A green roof is an ecosystem that covers a flat surface on the tops of buildings or external decks whose purpose is to retain (hold onto) or detain (slow down) stormwater and move it off the roof in controlled fashion using plants, growing media and specialized materials.

While a green roof is an artificial environment, its goal is to mimic the natural processes of its microclimate as closely as possible, providing stormwater services, air purification, and habitat for plants and animals.

Today, green roofs are common enough to have pervaded the public lexicon. Like “janky” and “Bye, Felicia,” they have a permanent place in global culture.

Still, that doesn’t mean green roofs are understood. The basic concept of Plants On A Roof contains multitudes. In order to do it right, you need to take all sorts of factors into account. At minimum, a good green roof design must:

• Keep water and roots from penetrating the building envelope

• Reroute water elsewhere, into soil, plants or rainwater collection systems, or down to the ground

• Filter stormwater to remove debris and pollutants before it leaves the roof

• Provide a medium for growing things

• Nourish those growing things so they can spread, naturalize and do their jobs well

• Leave room for green roof professionals to move around and provide care without damaging the ecosystem

• Optionally, provide a space for humans to hang out and decompress

That’s a lot to expect from a few inches of soil (or more likely, soilless media) and growing things. As anyone who’s ever thrown a bag of compost and a tray of succulents onto a roof can attest, it’s not as easy a proposition as you might think.

A green roof is so much more than a thin membrane of plants. In fact, the visible green is just the icing on the cake; equal magic happens beneath the surface.

As Green Roofs for Healthy Cities explains, “A green roof system is an extension of the existing roof which involves, at a minimum, high quality water-proofing, root repellent system, drainage system, filter cloth, a lightweight growing medium, and plants.” (1)

We will talk more specifically about this in the section below, “What Are the Layers of a Green Roof?” For now, suffice to say that each component of a green roof ecosystem has a job related to the detention, retention and movement of water.

Now let’s turn our attention to another point of green roof confusion: the difference between that and a rooftop garden.

What’s the Difference Between a Green Roof and Rooftop Garden?

Understanding the difference between green roofs and roof gardens also means understanding green roof design. At its most fundamental, a roof garden is considered a green roof – in the sense that it is an ecosystem on top of an external roof structure – and they therefore share many characteristics.

For one thing, both green roofs and rooftop gardens try to mimic nature in order to manage the water cycle, support plant life, and provide a home for other organisms – birds, insects, small mammals, microbes and fungi, for instance.

For another, architects and engineers must take many of the same considerations into account when building a green roof or rooftop garden, such as structural loading capacity, dry and wet weight (discussed more below), local climate and rainfall, and so forth.

From true, lush rooftop gardens to blankets of clover or grasses, any system that replaces impermeable rooftop surfaces with a green and growing environment is a green roof.

That said, the depth of the growing media, the plant palette and the overall design of the roof can lead to very different rooftop environments. Let’s take a look at those now.

Extensive Green Roofs

Extensive green roofs are those that utilize roughly 4 to 6 inches of growing media. Some green roof professionals can successfully grow plants in as little as 2 inches.

Typically, extensive green roofs are low-growing and low-maintenance. They use shorter plants than you would see in a traditional garden, ranging anywhere from a few inches tall to 16 inches at the highest. At that point, they usually get too top-heavy and fall over. Plant down!

Despite their thin profile, extensive green roofs are awesome at snatching stormwater out of the air and hanging onto it, either for a period of time or forever. In addition to air purification and habitat, this is the main service they provide. Extensive green roofs aren’t created for amenity use. (Although it’s quite nice to catch sight of one from the deck of an adjacent building!)

Increase that planting depth just a little bit, though, and amazing things can happen.

Intensive Green Roofs

Any rooftop ecosystem that requires more than 6 inches in planting depth is considered an intensive green roof. Intensive green roofs may have media as deep as 24 inches, or sometimes more. The deeper that media, the deeper plant roots can grow – and therefore the larger a plant can get. Trees, shrubs, herbs, grasses and flowers can grow many feet tall on green roofs with appropriate media.

