Clip-on Architecture: Reforesting Cities

Much of the literature related to the role of the built environment in climate change has focused on new technologies and new ideas which might be implemented in new buildings. Tabula rasa eco-cities trumpeting their green credentials and high levels of environmental sustainability are being planned in the U.S., China, and Abu Dhabi, among other places, and green is the word of the day. Despite these ambitious plans for new cities, one might ask, with all the urban fabric which currently exists, why build at all, and most especially on such a massive scale?

Starting from scratch is not the only way. Given the urgency of the massive changes to our way of life that must take place over the next seven to ten years, I believe that strategies which involve a retrofit or a clip-on to our existing structures and infrastructures deserve a serious look.

Retrofitting our urban building stock to address climate change need not be limited exclusively to increasing their energy efficiency. If “one of the primary causes of global environmental change is tropical deforestation” (Geist & Lambin, 143), then we should approach the adaptation of our buildings as an exercise in reforestation. Deforestation is too often divorced from urban discourse around climate change. In an attempt to redress that, my investigation into sustainable retrofits has included research into some causes of and solutions to deforestation, including a list of interventions already being implemented in the developing world (click here to read more). We must learn from both the causes of climate change and attempts to combat it as we attempt to reforest the city.

It is estimated that the Amazon rainforest absorbs approximately 28% (7.34 billion metric tons) of carbon emissions from the burning of fossil fuels every year. Tropical rainforests are the planet's defense against global warming. Deforestation is one of the greatest sources of carbon dioxide which is the principal cause of global warming. If converted to cattle operations, rainforest land yields about $60 per acre. If timber is harvested, the land is worth around $400 per acre. But if its renewable and sustainable resources are harvested, the land could yield an average of $2,400 per acre. Each acre of rainforest prevents 75,000 to 100,000 pounds of carbon dioxide from being released.
It is estimated that the Amazon rainforest absorbs approximately 28% (7.34 billion metric tons) of carbon emissions from the burning of fossil fuels every year. Tropical rainforests are the planet's defense against global warming. Deforestation is one of the greatest sources of carbon dioxide which is the principal cause of global warming. If converted to cattle operations, rainforest land yields about $60 per acre. If timber is harvested, the land is worth around $400 per acre. But if its renewable and sustainable resources are harvested, the land could yield an average of $2,400 per acre. Each acre of rainforest prevents 75,000 to 100,000 pounds of carbon dioxide from being released.


A densely populated city replicates its ground surface area many times over in the surfaces of the buildings that populate it. New York City, for example, covers some 309 square miles (801 square kilometers) of land area, much of which is built up. As of the 2000 census, there were 7,679,307 housing units in the five boroughs. A recent New York Times article quantifies the amount of available roofspace in the city alone as 944 million square feet, 11.5 percent of the total building area the city holds. Given that the population on the planet is rapidly increasing and due to double over the next 100 years, we may soon need all the available arable land for growing crops, with marginal lands where food crops provide inadequate yields relegated to biofuel crops (Killeen, 39). As the available space for the necessary green technologies is limited, it makes sense, therefore, to consider the city as the surface for our intervention.

A hypothetical six story apartment building has a footprint of approximately 2,100 square feet. The vertical surface area available on the facade for the deployment of green technologies using wind and solar power, or green screens for vertical gardening, or water walls for cooling, is approximately 12,000 square feet if the building is freestanding, and around 3,600 square feet if it is in an infill condition. Add on the roof area, much of which remains unused, and you get 14,100 square feet for the freestanding and 5,700 square feet for the infill building. Multiply that by the sheer number of buildings occupying any densely populated urban condition and the number becomes more significant still. (Buildings are only one field of action among many: New York State also has 113,000 miles of highway, another overlooked infrastructure to which clip-ons may be added.)


Even something as simple as painting roofs white, instead of black, has been shown to provide a significant savings in terms of the amount of energy expended to cool buildings, as well as reflecting heat away from the city rather than absorbing it. Read more about other sustainable solutions currently being implemented in the developing world here.


