GRP 002: The Types of Green Roof and What to Expect from Each

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In this episode: The difference between extensive, semi-intensive and intensive green roofs; modular versus built-in place

Links Mentioned

GRP 002 Green Roof Types (right click and select “save” to download this episode as an mp3 file)

Raw Transcript

Will: This is the Green Roof Podcast, Episode Two.  Hi everyone, this is the Green Roof Podcast episode 2, the podcast dedicated to helping your home or business get on the road to a beautiful, energy saving living roof, even if you’re just starting today.  Welcome to our second episode, my name is Will Ward

Kevin: And I’m Kevin Songer

Will: And today we have a special announcement that we will be bidding farewell to good ‘ol Dave Hilary. Dave Hilary was a pen name that I used for the first year of Green Roof Plan.  And when I started the site it was kind of a side project, and I wasn’t quite sure what would happen to it; it was an article based site.  And for those two reasons I decided to publish the articles under a pen name. But as the site has grown over the past year and I’ve been interacting with more and more people, it just became less comfortable to be using a pen name, and I felt that it was getting kind of ridiculous,  and I should just switch to my real name, which is Will.  So that’s me and I apologize if anyone is uncomfortable with that, but I’m hoping to make a fresh start today.

In today’s episode we are going to be going over the different types of green roofs and what you can expect from each. And I know when I was first starting the research for Green Roof Plan, I was a little bit confused by all the different distinctions between extensive and intensive green roofs, and semi-intensive, and it seemed like each source had their own definition of what they were, and there was a little bit of overlap, and the borderlines between the various types weren’t very clear.  Kevin, is that what you found as well?

Kevin: Absolutely.  There are a lot of descriptive terms with respect to green roofs, and some are quite technical.  However the best way to remember it is that green roofs are really just plants on a roof, and we’ll talk about some of the more descriptive terms today.

Will: So why don’t we start with the two main classification of green roof which are extensive and intensive, so what’s the difference between those two?

Kevin: Very simple.  Extensive green roofs are shallow green roofs, and intensive, they’re just the opposite.  You can think of “intensive care,” a room in the hospital, very detailed, very thick in soil media, and quite complicated.

Will: And what type of projects would the two different types of roof be suited for?

Kevin: Will, that’s a great question. Really both roofs have very specific applications, and an extensive green roof is typically used on a residence or a roof that cannot support the extreme weight that you typically see on an intensive green roof, so extensive green roofs might be found on a shed, a garage, a commercial building, a café, a restaurant, whereas intensive green roofs are more suited for large commercial buildings that are structurally and architecturally designed to handle the extra loading.

Will: Another borderline between extensive and intensive roofs is the need for irrigation, so how does that break down in your thinking?

Kevin: Good question, Will. There really are no specific requirements for irrigation on either extensive or intensive green roofs, however irrigation is a very important component on both. We typically see that extensive green roofs are designed to accommodate what I call “nature irrigation” or rainfall, precipitation, water vapor, dew and fog, whereas intensive green roofs, because of the thickness of the soil media, they typically have a micro-drip type irrigation or recycled rainwater irrigation system installed.

Will: Another way that these break down – and this is the way I have it explained in the ebook on my site- is by soil depth as the main variable that separates these out, and the way I had it organized was an intensive roof is going to have around 8 inches of growing medium, whereas a semi-intensive, which is the middle zone would be 4-8, and then the extensive, the shallowest, would be something like 1-4 in terms of growing medium depth – does that sound about right?

Kevin: That’s correct. The weight of the roof increases exponentially with each additional inch of soil media, especially when saturated, so typically the industry has delineated extensive roofs as those living roofs that have soil depths of less than four inches and intensive as having eight inches or more, and of course the semi-intensive or semi-extensive, however you wish to refer, as hybrids somewhere in between the four and eight inches.

Will: So, with these different soil depths of extensive versus intensive green roofs, naturally different types of plants are going to be able to grow in them, so what are the guidelines you think about for intensive versus extensive roofs?

Kevin: There’s been a lot of research done around the world by the Germans, the Europeans, who really have a considerable number of years of research and experience in planting green roofs. As to which plants to use, this is one of my favorite topics. I love to look around the world and talk to people from around the world and see their experience as to which plants have done best on extensive roofs, and which are more suited for intensive roofs.  And, of course, the extensive roofs, having a shallower soil media, requires plants that can grow in soil that has depths of – as we said – less than four inches. A good place to look for these types of plants are those plants that do well on rocky crevices or outcrops, or in very sandy soil. Intensive green roofs, the plants that can grow there, the choices are virtually unlimited, so you have a lot more, you can adapt the soil composition to exactly what you need, and install those plants to suit your preferences.

Will: And the last variable, which a lot of people are going to be thinking about with extensive versus intensive is the cost, so with an intensive roof, very likely you’re going to have drip irrigation built-in, you’re going to have deeper soil, you’re probably going to have a wider variety of plants, so I imagine all that would add up to generally a higher cost for the intensive. But do you have any guidelines when you’re thinking about costing out an intensive versus extensive project?

