GRP 003: Getting to Know Your Green Roof Components

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In this episode: waterproof membranes, options for drainage systems, and the essential functions of green roof soil mixture.

Links Mentioned

GRP 003 Green Roof Components (right click and select “save” to download this episode as an mp3 file)

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Raw Transcript

Will: This is the Green Roof Podcast episode 3. Hi everyone, welcome to the green roof podcast, the podcast dedicated to helping you get your home or business on the road to a beautiful energy saving living roof. Even if you are just starting today, my name is Will Ward

Kevin: My name is Kevin Songer and hello from sunny and hot Florida today

Will: Can you believe that we are on episode 3 already?

Kevin: Time flies

Will: Anyway today on Episode Three we are going to be talking about the essential components of a green roof  system and when I say essential there is an asterisk by that because there is really no world standard for a green roof system and  you can have something as simple as a home built into the side of the hill with sod covering it or various types of economic or likely green roof systems that have very few components are extremely simple and I hope we will be able to get into those in more detail in the future episodes. But what we are going to be talking about today are the generally accepted standard components of a green roof that you would find in a house, in a single family house in North America. Is that how you describe this Kevin?

Kevin: That is right Will, absolutely. There are as many different ways to build a green roof as you can imagine but there are some good tried and proved methods and those are the ones we will be talking about today.

Will: So, the way we decided to structure this is starting from the bottom up so you have your underlying structure, the roof surface and above that the first layer of your green roof system is going to be a water proof membrane, which in most cases will also function as a root barrier to keep the roots of your plants from penetrating and going below the bottom of your up green roof system. So what are, what is the general purpose of this layer, Kevin, and what are the things you look for when you are thinking about designing this layer?

Kevin: Right Will, we have to remember that green roofs are friends to our existing roofs, they are not enemies. However there are some factors associated with green roofs that do call for some protection of the underlying roof system. We want to keep water off of the underlying root system and we want to keep roots from growing into the existing roof system or after so a water proofing membrane root barrier is the first, the first layer you will see on most green roof systems and they come in, the membrane comes in propylene polyethylene or a sprayed on elastomer. The root barrier is typically the waterproofing membrane with extra engineering and design to prevent roots from growing through the membrane.

Will: And if you could walk me through, say I have some people coming over this afternoon to install one of these water proof membranes, what is the process, how did it happen, what is it look like and about how long is it take? That stage of the process

Kevin: Sure Will, if your system that you are purchasing uses a water proofing or root barrier membrane then you are more than likely it will look like a big sheet of black plastic and the roofers will carry it up and attach it to your roof by a number of different methods typically the most common is a low Volta organic compound, a low VOC adhesive and membrane is laid directly down on your roof. If it is a sprayed on elastomer then they will have a bag packed of swords with a hose and spray nozzle and they will be spraying the, the membrane directly over top of your existing roof so there is a variety of products. Typically the application or installation of your water proofing membrane and root barrier is a very quick process and should not take too long

Will: And just looking around on the net there are dozens of companies that offer this type of product but you have any tips for someone looking and trying to choose between all of these.

Kevin: Well it depends on their budget of course. I will talk to different representatives from the products and find out what they advantages and disadvantages are of each product. There a very unique Hydrotech system that say a sprayed on type system, there are couple of great fire stone liners, so again it just depends upon what the preference of the client is and their budget. I would always recommend several bids, several quotes to make sure that you are getting the best bank for the buck with respect to the water proofing membrane.

Will: Okay so we have got out water proof membrane down and that is also functioning as a root barrier to keep the roots from penetrating through that membrane and interfering with the structure of the root. And the second layer of the green roof system is the moisture retention and drainage layer. So its two functions are number one to retain some water to allow the plants to grow but then also to drain water away from the house and filter it down to your drainage system so that it keeps a lot of weight from humiliating on from the house structure. Is that generally how you look at it?

Kevin: That is correct Will. Green roof plants typically don’t like to have what I call wet feet. So, after it rains, they don’t want to sit in a puddle of water. These plants are designed to withstand drought typically or long periods of drought. Even those that have your irrigation systems and most of the more drought color plants such a succulents like sedum, they don’t like to have what I call wet feet so it is important that we move any kind of rain fall or storm water away from the plants and off the roof as quickly as possible. Having said that, there are a couple of products and moisture retention products that either a fabric that absorb water and hold water much like a diaper or fabrics that have little cups simple, excuse me molder plastic panels that have dimples that serve to hold water and so the plants can take advantage of the water in the future. From a stormwater perspective it is also important to be able to move water quickly out through roof because you, water is very heavy Will, you don’t want to have too much of a water build up because your structure could be comprised if you had a lot of water standing and pooling on the roof so the drainage layer is very important.

