaches that integrate living, working, and growing spaces in innovative
ways.
Minimum cost does not mean cheap, just thoughtful—superior—greening solutions that also are
cost effective, and seek to address optimize land use in an equitable way. Municipalities and
local governments are thus presented with several greening alternatives, of which a minimum
cost, participatory and community-based approach should also help the spread of green roofs,
especially for retrofit of existing municipal and medium density residential housing stock.
- 13 -
Appendix 1: Challenges and benefits of greening roofs with greening at grade
Roof greening
technologies:
Retro-fit
Challenges:
Benefits:
New roofs
Challenges:
Green Roof
(soil on top of
drainage mat)
Who has access
Benefits:
Privacy, security
Who has access
Privacy, security
Building codes
and permits
Efficient way to
retain water
Zoning, Permits
Tax credits
Insurance
Reduce heat
Incentives for
small projects
LEED “points”
(large projects)
High up-front cost
Overall large
impact if existing
building stock, e.g.
housing, is
greened
Limited impact on
city greening as
new construction
is limited, and
favor costly
solutions
Costs included in
overall building
project
Creating green
“oases” within
urban fabric
Little/no input from
future residents or
the community on
the garden’s
design and its
uses
Residents move
into a ready-toenjoy garden
Incentives needed
for the owner or
entrepreneur
Building code
could specify
green roof
requirements
Not easy to do
yourself
Structural re-work
may be needed
 Intensive roof
(thicker soil layer,
for walking)
Plant selection
and garden design
needs to be upfront
Repair or find
leaks
 Extensive roof
(thin soil layer,
usually no
walking)
 Container garden
(on top of
bitumen
membrane or
IRMA)
Difficult to repair in
case of leak
Incentives to
owner: How to
account for
community
benefits.
No need for
planter boxes,
A “meadow” feel
Address large
commercial strips
with large dark or
reflective roofs
Aesthetic visual
improvement for
neighbors
High visibility: may
inspire further
action
Satisfies a “highend “niche’ market
Market for lowcost (modular)
approaches to
reach home
builders
Protection of
membrane with
Low up-front cost
and maintenance
Building codes
and permits
Structural needs
Incremental
Access, Water
Do-it-yourself
Light-weight soil
and materials
Retains water,
cools roof
Owner(s) carry the
cost, need for
incentives
Potential publicprivate partnership
Account for social
benefits.
Increase real
estate value
Incentives,
matching grants
Personal
satisfaction
In-kind support
material, plants
Joint effort
Participation of the
community in the
design, planning
and building
Include low cost,
modular solutions
in initial design
Retains water,
cools roof
Roof ownership:
 Private buildings,
Condominiums
Educate owners of
green roof merits
and technology
 Community
facilities, and
social housing
Community-driven
design
Community
Public Funding
Community
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Market for costefficient roofing
and garden
solutions
Public institutions,
community
centers
Roof greening
technologies:
Retro-fit
Challenges:
commitment
Benefits:
access
New roofs
Challenges:
processes
Container garden
(bitumen
membrane or IRMA
Protection of
membrane
Low up-front cost
and maintenance
Building codes
and permits
Access, Water
Incremental
Light-weight
materials
Do-it-yourself
Greening at Grade
Public or private
parks or gardens
Open (new) land
Challenges
Ownership and
access
Ownership and
access
Decontamination
Security
Water and
sanitation
 Community
gardening and/or
urban agriculture
Access, security,
maintenance and
legal framework
Get community
buy-in, training
 Tree planting
Retain water,
Reduce heat
Add value to real
estate, Leisure
Low cost
Leverage existing
infrastructure
Large community
impact: social
interactions,
health, quality
food and income
Snow/leaf removal
and maintenance
Few structural
issues
Reduced space
for cars or parking
Shade for
pedestrians
Low cost
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Include low cost,
modular solutions
in initial design
Retains water,
cools roof
Retains water,
Load capacity
colds roof
Existing lots and streets
Challenges
Benefits
Scenery, shade
Benefits:
Benefits
Public use/access
Low cost
Land tenure
Existence, or nonexistence of public
infrastructure
Who decides what
is optimal land
use, how to
prioritize between
alternative uses
Regulated
access/use for
specific needs
Low cost
Engine for urban
renewal
Facilitate and
provide incentives
for individual
owners to plant
own streets
Introduce greens
on all new
construction sites
Low cost
Appendix 2: Case studies
The Hilton Hotel International Rooftop Garden (Montreal, 1967)
This example considers the construction of a new green roof for a high-end, niche market of
hotel patrons, and subsequent maintenance and retrofit. It is a costly, non-participatory,
intensive rooftop garden erected to receive visitors for world exhibition of 1967. The garden
was designed to conceal the black roof of the exhibition hall and the parking space below. The
garden is divided into quadrants, has winding paths, a continuous waterfall and swimming pool.
Given the severe freeze and thaw in Montreal, drainage gravel 8-14 inches sits on a base
membrane over a concrete structural slab. Plant materials were selected to withstand the cold
conditions in Montreal and trees were put on structural columns. A recent retrofit effort on the
roof cost CAD $4M (18). The garden occurs above grade on the 17th floor of the huge concrete
complex, one of the biggest in the city and in the world. The roof garden, which occupies the top
three floors of the multiuse exhibition hall, is 2.5 acres in size and provides both private and
shared access to hotel patrons (19). It was in line with the mega city concept of the 1960’s and
made the hotel highly visible and attractive. The design conception eliminated the need for a
conventional tower plaza at the ground level. In the interior of the garden are public rooms while
those on the exterior perimeter are guests’ rooms (20). The hotel is one hotel in Montreal that
maintains a higher than average occupancy rate, about 70 percent compared to 63 percent for
others in Montreal in 1996. Putting green space on the top three floors of the 17-stories was
innovative and revolutionary, and made the hotel attractive for locals and tourists alike. The late
Ray Affleck, of Montreal firm ARCOP was the lead architect responsible for the project. The
example is a success-story in terms of providing a beautiful city space away from the city.
Residents can interact with Montreal nature, local tree and plant varieties, and water, and with
each other, offering both private and shared spaces.
Nun’s Island Condo Rooftop Garden at Elevation and Grade (Montreal, 2003)
This rooftop garden is an example of new construction at moderate-to-high cost, with limited
participation of end-users during final design stages. A collaboration between the developer, the
project architect, the builder and some of the prospective homeowners, the project sought to
create superior value through high quality design and catering to the tastes of well-to-do endusers, incorporating both private green rooftops and terracing, with shared green spaces at
grade. Les Somets sur Le Fleuve, the first of two such developments, have units ranging in size
between 1050 and 3500 square feet and sold between CAD $230,000 to $1,800 000 and cost $
- 16 -
2 million to construct. The project is owned by Proment Corporation in Montreal, and occupies
an area of 130,000 square feet on the Nuns Islands. The roof garden complements the natural
features of the site, which include St Lawrence River. The green roof consists of flat bed slabs
and 12 inch of soil for the growth of plants. The cost of landscaping was CAD $45,000. For the
purposes of this paper, this project demonstrates both that when applied to a niche market, a
green roof can contribute to the overall attractiveness of a project in a cost-effective manner,
and that offering future occupants design choices of a private green space, increases the
successfulness of the garden (21).
The Skyturf green roof (New York, 2002-2003)
The project is an example of a high-cost retrofit roofing project, with limited end-user
participation. Planning and design of this owner-initiated project began in 2001, with the
sourcing of both German and American green roofing solutions. While design, planning and
construction phases were completed, the building was sold before the soil was installed. Thus
landscaping was not completed. The proposed garden area was a total of 1,500 square feet on
two levels, including a canal, a water basin, built-in irrigation, a gazebo, and soil depths varying
between 8” to 24.” The project involved tearing off several layers of old bitumen roof covers,
asbestos removal, some masonry and all copings redone. No structural work was required on
this 90-year-old industrial building, as the roof is a strong concrete slab supported with 10”
structural steel beams every 5 feet. The owner9 had first selected a German green roofing
solution, which featured a zero-pitch irrigation system whereby rainwater sit on top of the
drainage mat and passively irrigate the garden through a capillary system vertically connected
with the growth medium. Employing both the German an US teams proved too expensive,
however, and the zero-pitch10 slope, too risky as warranties and insurance proved difficult to
obtain. A sloping solution from the Garland Company was finally chosen. The plan was to
provide shared access to five luxury lofts, some of which were under construction at the time.
One new tenant decided to rent the penthouse loft, despite a slump in the luxury rental market,
because of the proposed garden. However, the garden development had many delays:
Because of the high cost associated with the project, other renovation projects took precedence
over the garden project. The cost per square foot of this retrofit project was approximately USD
$42—($34 without landscaping).
9
Rune Kongshaug, same as the author of this paper.
Roof without a gradient
10
- 17 -
Unit
Who
When
Design & Planning direct expenses
Filing and Building Dept. Approvals
Tear-off, Asbestos Removal
Masonry, New Coping, Irrigation
Green Roof Components & Install
Landscaping (supplies and install)
Other materials
Total
Owner (RK)
Architect, Expeditor
Specialized Firm
Roofing Contractor
Roofing Contractor
Other contractor
Miscellaneous
Throughout
Aug-2002
Sep-2002
Sep-2002
Oct-2003
Not completed
Throughout
Cost
$4,794
$3,381
$5,000
$11,100
$25,900
$12,000
$338
$62,513
Table 5: Green Roof Budget of Luxury Dwelling, New York.
