Vegetated roofs aren’t just beautiful—they’re valuable tools for managing stormwater, reducing the urban heat island effect, and providing more usable space for building occupants.
In the second installation of the Carlisle Roofing Series, BUILDINGS editor-in-chief Janelle Penny chats with Chris Kann, roof garden product manager for Carlisle, about best practices for green roof projects.
*This podcast was created in partnership with Carlisle SynTec Systems.
Read the transcript below:
Janelle Penny: Hi everyone, this is Janelle Penny. I’m the editor-in-chief of Buildings. And I’m here with Chris Kann. He’s the Roof Garden product manager for Carlisle SynTec Systems. And today, we’re going to be talking about green roofing solutions. Welcome, Chris.
Chris Kahn: Thank you.
Janelle: So, what is a roof garden exactly?
Chris: So, a roof garden, they can go by many different names. They can go by the name of green roof, vegetated roof, living roof, but basically, it’s a contained green space on top of a man-made structure, which can be above, below or at grade.
Janelle: Interesting. Why might somebody install something like that?
Chris: Well, there’s many different benefits of installing a roof garden on the rooftop of a building. Some of the primary benefits are the ability to retain stormwater that would otherwise flood the sewer system and potentially cause flooding.
This is especially important in older cities where the sewer system does not have the capacity to handle those heavy downpours when they occur. Additionally, roof gardens provide useable space for the building occupants by creating a natural environment on the roof. They can also help to reduce urban heat island effect.
Janelle: Makes sense. So, we’re based here in Iowa and I’ve seen a few of them locally, but are there other areas of the country where roof gardens are better suited?
Chris: Yeah. Absolutely. As I mentioned, one of the biggest benefits of a roof garden is that stormwater management. So, there are areas in the country that are better suited where they have an older sewer system, such as cities like Washington, D.C.; Baltimore, Maryland; Philadelphia, Pennsylvania; New York City and Chicago. And those are also areas where incentives exist to basically manage your stormwater.
There are other areas of the country such as the Pacific Northwest which have ideal climates for promoting the vegetative life. And then there’s certain other areas of the country such as the desert Southwest that aren’t so ideal for green roofing necessarily because of the shortage of rainfall in the area. But you can use drought-tolerant vegetation and you can create a green roof in those areas as well.
Janelle: I’m glad you mentioned drought-tolerant vegetation because I was wondering if there were specific plants or certain types of vegetation that you’d want to use?
Chris: Yes, absolutely. For the most part, pretty much anything that can be planted on the ground can be planted in a roof garden system, provided that the plants are suited for that particular building’s USDA Hardiness Zone.
However, sedums are the most popular option for vegetating a roof garden. And the reason for that is sedums [MO1] are very low growing, so that means they require very little maintenance. You don’t have to go up there and mow like you would with like a grass or something like that. And they’re succulents, which means they require very little water for survival. They can also be grown in extensive or shallow roof garden assemblies, which are the most inexpensive systems.
For plants or trees with more complex root systems, you need to use more of an intensive or a deep roof garden assembly, which requires a little bit more maintenance. But, generally speaking, sedums are the most possible plant. However, you can use pretty much any vegetation you would use around the building.
Janelle: What do you mean by extensive and intensive?
Chris: That’s a good question as well. So, roof garden systems are classified by the depth of the growth media that’s used. Extensive systems, which are also sometimes called shallow systems, utilize between 3-5 inches of growth media.
And they’re designed to be low maintenance, utilizing mostly sedum vegetation. And these extensive shallow systems are really the most popular roof garden assembly that’s going to be out there in the industry.
There are also semi-intensive systems that are also called medium systems. And they utilize between 5-8 inches of growth media. And because of the deeper amount of growth media there, you can start planting a little bit more local plants, perennial flowers, those types of things.
Then you move into the intensive systems, sometimes called deep systems. And they utilize growth media that is depths greater than 8 inches. And they require higher levels of maintenance, obviously.