Will: Some of the dimple products are very interesting design because they allow water to go only up to fill up the small little cup of the dimple and I guess the easiest way to demonstrate this would be with a picture which we will have in the show notes, but once the water fills up that indentation in the dimple any access water will flow over the top and then it is filtered off into the regular drainage system and that there are a number of companies that do that. Are there ones that you would recommend or ones that you work with a lot when you are doing green roofs?

Kevin: Again I would say do you research on the Internet. I think that water retention products are very important but honestly Will I think specifying the right plants for the roof is probably more important because there is a fundamental problem with irrigation. When you have high desiccating winds like many cities do during the winter time when it is cold and the air is very dry, doesn’t matter how much water is around the roots of the plants, green roof plants, doesn’t matter how much irrigation you apply to it, doesn’t matter how much water is absorbed in fabric on in the dimples of water retention panels. If the wind is too, too fast and too desiccating, the vascular system in the plants are not going to be able to provide enough water to the leaves and they are going to have plant desiccation occur. So the panels are important and the fabric is important but I typically focus on other aspects of the green roof rather than the water retention aspects as more of an area to understand and make sure your design is correct.

Will: And the other function of this layer is the drainage function so what are the different ways or structure that accomplish the drainage function, how does that work?

Kevin: There is two basic types of drainage systems for green roofs Will. The first is arrogate or gravel based drainage system, much like a drainage field in your septic tank drain field and then there is a polymer type woven drainage mat which is like a big sponge or a big piece of half inch thick walcrow that allows the water to flow down, keeps the gravel above and facilitates drainage but as we mention drainage is very important aspect of a green roof system.

Will: Okay and if you are looking at the aggregate system what are some of the tips that you have when you are looking for that type of system. What are you looking for?

Kevin: Well there is a lot of material that will work well as arrogate in a drainage system for a green roof. You can use a gravel, you can use a crushed brick, any other type of recycled crushed glass, what you want to make sure of a couple of things. You want to make sure that the material is free from very fine dust and I typically say keep your fines, fines being the amount of material that we pass through a number two hundred sieve, keep those less than seven percent, five to seven percent and you don’t want the material that you use for drainage to either lower or elevate the PH of your stone water and you don’t want it to contribute to any kind of a acute toxicity either so ASTM has a great standard D448 size number 8. I would check out that standard when looking that gravel for a drainage layer

Will: Okay and if you are going with the filtered fabric type system what do you look for in that type system.

Kevin: Typically Will a filter fabric goes above the gravel and excusive swarm media which may contain some organic material from clogging up the drainage layer so a filtered fabric goes on top of your poly drainage mat or your gravel and what I see most of in filtered fabric is it is made from a non woven geosynthetic material much like a rock wall or a wool blanket will sit on top of your drainage layer and basically all it does is keep the drainage layer from clogging up with the soil from the green roof soil media but it is a very important component within the green roof system so after you see your roof will lay down the water proofing membrane and the drainage layer, then on top of drainage layer goes the filtered fabric.

Will: Okay so we have got our water proof membrane, we have got our moisture retention layer, we have got the drainage layer, we have got the filtered fabric to prevent the soil media from going down into the drainage layer. Let’s talk a little bit about the soil media. What are the main things that we are looking for in a soil mixture for a green roof?

Kevin: Well you want your soil media to be light weight number one, keep heavy things off the roof. The lighter the better, number two you want it to be able to hold water, retain water from a storm water perspective and from a watering the plants perspective. You want it to be able to grow plants so it has to have a wide range of characteristics that enable plants to dry such as, we want our soil media to have a PH of somewhere between six or six and a half and eight, six and half being the lower side, eight being the highest side. We would like to see soluble salts less than three or four grams per liter potassium chloride. Importantly we want that soil media Will, to drain water well. So the hydraulic conductivity, we  look for a value of at least one two inches per hour which is 30 to 55 millimeters per hour and you don’t want your soil media to have a lot of large stones in it and this is particularly true of the cyclone and hurricane areas like we live here in Florida but also as we see more and more tornados come across the Midwest, you want that green roof soil media to be small and nothing large that could be picked up in wind storm and damage adjacent fenestration or windows so this is some of the qualities that we look for in green roof soil media. Certainly we want it to be able to support plant growth that is very important and through trial and error, talk to some good gardeners, you know, there is an extreme science involved in development of a green roof soil media and it is very interesting.