White Roof: “Hashem Akbari, a physicist at the Lawrence Berkeley lab, just released a study showing that the average American 1,000-square-foot white roof could offset 10 metric tons of carbon dioxide. According to his data, roofs constitute 20 to 25 percent of urban surfaces, while pavement is about 40 percent. Therefore, if all of those surfaces were switched to a reflective material (or color) in the 100 largest urban areas in America, his calculations show, this would offset 44 metric gigatons of carbon dioxide. That’s more than all countries emit in a single year. Further, that’s worth about $1.1 trillion at current carbon trading rates.”[1]

Greenscreen: By integrating more trees and photosynthesizing plants within the fabric of our existing cities, we harness the power of plants to absorb carbon from the atmosphere. The surface area of buildings multiplies the ground footprint of the city many times over, making vertical gardening and the integration of growing walls into our buildings an interesting practical solution. The roofscape of most cities is an area that is often forgotten but that could easily be used for the application of green technologies beneficial to all. Greenscreen is a type of metal structure that can be attached to existing walls or used to create freestanding growing walls.

Windbelts and Green Roof: Wind belts are a recent technology which harness the power of the wind to generate electricity. They are relatively inexpensive and suitable for both developed and developing countries and are the first wind technology not to employ turbines: according to Matt Vella, “About the size of a cell phone, the final Windbelt prototype employs a taut membrane that, when air passes over it, vibrates between metal coils to generate electricity.”  Windbelts could be used on the facades and roofs of existing buildings as a sculptural element, taking advantage of the building envelope as an available surface upon which to attach. Trees may be planted on the roof by using either planters or by using a new Japanese soil substitute, Pafcal, which is much lighter than earth.

Windbelts and Green Roof: Windbelts can also be attached to functional structures such as canopies which are normally used to protect the building entry from rain.

Roof Pond: Roof ponds can be used for cooling in areas that are warm and not very humid. This technology has a lot of potential, but has been underused to date because of a fear of leakage on the part of architects and clients, however, if properly detailed it is a promising strategy and can help to reduce the heat island effect in cities. Water is placed between two layers of insulating material. The area covered with water should be 85 percent to 100 percent of floor area in places with winter temperatures between 25 and 35 degrees Fahrenheit (-4 to +2 Celsius) and 60 percent to 90 percent of floor area in places with winter temperatures between 35 and 45 degrees Fahrenheit ( +2 to +7 degrees Celsius). Average pond depth is between 3 and 6 inches (Stein & Reynolds, 2004.) Insulating panels cover the roof and are opened during the day in the winter to absorb the heat of the sun, and at night, the panels are closed, allowing heat to radiate to the building’s interior. In the summer, the process is reversed.

Roof Spray: This is another method for cooling which could be employed in a retrofit of existing buildings. It can be used in combination with the roof pond, or independently with the water being stored in a tank. Here water is cooled by spray at night, via evaporation and night sky radiation, and then stored for use during the day in the building’s cooling system (Stein & Reynolds, 379-80).

Water Wall, Water Collection and Solar Pipe: It is well known that electricity can be generated from fast moving water. Here, we propose that a water wall be added to a blank facade on an existing building as a means of generating electricity. Water can be collected via a system of gutters on the building, and then can be piped and recycled to generate the necessary flow. This water can also be used to flush toilets and for other non-potable applications. In addition, the water provides cooling to the building’s inhabitants.The roof in this scheme is envisioned as a space in which the entire surface area is covered by solar coils. This is a recent development: “Solyndra’s panels employ cylindrical modules which capture sunlight across a 360-degree photovoltaic surface capable of converting direct, diffuse and reflected sunlight into electricity. This self-tracking design allows Solyndra’s PV systems to capture significantly more sunlight than traditional flat-surfaced solar panels…”