The vegetation you can plant in a deeper assembly is essentially limitless. Ranging from shrubs to trees. However, the thing that’s always gotta be remembered is as you get to deeper and deeper assemblies, the weight is going to increase, but also the amount of maintenance is going to increase.
Janelle: Okay. So, you mentioned growth media, too. Isn’t that just a fancy word for “dirt”?
Chris: Well, when discussing roof garden systems, it’s very important to make the distinction between growth media that’s used for rooftop applications like we’re talking about here and dirt found on the ground.
Growth media is engineered to strict FLL guidelines. And just to make it clear, FLL is some long German word that I can’t pronounce. But it’s basically an organization that sets standards and guidelines for blending growth media.
But again, growth media is mostly an engineered media that is blended out of inorganic aggregates and some organic matter needed to sustain the plant growth. Growth media is engineered to be lightweight and maximize the water holding capacity of the system. But also, once it gets saturated, it’s designed to drain very quickly as well.
The use of inorganic aggregates provides structural stability, minimizes shrinkage, which is a big common problem with dirt. Standard dirt is also very heavy and compacts over time. So, if you use standard dirt, typically you’re going to be left with much less depth of that dirt over a long period of time because of the compaction that occurs.
So, again, growth media is just designed to kind of retain its initial depth, hold a lot of moisture, but also drain very quickly once it becomes fully saturated.
Janelle: Could you elaborate a little bit on the weight and water holding capacity aspect? What does that look like for a typical roof garden system?
Chris: Yeah. So, the weight of a roof garden system is completely dependent on the depth of the growth media installed and the water holding potential of that system.
So, most roof garden systems that are installed, as I mentioned earlier, are a shallow system. That’s the majority of what we see being installed out in the industry. And those types of systems utilize drainage boards and retention fabrics and typically around 3 to 4 inches of growth media. And then they’re going to be using sedums, like we mentioned earlier.
A system like this will weigh somewhere around 10 to 15 pounds when it’s dry. However, they’re never fully dry or at least we don’t want them to be fully dry. So, when fully saturated, a system like that would typically weigh somewhere between 30 and 34 pounds per square foot.
So, what that means is that a system like that is gonna retain around 20 to 25 pounds of water when it’s fully saturated. Or 2.4 to roughly 3 gallons of water. And on a year-round average, 55 to 80% of all the stormwater that falls on a shallow system like that will retain about 55 to 80% of the stormwater that falls on it. And it won’t be released to the drains as run-off. Which, again, is one of the biggest benefits of these roof garden systems in areas with older stormwater systems, like we mentioned previously.
Janelle: Great. No wonder you can get those stormwater incentives.
Chris: Yeah. Absolutely.
Janelle: What are the different components you would need to make sure this roof garden installation is successful? And how would you install something like that?
Chris: Roof gardens are available in two main installation categories. You’ll often hear them being called either a traditional system or a modular or tray system.
For the purposes of explaining, I will focus more on a traditional system. Again, a traditional system is going to be one that is layered on the roof, so it’s actually going to be built in place. So, before any installation occurs, we must ensure that the roofing system itself is designed for the roof garden components.
So, at Carlisle, we have some minimum requirements that are needed for putting a system like this on top of the roof and in place. What we like to see is a minimum half-inch thick adhered coverboard overtop of our insulation layer in a traditional roofing type assembly to provide a good surface for membrane application. It also provides better impact resistance for the weight of the system that’s going to be installed on top.
Next, Carlisle requires minimum of 60-mil membranes on your roof garden systems. So, 60-mil membranes for our shorter warranty durations, and then we like to see thicker membranes, such as 80- and 90-mils for longer duration warranties.
We also require that the seams of that membrane system be overlaid with cover strips prior to the roof garden installation. So, the point being here, the reason behind that is we want to have that belt-and-suspender approach to the roofing system that’s going to be fully buried underneath a roof garden system so we know we’re good long term.