The Rooftop Wastelands project (Montreal, 1974 – 1976)
Low cost rooftop gardening is not a new phenomenon. This example also illustrates high levels
of community participation. This federally funded project took place at McGill and was
supported by the Canadian Department of Supply and Services. The Rooftop Wastelands
project in downtown Montreal, a 1,000 square meter roof was developed as a community
garden intended to serve as both a demonstration of rooftop potential as well as a living
classroom that formed an important part of the activities. Comprising about 250 garden
containers, a dozen cold frames, three small greenhouses, a compost bin, and a shaded sitting
area, about 200 people would partake in gardening and nutrition classes, of which 80 would
participate in gardening. The project brought together a team of carpenters, architects, McGill
staff and students, and participants from the community. The project had, as its original
objectives, both social and technological goals. It was intended to examine the feasibility of
small-scale, individually operated food production within the downtown area of a major city. As
a result of the activities, a Gardening Supply Store was started where soil, seeds and organic
fertilizers were made available to community residents at cost. Response from the community
was so enthusiastic that extra courses had to be given and a waiting list was made for those
wanting gardening boxes. The project was successful, but the rooftop component was very
expensive – we had to build a metal access stair, had to make wooden decking to protect the
roof membrane when people walked on the roof, we also had to build guard rails all around the
edge of the building to prevent people from falling off the roof – it was also a legal requirement.
We made every effort to make the project as inexpensive as possible by: using old wooden
containers – often retrieved from garbage, we used new materials to build cold frames but
covers for it were made using old windows which were refurbished and recycled, one of the
greenhouse was a do it your self geodesic dome, we tried our best to build most of the
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installations ourselves, but still the initial cost of making the roof operational was quite high
compared to promoting small front or back yard or small container gardens on the ground.
Unit
Cost
Garden containers
Cold frames
Greenhouses
No cost (Recycled boxes)
Only frames required new wood as recycled windows used as covers
1 greenhouse do-it-yourself (less than $200) 1commercial greenhouse
cost about $1000
Self-built with new materials
Compost bin
Table 6: Costs of gardening elements in container rooftop garden
City of Montreal Community Gardens (Montreal, 1974 – current)
During the 2002 Edible Landscapes seminar a review of urban agriculture for food and pleasure
was studied in Montreal where about 25,000 people—about 1% of the urban population—
participate in government-sponsored community gardens, peri-urban partnerships or other
forms of collective urban agriculture, not counting other private forms of enjoying gardening for
relaxation and recreation, in which 72% of Canadians participate (22). Currently the number of
gardens are around 76, which corresponds to approx. 6,000 garden plots and 10,000 familymembers involved. Each garden has a Garden Committee and an Instructor supplied by the
City of Montreal. Four t of these community gardens were studied in detail, with important
lessons that could be applied for rooftop gardening.
Budget of Gardener
Yearly fee for (9’ x 18’) plot
City Maintenance Fee
Yearly seed budget
Conserving, canning
Total
CAD
Budget of City
$10
$2
$25
$250
+/- $287
2001/2002 Budget, of which:
Admin, coordination, publication
Per plot, start-up
(not including decontamination)
Annual maintenance per plot
CAD
$770,400
$115,000
$2,000
$128
Table 7: Cost-Effective Gardening and Provision of Recreational Services
Historically, the community gardens in Montreal have been a populist phenomenon, initiated by
a small group of individuals, generally in poor neighborhoods (23). The phenomenon of
community gardening was neither initiated as a policy goal nor was it created, or supplied,
through government programs. However, management functions were first assumed by the
Botanical Gardens, and the City of Montreal played an important role in securing the terrain and
- 19 -
providing initial capital investments. The City of Montreal thus played the role as a “facilitator”
from the very first community garden, created in 1974, supporting a movement initiated by the
neighborhood residents. This basic structure has been maintained in the sense that for every
new gardens, a group of residents need to organize themselves and form a nonprofit
corporation for each community garden. The number gardens grew in waves corresponding to
the energy crisis (1974-1981) and due to growing concerns for the environment and quality of
life (1982-1996).
In addition to preparing (and decontaminating) the grounds and providing basic security by
providing gates and fences, the City of Montreal, regulates access by collecting member fees,
maintaining rules and dedicated staff for garden management and training. A similar use of
existing infrastructure in conceivable for controlling access and leveraging municipal resources
for rooftop gardens in situations where neighborhood topographies and type of housing stock is
conducive to rooftop gardening. Also, grower-dwellers must agree to organize socially and
commit to the process of setting up and maintaining the rooftop garden. Another element is the
increasing costs associated with the decontamination of soils at grade, which could prohibitively
expensive (ranging from CAD 400K—1MM per garden in Montreal), contributing to making
rooftop gardening the preferred greening solution.
- 20 -
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Canada. 2004.
(2)
United Nations Center for Human Settlements (2001). The state of the world cities. Nairobi
Habitat. 2001.
(3)
Bhatt, Dubbeling, Kongshaug (2004). Making the edible landscape: Planning and design for
urban agriculture (UA). Project for 2004-2006. Submitted to: Cities Feeding People Program,
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Canada. 2004.
(4)
Rybscinsky, Witold [et al.](1976), Rooftop Wastelands. McGill University. 1976.
(5)
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