Kevin: Yes, Will, absolutely. We see intensive green roofs more and more being installed on municipal buildings or government-funded projects, those projects that have a larger budget and can afford the additional architectural support that’s required to successfully hold an intensive green roof. You’ll recall we just had a green roof collapse in Chicago from a snow build-up. So it’s vital, as you mentioned in the last podcast, critically important, that your structural system in the building can handle the heavy weight of an intensive green roof, and, as such, intensive green roofs are going to be a lot more expensive because you have not only the structural components, but you also have a variety of system components that are installed such as irrigation. And we see the cost of intensive green roofs ranging anywhere from $20 per square foot to $100 per square foot or more, depending on the plants and the type of system, and of course the structural requirements. And extensive green roofs just the opposite: I look at extensive green roofs, Will, as being more a cost-effective green roof or living roof available to most people and a DYI kit can run from $5 a square foot to as much as you want to spend. So extensive green roofs are a lot more cost effective, they’re a lot lighter, and require much less support architecturally. Of course, it’s very important when installing a green roof that you always check with a structural engineer or architect to make sure that your structure can hold and support the green roof.

Will: Yeah, and that’s really a whole other dimension of cost that can potentially be added to a green roof. When you have a structural engineer out and you need to do reinforcement to the underlying structure, have you had any projects you’ve worked on where that’s been an issue and you’ve had to rework an underlying structure to make it able to support a green roof project?

Kevin: That’s all based on the desires of the client, Will. If the client does not have the funds to beef up the structure then generally what we do is scale-down the roof to match what his existing structure possesses. However, in a situation where there’s a commercial building and someone wants a very nice rooftop garden or living roof, then if the funds are available it’s certainly very easy to go in and retrofit the roof to accommodate the additional support requirements.

Will: Okay I think we’ve covered extensive versus intensive pretty well. The other major category that green roofs break down along is modular green roof systems versus built in place green roof systems. How would you explain that to someone who is first getting into green roofing for the first time?

Kevin: Sure, Will. The modular ones are ready to install. You have beauty, you have greenery, you have habitat, storm water control, ready to go, just set in place, much like a set of building blocks children would use. However, a build in place-type system is more, you bring the dirt, the soil media in, you typically have an array of smaller plants, generally plugs, and it may take a while, some time, for those plants to grow-in and give you solid coverage, so I would best describe modular as a ready to go, fully grown system that you can just set on your roof, plug and play type, and go, whereas a built in place system would be much like building a race bed to your garden adding the soil media, adding the plants, watering, and watching them grow.

Will: So if you were an individual homeowner, what are the guidelines you would be thinking about if you were trying to decide between a modular system or a built in place unique system?

Kevin: Sure, there are advantages and disadvantages to both. Modular systems are great. They don’t work in Florida as well as they do in other parts of the United States because we have cyclones and hurricanes here, and a modular system may blow off in a Category 3 hurricane, whereas a built in place is more designed to withstand more environmental factors. However, modular systems now are being designed to also withstand environmental issues, so you need to check with your building code. But if I was a homeowner, I would look at both. It depends also on how much I want to get involved in the design, installation of the living roof. There’s advantages to really understanding in depth and in detail your system, so if I just wanted greenery and didn’t really want to become involved in design I would really go with a modular system. However, personally I love to look at how soil media is working and understand root architecture and see how the plants are doing and look at the thermal efficiency, so again, it’s purely the homeowner’s choice depending upon their level of interest and involvement.

Will: And if you’re interested in more information about modular green roofs, usually some of the bigger green roofing companies offer ready to go modular systems, so just two of those, if people want to go check those out: and are two companies that make modular green roof systems. And what about, you know, looking at some of the modular companies it seems in general, their projects are tailored towards bigger urban buildings, doing an entire rooftop of an apartment building, that type of project, but there are modular systems that can be tailored or scaled-down to an individual home, is that correct?

Kevin: That’s correct. In fact I saw an advertisement on the internet that Toyota, I believe it is, actually has a small carpet-like grass that they’ve developed that comes in 2 foot by 2 foot or 1 meter by 1 meter squares that you can just lay on top of your roof, so there are modular systems that are available for residences, and certainly the internet is a great way to find out about those.

Will: Alright, well I think that wraps up our discussion today about the different types of green roofs: extensive versus intensive, and modular versus built in place, and I think Kevin has our quick tip for today.

Kevin: Absolutely. You know, Will, last week we talked about the benefits of living roofs, and there’s a great article that’s just recently been published by the Columbia Daily Tribune out of Columbia, Missouri, and if you go to Will’s website I will pass that link on to Will and it’s well worth your time and reading.

Will: That, I think, is going to wrap it up for today. I want to thank everyone for listening and thank Kevin, and don’t forget to check out his website, which is which has all of his green roof research and pictures of projects that he’s working on, and a ton of other useful information, and as always, please if you have a comment or question, or suggestion for something that you’d like us to cover in the podcast, please leave that in the comments. Thank you all for listening to the Green Roof Podcast, and we’ll see you next time.

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