So, once the roof system itself is designed and ready to go, the next step of the installation is we are going to frame out the roof garden area with specifically designed edge metal that gets adhered to the roofing membrane.
Then a protection fabric is installed and a drainage board along with moisture attention mat and filter fabrics. These materials can be installed as individual components, or materials like Carlisle’s MiraDrain G4, which incorporates all four of those materials in one easy-to-install product.
So, MiraDrain G4 just incorporates that protection fabric, the drainage board, moisture attention mat and filter fabric into one easy-to-install product.
Next comes the growth media. And so, growth media can be supplied in either super sack form, which typically includes two cubic yards of material. Or it can be supplied in bulk. Bulk just basically means it’s delivered by a dump truck, and then that media would have to be blown to the roof.
Typically, it’s going to be done with a super sack. And then what happens is they will crane that super sack up to the roof, hover it over the drainage board and (the fabrics that were already installed) cut it open and just allow it to kind of fill out on top of that system.
It’s going to be raked around to the desired depth. It’s going to be compacted. And then next, for an extensive system where sedums are typically installed, you would either be doing sedums in plug form—they can be planted by cuttings—or the most popular option are the fully vegetated systems like the tiles and mats. And then those are simply placed on top of the media and made sure that we water that system fully after the day of installation and thereafter.
Janelle: Now, can you retrofit these things onto existing buildings? Or do you typically only use them on new construction projects?
Chris: That’s a very good question. We get asked that quite often here at Carlisle. Many of the roof garden systems are installed over existing buildings. We see a lot of that occurring. Especially because, as I mentioned earlier, a lot of these incentive programs are going to be in older cities where they have those poorer storm water systems in place, right?
So, they’re not building a lot of new construction in those areas. So, retrofitting is definitely something that’s going to occur. However, a structural analysis will need to be done on the building and the roofing structure by a certified and licensed structural engineer. And they’re going to look at the live and dead loads of the system to make sure that the roof structure is going to be able to support the weight.
If it cannot support the weight in the current state, additional bracing must be provided to allow for the roof garden installation. And, in some cases, if the weight is going to be too much and they’re not able to brace that system accordingly, other options would need to be looked at for stormwater management.
Janelle: Back to the components that we talked about. You mentioned the 60-mil membrane requirement as a minimum, but is there a specific type of membrane that you would suggest for a roof garden system?
Chris: Any single ply roof membrane can be used under a roof garden system. That includes EPDM, TPO, PVC and KEE membranes. It’s also a very popular option to use a fluid applied membrane, such as hot rubberized asphalt.
Depending on the roof membrane and vegetation being used, a roof barrier may be needed to prevent damage to the membrane by intrusive root systems. One of the benefits of using a roof garden system is that they protect the roof membrane below from any damage and degrading UV light. So, the roof membrane can oftentimes last much longer than a conventional exposed membrane system.
Janelle: What does the maintenance look like for a roof garden?
Chris: That’s a very good question as well. One of the biggest misconceptions about roof gardens, especially extensive systems that we’ve really kind of talked about here when they’re using sedums is that, once they’re installed, they’re good to go and they don’t require any type of maintenance.
And although that can be the case, regular maintenance is required to ensure the success of the vegetation being used, just like the maintenance provided for vegetation at ground level. So, this includes weeding and supplemental watering at times of low rainfall and drought. Just generally getting up on the roof and making sure things are going properly, pulling weeds if need be.
So, Carlisle actually also publishes a traditional roof garden and care and maintenance requirement document[MO2] that provides additional information on maintenance. But again, one of the things that you always have to consider when you’re designing a roof garden system or you’re considering one for your building is it’s not “no maintenance,” it’s “low maintenance.” So, we really try to make sure that our customers are aware of that.
Janelle: Okay. What about the insulative value? You’re putting on these extra layers on the roof. What kind of insulative value would you get from a roof garden?