Will: Ya and one of those areas which we touched on a little bit in our discussion before recording is the controversy I guess we should say, over whether or not to use shale or expanded shale as part of this mixture of your green roof substrate. So can you talk a little bit about that controversy and how you see shale factoring in or not factoring into the sustainable green roof system.

Kevin: Great, great thought Will. There some wonderful arguments for both the use sim for the non use of expanded shale in green roof soil media. Referring to the argument first, it is a very prominent material. Once you put expanded shale on to a green roof you don’t have to worry about it. It is there forever, very light weight and has a great water absorption capacity so there is many benefits to using expanded shale. However on the downside, some of the, I have seen some articles recently that talk about the extremely high carbon footprint of expanded shale because shale is mind out of the ground so you have a strip mining component. Now it is a side product of coal and other petroleum type manufacturing processes but still you are involved in strip mining and then you take and you use petroleum based fuel and furnace to fire the shale up a thousands of degrees to where it just kind of expands like pop corn so you have a huge carbon foot print associated with expanded shale now I have heard the arguments well that carbon foot prints are a onetime cost and because it is permanent you don’t have an ongoing carbon foot print however I have also seen articles that talk about the virtues of using something a little bit more sustainable like sand or even organic compost. So the jury to allow or see which way the market takes the green roof industry with respect to expanded shale.

Will: Ya we will have to get into that a little bit deeper in a later episode but I guess if I were to pull out the top three ideas from what we are looking for in the growing media is, we want it to be lightweight enough so that we are not adding a ton of extra load to the roof surface. We want it to facilitate drainage so we want it to not retain water and we also want it to have an extra top soil so that we have enough nutrients to grow the plant. Does that sound right?

Kevin: That is right. You can use the term organic material instead of top soil so could either be top soil or could be compost. We are doing a project right now where the green roof soil media is being manufactured on site from similar local materials because it is any place that is, does not have access to transportation to get the shale or some of the other products in and in addition to the crushed limestone that we are using, we are using some local homemade organic compost so top soil or compost, yes you have to have that organic component in your green roof soil media for the plants to be able to grow and thrive.

Will: Okay so we have covered the water proof membrane and the root barrier, we have covered the moisture retention and drainage layer, we have covered the growing media mixture, now we are going to get into some of the things that you won’t find on every green roof or some things that are a little bit less common depending on or dependant on the structure that you are building on your particular project. So what are some of the other components that you may or may not find on say a residential green roof in North America?

Kevin: Well if the green roof is going to be accessible then certainly you want to make sure the design is in accordance with all safety standards, all local and state federal safety standards, hand royals potentially, definitely anti slip devices, there is also a very interesting new science, it is not new science, I have just seen a research of interest in it lately, it is fog catchers and dew catchers where people are taking advantage atmosphere of water vapor, capturing the water vapor and irrigating the roof with the water vapor and then you know, the typical amenities that you would see on a garden roof in some areas, benches and containers, again you need to be careful and check with your building code and make sure that what you put on a roof is allowed and it is not going to become air borne during that wind storm.

Will: And speaking of wind, what about, if I am in an area, you know, high windy area and I am worried about soil erosion or you know the soil blowing off the roof, are there any devices that help control that?

Kevin: There are Will, there is some very interesting biodegradable wind erosion mats, many of them are easily found on the internet just type in erosion mat or wind mat for green roof and it is a very interesting products and what they do is hold the soil in place, hold the green roof soil media down until the plants, the root systems can become established and once the root systems on the plants are established the erosion control mats are bio degraded if at time.

Will: Alright, well I think one thing that this absolutely has shown me that we are just scratching the surface with all the different variations and types of materials and options that this home owners or business owners who are putting in a green roof and we certainly have a lot of material for future episodes but I think that is all the time we have for today so I wanted in today’s quick tip point you to a page that I put on which is a survey so if you go to a you will find a five or six question survey that I hope you all will just take a couple of minutes to fill out and that will let us know a little bit about what topics you are interested in and what you know, are you home owner, are you business owner, are you someone working in the green roof industry. It will help us guide a little bit more in what topics we cover and what kind of information we provide. So again, we encourage you all to go to and that will help us provide more relevant and hopefully interesting green roof podcast as we go forward. So thank you all very much for listening and we will see you next time.

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