Friday, February 6, 2009

Green Roofs?

I have remained on the sidelines with regard to promoting green roofs. I'm hesitant partly because many people in the multifamily industry have this saying that goes "flat roofs always leak".

In my own experience, the most frequent and worst leaks occur at the low spots where ponding occurs. A pond on your roof causes dust in the air to stick to the surface of the water. Over time, this dust builds up and starts looking like dirt, or soil. Seeds get blown into this muck, and sprout. I know this is a poorly designed ad-hoc green roof, but the fact remains that leaks never start in the dry spots on flat roofs.

I guess some sort of roof garden would be nice if you also had a roof deck and actually used it.

I'll let Dr. Joe L., every building scientist's hero, explain it:

"Green roofs? Grass and dirt are not energy efficient.Work with me here. Which saves more energy—2 inches of dirt or 2 inches of insulation? Which saves more energy—grass or a white colored membrane? Which is more expensive and does not save energy—grass and dirt or insulation and a white colored membrane? Which needs to be watered to keep the grass from dying and blowing away? But they are beautiful and look cool. And that apparently is more important than cost and energy savings. Okay, I can live with the beautiful and looking cool argument if that is in fact the argument—but don’t clutter it with half-truths such as heat island effects and water run-off. There are other (better) ways to deal with each."

He goes on to admit he won't win the argument, so he provides the proper drawing detail that works.

In semi-arid Denver, I'll posit that the money spent for the water needed to keep your roof alive will be far more than the dollar value of any perceived benefit.


  1. As an architect, I have been designing 'flat roof' buildings for over 20 years. But none of these roofs are actually flat, and we have found that two areas are especially vulnerable to water penetration. The first is at roof penetrations, where improper flashing can cause serious leaks, and the second is at parapet penetrations for downspouts and overflows. Based on my own empirical research, I think that flat roofs can be water tight if the correct system is installed and if the workmanship is at a high quality. We spend a great deal of time detailing and inspecting the work on projects we design.

    Now, regarding green roofs and watering. In California, because of our relatively benign climate, we can use drought tolerant plants, eliminating the need for watering.

    We are currently designing a complex of 120 residential units in 7 "mini towers" in the Oakland Hills and we are incorporating "green roofs" over both the parking podium and on a portion of each building roof. However, we are not incorporating them for insulation value, but rather for their value in reducing impermeable surface area so that we can meet strict California "on site" water treatment requirements, reduce the visibility of the structures within the landscape, and to create the outdoor gardens you spoke of.

    As far as cold climates, like Lake Tahoe, and insulation, we still like to design relatively flat roofs so that we can trap the snow and use its insulation value. It is ironic to me that much mountain architecture today still uses high pitched roofs. Not only do they shed the snow and eliminate the insulation value, but they also usually dump it at the side of the building into undersized gutters. Water penetration and damage at eaves and fascias is common.

    Perhaps the most interesting thing to me regarding the long term potential of "green roofs" has to due with the industrial brownfield areas I provide planning services for. In every major urban center, there are thousands of acres of industrial roof tops, unshaded by surrounding buildings or vegetation, that can be both green and productive. I picture flying into one of these communities and instead of seeing acres of flat white roofs, seeing a patchwork of small organic rooftop gardens generating both local food and oxygen.

  2. Thanks for some great info. and new ideas. I've done a lot of thinking and research about snow country roofing and guttering. Basically, in the Colo. high country, gutters just don't work. They either freeze up or get ripped off by ice and snow. The ubiquitous heat cable needs replacing every 3 years and has high operating costs.

    As far as all the commercial roofs, I think PV solar will eventually make more sense than growing stuff. PV would offset way more carbon than plant life. And the adjacent parking lots are a better/easier place to grow food.

    A low slope roof in snow country makes some sense as you say, but glaciation is a problem to watch out for. I've seen 24" flue pipes ripped completely off the roof from that effect.

  3. Most green roofs use Sedum. NO watering (except for installation day)and NO mowing. In theory, the Sedum roof just needs weeding once a year.

    That being said, my roof is steeply pitched.

  4. Sedum also sounds like the perfect yard. Isn't everyone's goal something green, no watering, and no mowing?