How might all this begin to be implemented? Below you see a piece of New York City to which we have applied some of the technologies discussed above. Large scale urban farming which takes place indoors and on large expanses of roof, greenscreens to let plants to climb the vertical surfaces of the city, trees which are now able to grow on the city roofscape. Roof ponds and artificial waterfalls for cooling and electrical generation. Solar and wind devices which form sculptural elements in the city, performing a function as well as having an aesthetic. Ports for plug-in electric vehicles which gather energy from photovoltaics. Solar panels incorporated into street poles, and vertical wind turbines which form a rhythm in the streetscape. Bicycle lanes, room for walking and the incorporation of still more trees.

The government of New York City might choose to provide tax breaks that would give economic incentives to building owners who implement the strategies we have illustrated. A demonstration project of a square city block could be facilitated in this way, perhaps involving the participation of large scale property owners or local community groups such as PACC5th Ave Committee, and HCCI, which are already spearheading initiatives aimed at reducing the city’s carbon footprint. Justin Garrett Moore, an Urban Designer at the Department of City Planning, suggests targeting groups with large property holdings, such as the New York City Housing Authority or the Abyssinian Development Corporation.

A demonstration project may also fall under the aegis of HUD’s Choice Neighborhoods Initiative, which has as one of its goals the transformation and energy efficiency upgrade of housing in economically distressed communities. In addition to the tax break provided by the city, property owners involved in the demonstration project would see a significant reduction in their utility bills, and could potentially sell the energy they produce as excess capacity back to Con Edison. In the distant future, energy production may become a much more localized phenomenon, with the old, formerly centralized public utilities being rebranded into producers of alternative energy. Perhaps the current dilemma, rather than being seen as a death sentence or a depressing indictment of wasteful society, can provide an opportunity to rethink and retool our existing way of life. Our chance is now.


Research Team: Fay Alkhalifa, Marcus Brooks, Graziela Gimenes, Akiko Hagiwara, Anna Obratzsova, Gabriela Rodriguez, Allison Schwartz.

This article is adapted from “Clip-on Architecture: Reforesting Cities and Potential Solutions to the Climate Crisis” by Vanessa Keith, a PDF of which can be downloaded here. Download the bibliography as a PDF here.

All images courtesy of Vanessa Keith.

From “Paint Your Roof White, Save the Planet,” published on September 12, 2008.

Vanessa Keith, AIA is a principal at StudioTEKA. She is a registered architect who received her Master of Architecture degree from the University of Pennsylvania and her Master of International Affairs from the School of International and Public Affairs at Columbia University, graduating with a concentration in Economic and Political Development and a focus area in urban planning.

The views expressed here are those of the authors only and do not reflect the position of The Architectural League of New York.


Peter Sigrist January 13, 2010

Thank you for sharing these ideas and providing such clear explanations. White roofs, gardens, pools, windbelts, solar panels … all seem very doable. The gardens and pools could make rooftops even more enjoyable places to be. It’s great to see innovation applied towards retrofitting existing cities!

Jack Conviser January 14, 2010

Agreed, this is a very clear article which lists some of the ways that we can introduce more vegetation into the city scape, with all of the potential environmental benefits that accompany it.

However, this article may focus a little too much on exotic solutions which are either unworkable or cost prohibitive for much of our existing building stock. For example, as built few rowhouses or prewar office buildings could handle either the weight or waterproofing issues associated with a sod roof, and trellising vines up and down building faces requires some form of reasonable planting bed at the base that will not jeopardize existing foundations.

What is more welcome is a discussion of the groundscape solutions. While there may be 944million sf of roofscape, that leaves over 7 billion sf of groundscape within NYC including courtyards, roadways, sidewalks, and more. There are far more and far simpler opportunities to green the city and manage stormwater through those means, which including traditional street trees as well as the rain gardens, vegetated swales, and planting strips mentioned in the article. Additionally, not all can access the rooftops, but everybody can walk on the streets. Groundscape beautification is more egalitarian and does more for the public at large.

Before developers spend exorbitant amounts of money on roof top solutions, we should encourage them to spend smaller amounts on the streets sidewalks, parks and plazas around us.

Paul Nelson - StructureHub January 14, 2010

Thanks Vanessa. I now await your thoughts about financing!

Jack, I’d argue that Vanessa’s suggestions are actually not “unworkable.” Certainly, as you note, some buildings are structurally incapable of supporting the substantial weight of green roofs, among other improvements. But these same buildings are entirely capable of undergoing other efficiency makeovers – such as having their roofs splashed with white paint – to take another of Vanessa’s examples.

Moreover, it’s worth noting that the ultimate goal of these suggestions is not to beautify the streetscrape (beautification just happens to a happy consequence of our primary concern: energy conservation via “green” improvements). Rather, the suggestions are aimed at energy efficiency and reduced environmental impact. It just so happens that – at least in terms of square footage – Vanessa’s suggestions regarding the building envelope may not lead to the most-visible beautification (at street level). But the suggestions do address major shortcomings in our absurdly inefficient inventory of existing buildings.

Incidentally, roofs, being spatially and visually isolated from the sort of substantial human and vehicular traffic on sidewalks and roads, respectively, may actually be easier to transform over time. Practically speaking, lives would likely be less effected from many top-level improvements during construction and the stakeholders would be fewer in number during planning phases as well.

Kat Logan Smith January 20, 2010

I love all of this…but would have to resist trees on roofs- I’d suggest sticking with grasses, vines, sedum and other plants. It is because here in the Midwest, we often have straight-line winds that knock over full-grown trees rooted in the ground….many people have lost their cars or homes to sycamore limbs or pin oaks downed – and the prospect of tree limbs falling from high roofs- too much of a people/property damage risk.

jim January 23, 2010

Good information here. I enjoyed reading this and can’t wait for more. Keep up the good work.

Kelven Goodridge January 24, 2010

Fantastic article, nice writing and great images make it easy to understand for neophyte.

Pirijan Ketheswaran January 28, 2010

Hi, I really enjoyed your article on Reforesting Cities and the really great ideas I got from it. I do have a question though, I was interested in creating the style of 3d illustrations you’ve used in your post to visualize some of my own ideas for creating a comphrehensive ecological system within the urban realm. (I’m an urban design and planning student). I was wondering what software or specific techniques you use or would recommend?

Thanks so much

Vanessa January 28, 2010

Hi everyone, I have been out of the country for about a month and just got back to see all the responses to the articles–thank you! I have a longer piece that explains much of the missing bits in the research which we’ll make available soon. I will send it to the editors here at Urban Omnibus. Would love to see it implemented in a pilot project somewhere, so ideas on that are most welcome too. Regarding the software, we used Rhino 4, turned it into line drawings and finished it up with Adobe Illustrator and Photoshop.


Varick Shute February 8, 2010

Hi all,

The extended article that Vanessa mentioned is now available as a PDF. You can download the document here (the link is also posted in the footer text of the article above). Thanks for making it available Vanessa!


Carl February 23, 2010

I totally loved this article, I have been designing a 3d city on sketchup with the future in mind and have had some innovative ideas (which may have already been thought of and I haven’t read about it yet!). I’m glad to see that others have been thinking on the same wave length. I hope governments in developing countries consider these ideas in their plans for future urban developments. Along with our own too make living in cities more enjoyable and within guilt of damaging our planet through living unsustainable lifestyles.

jon yang February 24, 2010

Simple proposed stepping stone.

Change building code to require all new construction to be ‘green roof adaptable,’ similar to ADA type notion.

New buildings of x dimensions be required to have base structure be engineered to carry future loads of green roofing. This should require nominal oversizing of members for vertical and perhaps a little extra shear bracing for wind through the leaves.

Carlo Farroni February 12, 2016

see my project on my web site