My recent research shows that installing a green roof system on a project aimed at LEED certification has the potential to contribute to the following LEED credits:
SS 5.1 Site Development - Protect or Restore Natural Habitat (1 point) - extensive green roof only
SS 5.2 Site Development - Maximize Open Space (1 point)
SS 6.1 Storm water design quantity control (1 point)
SS 6.2 Storm water design quality control (1 point)
SS 7.2 Heat island effect: roof (1 point)
WE 1 Water Efficient Landscaping (2-4 points)
EA 1 Optimize Energy Performance (up to 19 points)
MR 3 Materials Reuse (1-2 points) and Recycled Content (1-2 points)
MR 5.2 Regional Material (1-2 points).
Thus, installing a green roofing systems of 50% of the roof area guarantees 2 LEED points and can contribute to 33 possible points which is 82.5% of the points required to be LEED certified.
That being said, why does LEED not differentiate between extensive and intensive green roof? SS5.1 is only eligible with an extensive green roof which is composed of native plants. An intensive green roof on the other hand, which has deeper substrate, foreign plants and requires watering (which creates evapotranspiration) would contribute higher to SS6.1, SS6.1, SS7.2 and EA1. Why does LEED assume any green roof will have the same affects? Shouldn’t different types of green roof systems contribute to different scores in particular categories?
Erin Holdenried
Sustainability Architect125 thumbs up
April 28, 2014 - 11:06 am
If your project is eligible, a green roof can contribute to SSc5.1 if it is intensive (not extensive). LEED does not assume any green roof will have the same affects. It depends on the credit, and what the credit it focusing on. For SSc5.1, the credit focuses on biodiversity, an intensive system allows for more plant variety. For SSc6.1 & 6.2, and intenive system may also help reduce stormwater impacts because it can hold more water, but your civil engineer will have to do the calcs to determine what system would be most appropriate if you are just looking at just stormwater management. Not all credits have multiple threshold levels for varying points. But, you can earn Exemplary Performance for this credit with an entirely green roof.
Not sure how to answer your question, but obviously you have done a lot of research, and know the properties of a green roof can contribute to many credits. That is the advantage of a green roof, over other roof systems.
Davis Kantor
LEED AP BD+C1 thumbs up
April 29, 2014 - 2:07 am
The problem is an intensive green roof is typically composed of foreign plants. yes, it can host a wide variety. but the optimal roof for habitat restoration is really a seeded green roof of native plants as SS5.1 suggests. Only plant species that grow naturally in the region are going to promote biodiversity. these roofs are generally a few centimeters unlike intensive green roofs with much thicker substrate which requires more maintenance and LCC.
Shouldn't a green roof that better promotes biodiversity and requires little to no maintenance be awarded more points in the LEED rating system? I ask because I am researching green roofs in Switzerland where there are many city wide mandates requiring unoccupied, new or renovated flat roofs to be extensive green roofs. I'm currently preparing a plan for a Masters thesis on this topic and i want to promote a change in the LEED system which awards more points to an extensive green roof because ultimately it has a better affect on the environment. I'm just not exactly sure how to go about it.
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11478 thumbs up
April 28, 2014 - 9:49 pm
Davis, a couple comments. One, I would be looking at LEED v4 (see LEED-NC v4 SSc5), because that is where LEED has developed to today. Not that it is much different with respect to the heat island credit, but you might see some changes with rainwater management that might have an impact.Two, I agree with the earlier comment to this thread, in that LEED does reward intensive green roofs for their better stormwater management potential.Three, your previous comment on this thread leaves me confused. At the end of your comment you say you want to award "more points to an intensive green roof because ultimately it has a better affect on the environment," but in the rest of your comment you seem to suggest that an extensive (i.e. thinner) green roof better supports biodiversity.Which roof do you think is better? If you're saying that extensive roofs are better, I'm confused, because it seems to me that a deeper substrate simply allows more opportunity for biodiversity and more choice of plant species, as well as potential for a more three-dimensional habitat space that supports more species. Look at the intensive green roofs in Chicago, for example, that support entire botanical gardens and prairie landscapes.I would say that opposite is true—only sedums and some grasses grow on extensive roofs, and those plants might very likely not be native to the area.Regarding maintenance, every installation will be different. No green roof will be maintenance free, just as no landscape is maintenance free.
Davis Kantor
LEED AP BD+C1 thumbs up
April 29, 2014 - 2:20 am
Hi Tristan-
Excuse me, I meant to say 'I want to promote EXTENSIVE green roofs because ultimately they has a better affect on the environment.' Thanks for catching that! I edited my previous comment to say what I intended.
So, actually extensive green roofs provide great habitat restoration because they are generally composed of native plants. Again, only native plants naturally growing in the region are going to improve biodiversity. This is why Switzerland has mandated such roofs in all major cities, and you can really notice the difference! Please read my experience of the top 5 green roofs in Switzerland. http://land8.com/profiles/blogs/top-five-green-roofs-from-switzerland-tour
I'll tell you that these roofs are naturally seeded and the only maintenance required is bi-annual mowing. I'm not kidding, these roofs are practically maintenance free because the plants grow naturally in the region! They don't ever water their roofs here, that is also important to consider when you talk about maintenance requirements.
True, intensive green roofs, with a deeper substrate, provide better storm water retention. More plant diversity will provide better biodiversity ONLY if they are native plants. However, intensive green roofs are generally composed of foreign plants. I say this because I have been a PM on the LEED project at USF. http://land8.com/profiles/blogs/under-construction-center-for-science-an...
You only need an intensive roof for native plants if say, you have trees and large shrubs growing. Does this make sense?
Tristan Roberts
RepresentativeVermont House of Representatives
LEEDuser Expert
11478 thumbs up
April 29, 2014 - 9:26 am
Davis, I am still getting tripped up by definitions. In the U.S. "extensive roofs typically are defined as roughly 4" (10 cm) of soil, and no more than 20 cm.Some of the projects you highlighted in your blog post have about that much, but some have a lot more soil.I think you are oversimplifying. Deeper green roofs, like any landscape, can have native vegetation if designed and planted with it. Shallow green roofs may have native grasses, but in the U.S. they often have sedums which are not native. These may effectively provide habitat, or might not. It's not a simple matter. I think the lack of simple prescriptions here may be a factor in LEED not preferring one specific type of green roof. Here is one study that indicates the variation that can occur.
Davis Kantor
LEED AP BD+C1 thumbs up
May 3, 2014 - 4:47 pm
Tristan-
Yes, in America storm water retention is the biggest motive for future public policies to incentivize green roofs. And the deeper the substrate, the more rain water which gets retained. This is clear.
All of those examples in my blog post are shallow, extensive green roofs. So, you are wrong in saying that the examples in my blog post 'have a lot more' than 20cm. The point is that a native plant palette does not need a thick substrate.
However, deeper substrate can potentially host native plants which would promote biodiversity and retain more rain water. According to LEED v2009 it seems that this would be the best strategy for a green roof design seeking LEED certification. Is this a strategy projects can use to earn SS 5.1 Protect or Restore Habitat, SS6.1 Storm water Design- Quantity Control and SS6.2 Storm water Design- Quality Control?
I'm curious as to what strategy a project team would take to maximize points/benefits from a green roof. If you know of any projects that attained ID points in conjunction with green roofs I would be very interested to know what they did. Thanks!