Chris: That’s also a very common question. I think it’s kind of a misguided question at that. Roof gardens do provide some insulative value to the roofing system due to their large thermal mass. So, that is one of the benefits of putting a system like that on a roof.
However, the insulative value greatly varies depending on the amount of water that’s contained in the system at any given time. As I mentioned earlier, we always want that system to be somewhat moist, because that’s what helps to allow that vegetation to flourish.
As you probably know, water is conductive. So, as the water content of the system increases, the R-value of the system will decrease. Any R-value requirement that is needed to meet energy code compliance for that particular project must be done with the roof insulation itself.
Any additional R-value that’s provided by the roof garden system should just be seen as an extra added benefit and not figured into the R-value of the roofing system.
Janelle: Can roof gardens help contribute toward LEED credits?
Chris: Roof gardens can contribute in up to five credit categories with a possibility of 10 total LEED points. Additional credits and points are available if the project is a school or a healthcare facility. But again, generally speaking, five credit categories, 10 total points.
Some of the examples of these credit categories where roof gardens can provide LEED points are under the SS Sustainable Sites. So, site development credit, protect or restore habitat, you can get a potential of two points. The open space credit, there’s a potential of one point. Rainwater management credit is a potential of three points. The heat island reduction credit can give you a potential of two points. And then there’s the WE (Water Efficiency) credit where a requirement for outdoor water use reduction allows for a potential of two points.
So, I just want to be clear here, too, a roof garden isn’t necessarily going to provide all of those points by itself. It’s just going to contribute to those points from the rest of the building and the rest of the site.
Janelle: So, Chris, to finish up, can you explain the roof garden product and warranty offerings that Carlisle has and why it’s so important to have a single source roof garden warranty?
Chris: Sure. As mentioned previously, I work for Carlisle SynTec Systems, where we manufacture many building products, including single ply roof membranes. Because roof gardens are placed over our roofing materials, it was only natural that we provide a roof garden system to be placed over these membranes.
Carlisle SynTec Systems offers a wide variety of roof garden systems, including traditional and layered systems or module systems. No matter which system is installed, an overburden warranty is available for 10, 15 or 20 years when installed over a properly designed roofing system.
And overburden warranty covers the removal and replacement of overburdened materials supplied by Carlisle. And that is if there is ever a need to access the roof membrane in the event of a roof leak. So, an overburden warranty is totally separate from the roof system warranty itself.
I’ll explain it like this. If you have a roof system warranty on a Carlisle roof membrane and you install overburden materials from another company, and let’s say down the line a leak develops, it would be the building owner's responsibility to remove that overburden material to provide access to the roof membrane for Carlisle to then go in, investigate and repair a leak. And then the building owner’s responsibility to also replace those materials.
So, the benefit of an overburden warranty is that Carlisle would then be responsible for that removal and replacement of the overburden materials in the event of a leak.
In addition to overburden warranties, Carlisle also offers a vegetation warranty, insuring 80% vegetative coverage of the system after two years, as long as the system is full vegetated during the time of installation, meaning we’re going to be using sedum tiles or mats, or full vegetated trays.
So, again, the main benefit of a single source overburden warranty is that you only have one company to call in the event of a leak. Carlisle will come out, investigate the leak, remove the materials needed to access the roof, fix the leak and put everything back in place when the roof system and overburden warranty are purchased together.
Janelle: So, where can we learn more?
Chris: For more information on Carlisle’s roof garden product offerings, you can go to www.carlisleroofgardens.com. Or you can go directly to the Carlisle SynTec System’s website, which is www.carlislesyntec.com and click on “Carlisle Roof Garden Systems.”
Janelle: Great. This has been Janelle Penny, editor-in-chief of Buildings. Thank you to everyone who’s listening to this today. And Chris, thank you again for joining me.
Chris: Thanks a lot for having me.
Don't miss these other episodes in this roofing series: