One Roof, Two Roofs, Green Roofs, Blue Roofs

city of green rooftopsContemporary designers continue to explore new ways that the forgotten wilderness of the roofscape can be utilized as usable space with a greater purpose. Roof designs can become an integral part of a network of sustainable systems for a green building to purify its connection with its surrounding environs (be them urban or rural.) New York City has recently pushed past its green roof initiative to include “Blue Roofs” in its new campaign for a cleaner city, but despite the endorsement, convincing residents to invest in roof systems may still face resistance.

Now in his third term (somewhat controversially), Mayor Bloomberg has toted sustainability as a priority throughout his tenure in overseeing one of the most densely populated cities in the world. In addition to creating PlaNYC 2030 and yielding power back to pedestrian traffic in numerous locations like Times Square and Herald Square, the tax credits for green roof construction were also passed under his administration. According to an article by Reuters, the Mayor is taking another swing with an evolved message that adds “Blue Roofs” to the arsenal of tactics to be employed throughout the city.

Bloomberg’s targeting of stormwater management as an urban issue could not be more appropriate.  In many of our cities the problem of our wastewater infrastructure is rarely given the level of importance and urgency that it is due. Like many older cities in the Northeast, New York’s wastewater system has been around for over a century and was constructed around the idea of Combined Sewer Overflows. CSOs funnel stormwater and municipal sewage into the same pipes—a two for one deal that helps reduce the amount of infrastructure that’s built as well as diluting sewage waste on its way to treatment.

The problem is that the growing population of the city has paralleled the growth of impervious surface, meaning that volume of both sewage and stormwater run-off have risen beyond the capacity of New York’s 14 wastewater treatment plants. When rainfall reaches a certain strength (sometimes as little as ½” per hour), the flow exceeds capacity of the infrastructure and is allowed to “overflow” into the East and Hudson Rivers. Raw sewage into public waterways. It sounds shocking, but it is far more common that most of us think and stands in the face of the Mayor’s goal of 90% of the city’s waterways suitable for recreation.

Short of replacing the entire system (a fiscal impossibility for the city any time in the foreseeable future) any efforts to stem the flow of stormwater into the sewage system will relieve the pollution from entering the natural watershed. On the contrary, Bloomberg claims that not only could the city save $2.4 billion over 20 years by utilizing sustainable roof systems but not doing it could force taxpayers to spend $6.8 billion on repairing the flooding treatment plants.

The Rainbow of Options

In some ways, Green Roofs and Blue Roofs accomplish similar things. Green roofs consist of growing plants on rooftops and can be divided into two groups: Extensive and Intensive. The former refers to systems that usually have 6” or less of growing medium and often are placed on top of the roof in either trays or bags. Intensive roofs are deeper installations that include full continuous soil over an integrated drainage mat and water proofing. These roofs can be anyway from 8” to multiple feet deep.

Conversely, the label “Blue Roof” has been coined to refer to systems that focus on rainwater collection. By using catchment pools, rain barrels and more discreet water-hungry plantings, the goal of the roof system would be to minimize the amount of storm water that a building site sheds to the rest of the city.

Both of these options help mitigate stormwater by gathering it as rainfall and releasing it gradually over time. They both offer an insulating layer on top of a roof to help trap energy in the winter and reflect sunlight in the summer (reducing the heat island effect felt in urban centers.) The difference is that green roofs offer an opportunity for biodiversity and food production while the water gathered by blue roofs can be used for irrigation, cleaning sidewalks or reducing potable water use by filling washing machines or toilets.

En masse, both would be valuable, productive additions to the top of New York or many other U.S. cities.

A Hard Sell

In my experience, these kinds of roof systems have not yet found a great deal of patronage outside of municipal installations. In laying out the rooftop options of Photovoltaics, Evacuated Solar Tubes or green roofs, PVs always gain the most interest and commitment. Although I have never pitched a “blue roof,” I feel like the results would be the same.

First of all, the entire notion of blue roofs is rather new. I have never seen one installed nor have heard of one being pitched to a client in the city. The concept itself seems a bit half-baked. For an architect, the idea of having standing water on a roof is usually something we try to avoid rather than encourage. After enough time, water has a way of finding its way into just about everything.

The other problem with green and blue roofs is that the benefits are not necessarily less material, but usually less visible and not as concentrated immediately on the space below them. While the benefits I mentioned are real, they are admittedly incremental as an investment with a payback period. Instead, their positive effect is felt more at the urban scale. Adding to biodiversity yields no dollars back to the client, but could bolster urban farming as well as a healthier city ecosystem. While reducing the heat gain of one home is relatively small, covering 20% of the city with green roofs could lower the ambient temperature of the island as much as five degrees in the summer according to one green roof installer. Even stormwater management is a city-scaled benefit that may eventually yield lower taxes but only after a certain critical mass has been achieved.

Unfortunately, too many people may need healthy incentives in order to invest in these new roofscapes. Only so many people are willing to fund a smaller piece of a larger system with minimal return. As of yet, no new details have been released concerning how the effort will be supported by the city. The current subsidy for green roofs is $4.50/square foot up to $100,000 which most likely does not cover more than 20-30% of an intensive green roof system.

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46 Responses to “One Roof, Two Roofs, Green Roofs, Blue Roofs”

  1. I just visited NYC and I have to say that for me decreasing the temperature by 5 degrees is a BIG incentive. I was so hot and so sweaty that my hair was basically wet all day. My friends kept saying things like, “Welcome to NYC.” Like I just had to deal with it and there was nothing that could be done. I applaud any efforts in this vein! (Not just because I can’t handle the heat, because I don’t live there, but for our society and environment as a whole.)


    • Hey Crystal, thanks for stopping by. It really is an issue. In the dead of summer the difference in temperature between 34th and 6th to Central Park is amazingly apparent–which is in itself a problem! There is an opportunity for the city to be cleaner, but more comfortable as well.

  2. Just accidentally found your blog on green roofs. I saw my first ones in Europe many years ago. Excellent blog.

    • Great to have you, Chris. Green roofs can be amazing spaces, even if they are for viewing more than occupying. When I worked at Cook+Fox I was there when their green roof was installed and watched it grow for two years. It’s really a rewarding experience and changes your perspective of what living and working in the city can be like.

  3. What a cool post-I lived in Manhattan for 7 years and it would be wonderful to see more green roofs there. My refuge was Central Park and without it, I don’t think the city would be liveable. I hope more roofs get planted and not just in NYC. Thanks.


  4. That seems like a perfect opportunity for a small vegetable and herb garden.

  5. I absolutely love the idea of green roofs. When I own my own house, I’m going to get a green roof and–totally agree with The Gates of Lodore–grow my own garden up there. :D

    • Absolutely. Small scale urban farms produce more food than most people would think. On a single brownstone roof in the city you can grow more produce than a family would need. On top of that, you insulate your home that provides savings on heating and cooling while the plants and soils helps to mitigate storm water runoff. If I had my own roof in the city, I would already be cultivating!

  6. i hope everyone would have the initiative to participate in this simple activity that may lessen the intense effect of global warming not only in your place but also for us and the whole world.

  7. I used to work across from this huge building that was completely covered with gray roofing material. It was big enough for a soccer field. We used to look at it during lunch and come up with ideas for things that would go well there. Unfortunately, I don’t think it had a great deal of structural support. Ideally, though, a soccer field surrounded by fence and caged in would be really cool. You could play soccer on the top of a building in the middle of the city.

  8. thats such a cool idea!!! green roof!!


  9. The outgoing mayor in Chicago is a big proponent of green initiatives. So much so that the building shared by the county and city has a split roof – one green, the other a regular roof. At one point they did a study on the differences and found the city portion to be cooler and work better in the long run. If Bloomberg can get this going, it would be great for a number of reasons.

    • Thanks for stopping by. I’ve only been to Chicago once, but I loved it. From what i have heard, Chicago may be clear winner in U.S. for the amount of square feet of green roof installed to date. Where New York may be “endorsing” these roof systems, Chicago has apparently embraced it. With all hope, Bloomberg can take what appears to be a working model and refit it to a larger city.

  10. When you look at NYC and land use, you’ll find that the outer boros are much bigger than Manhattan. In places like these, homeowners have a rather counter-intuitive tendency to pave their garden spaces. Tax incentives to jackhammer this concrete (and fines for new pavement) would seem to solve the runoff issue on a much greater scale.

    • An interesting suggestion. I agree that together the boroughs comprise a larger land area than the island, but I would think that roof space is still much more prevalent than yard space in any of those burrows–even the Bronx. Queens and Brooklyn have many large warehouses and their blocks are not really populated with the missing teeth of vacant lots. Try to stem certainly would help in some capacity, but I don’t know if its enough to trump targeting roof surfaces.

  11. Interesting article, critical mass of adoption is the biggest hurdle for any new technology. All the cats sit around the pond waiting for someone else to try to snag a fish first, nobody wants to get their feet wet. If I owned a flat-topped building in NYC, or anywhere else, I would install gardens at the very least. More plants = more CO2 sucking power = cleaner air :) Big cities need as many plants possible.

  12. Interesting piece on a very good idea.

  13. When I first visited NYC, I was shocked at the amount of green throughout the city…and that was outside of Central Park. There is of course room for improvement, but I think that the city has a good foundation. Just think of the opportunities for plantlife on balconies…greenhouses on rooftops. In the era of urban sprawl, this type of growth may one of the primary ways to help mitigate the urban heat island effect afflicting the city.

  14. When I saw Bloomberg on the Daily Show, I was amazed at what a difference in energy it made just to paint the top of a building white. It seems that these green roofs would have a tremendous benefit.

    • Welcome, Mandy. Talking about a true no-brainer that requires virtually nothing but produces results… People like myself can support green roofs knowing that they carry an expense while bringing many benefits, but painting roofs with reflective or high-albedo colors should simply be required as the new standard. In summer sun a black tar/rubber roof in NYC can easily reach temperatures of 125 degrees. Green roofs could lower that surface temperature down to 70 degrees, but reflective paints can take off a meaningful chunk as well.

  15. Such a great idea! Love thy environment, and it will give back positive effects on us. Kudos to the people behind this and good luck to them! May each company or those high-end building owners cooperate on this project.

  16. Green roofs are refreshing in the eyes. It’s a good idea to have one, it will promote eco-friendly nature. Nice blog!

  17. Great topic here.
    Work of many people on this issue of plastic, there are several plastic materials recycling organic-based view. In February, for example, Imperial College London and bioceramic drug polymer biodegradable plastic from sugar derived from the decay of lignocellulosic biomass. There is also an existing plant more corn starch and plastics based on paper, including household goods and food packaging, bioplastics toys, plastic dynamic Cereplast. Metabolix also several lines of plastic products from corn, in cooperation with partner companies.

  18. this sounds pretty good to me. though it will take ages to get done.

  19. Wonderful post. A few points:

    – Many businesses and NGOs in the DC area are beginning to use green roofs in their even on larger buildings because they do in fact provide reductions (sometimes up to 30%) in heating and cooling costs – so some benefits do accrue directly to the building owners.

    – another good place to start is condominium owners, who I’ve found are increasingly willing to pay a bit more upfront for a benefits of a usable green roof

    -finally, green roofs can also be used in some cases to collect usable rainwater (since the lightweight substrate material does not retain as much water as topsoil). There is a debate over the usability of rainwater runoff from green roofs, which I’ve attempted to sum-up in this article:rainwater and your green roof, but it seems that the water can be usable given the right plant/substrate mix.

    • Thanks for swinging by, Dave. I think that energy reduction costs are definitely real, but we are in still in the phase of the process where numbers are still being gathered. I imagine the type of program (use groups) in the building makes a difference as well. More technology-laden occupants have things that use energy but produce heat anyway which may skew numbers as a percentage of savings.

      Agreed, sometimes rainwater catchment from green roofs is a possibility but the sediment can complicate its end use if you are not including greywater filtration (even something as simple as a sand/gravel based filtration system.) Large buildings like One Bryant Park can use greywater for cooling tower make-up, which helps with efficiency and has cost reduction implications–but it requires filtration to avoid damaging the equipment. Even flushing toilets or custodial sinks will want some primary filtration as well.

      I think it depends on the size of the building, the size of occupancy, how big the roof is in comparison to the overall square footage, and of course how much rainfall it gets. Certainly an option though. I think greywater filtration is a good system to have anyway, regardless of green roof installation. Thanks again for stopping by. Hope to see you around again!

  20. I agree the benefits alone far out way the cost of the system. Although it wouldn’t hurt researching a cheaper way to produce the same effects, if possible.

  21. The use of stormwater in highrise buildings is an important issue. One way to address the issue without having water on the roof just waiting to damage the building, is to actually use the water.

    The Head Office of the Department of Conservation in New Zealand has used a lot of ‘grey water’ for it’s toilet system and water features. This makes sense. Why do we need drinking standard water for fountains and flushing? It also saves the Department money. These issues just take a little thinking outside of the box.

    • Absolutely. Supposedly the administration is trying to vie for systems that push away from grey technology, claiming that they are less costly and require less maintenance (debatable perhaps). Personally I think grey water systems are an asset that allows this kind of infrastructural campaign to have a bit more breadth and depth, allowing it to solve multiple problems. For as troubles as our sewage and stormwater situation is, we are amazingly wasteful with our potable water usage. Some buildings like One Bryant Park are seizing these kinds of opportunities by not only capturing every drop of water that falls on their site, but using on-site grey water systems to use that water for flushing toilets as well as their cooling tower make-up. Personally, I think it’s worth the investment. Thanks for stopping by!

  22. A great way of embelishing city panorama and helping our ecosystem. But is there any chance that this kind of roof gardens can damage the building with excessive humidity ?

  23. as a native of New York City, I think that an increase of green roofs would vastly benefit the city. Is that picture of green roofs rendered, or is that an actual shot of the city?

    I was at the Metropolitan Museum of Art this summer on one of the hottest days. I was on the roof of the met, checking out this bamboo sculpture, and it was SO unbearably hot up on that roof and it made me think of how this heat was reflecting right back into the atmosphere.

    I live and write up in Rochester, where some schools here have installed a green roof and the kids are collecting data from it. You can read it here:

  24. A green roof is a fantastic investment and a great way to help the environment.


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