For obvious reasons, the weight and stewardship requirements of an intensive green roof are significantly more ... intense. Plants may impose more demands on the green roof professional to stay healthy, though they can still naturalize nicely if they fit the microclimate well. No matter what, they usually have greater water and nutrient requirements.

Semi-Intensive Green Roofs

Some roofs split the difference between extensive and intensive.

So-called “semi-intensive” roofs usually sit between 6 and 12 inches deep, and you’d be surprised what a difference those inches can make. Semi-intensive roofs can significantly expand the range of grass, groundcover and shrub species available to the designer. It can also increase the water-holding capacity of the roof.

(Note: It is not the case across the board that deeper profiles hold more water. An extensive profile using a cutting-edge water retention layer capable of holding 90 percent of its volume in liquid could equal a significantly deeper standard media green roof profile in water-holding capacity, even with only 2 inches of media on top. Head spinning? That’s where a quality green roof professional comes in.)

Rooftop Gardens

While rooftop gardens are really intensive green roofs, they typically earn the “garden” moniker if they have many of the features of a true garden.

For instance, intensive green roofs can have deeper plantings and larger specimens, but if they’re not accessible to people and not cultivated with an eye toward human aesthetics, they don’t really fit the bill.

Rather, a rooftop garden is a place meant for people to enjoy. It will typically incorporate architectural features as well as plants, such as fountains, walkways, benches or even outdoor kitchens.

As with any other living system, the individual space, building and climate will dictate what works best on a particular rooftop. In our personal and ever-humble opinion, however, any space to which people have access – and which meets the engineering requirements – would make a nice candidate for a rooftop garden.

But that’s just us. (You can read more about whether or not to get a rooftop garden here.)

Now on to another critical question: Just why do we need green roofs at all?

Why Do We Need Green Roofs?

The best information in the world can’t make up for motivation. We won’t get very far in convincing anyone of the efficacy of green roofs without first convincing them of their worth. Ahem ... convincing you. And honestly, when it’s the dead of night and we’re having why even bother? type thoughts, convincing ourselves.

It is very, very important to keep in mind all the benefits that green roofs bring to our cities and world. Those tiny bennies – each raindrop snagged and each pollinator plied with pollen – add up over time. It’s how we convince our wildlife that cities are just as good a place to make a home. It’s how we convince the sky not to destroy us all.

Okay, that’s a stretch. But only kind of.

So Why Green Roofs? Stormwater, Baby

Why should we install, grow and heal green systems in our cities and suburban areas? Why should we continue to improve industry standards and educate hoi polloi? It’s not because it makes life easier, that’s for sure.

It’s because without green roofs, we compromise our ability to address major issues inside and outside of our cities.

The main issue is stormwater.

First and most importantly, green roofs form the vanguard in the fight against stormwater. Each rainstorm or blizzard dumps billions of gallons on cities like Chicago, St. Louis and Detroit. When it has nowhere to go, stormwater pours off of buildings and flows through the city, quite literally “storming” the streets and sewers. This causes a huge range of harms, including:

• Pollution: Nasty toxins such as jet fuel, carbon emissions, fertilizers and pesticides gather on impermeable surfaces such as rooftops and streets. Stormwater washes them away into sewers and natural waterways, to the benefit of no one. Obviously.

• Disease: Heavy rainfall also causes combined sewer overflows, which is when excess water flowing into sewers (called “combined” because they aggregate both waste and stormwater) causes them to overflow, spilling out onto the streets. It’s no secret that human waste carries mad diseases, which now coat the streets and flow every which-a-where. Gross. And NSFL, we might add.

• Erosion: Fast-flowing water strips streambanks, hills, coastal areas, fields and gardens of soil. Not only does this harm the plants and animals living there, who need that soil, it carries sediment downstream, where it deposits it in other waterways, mucking them up.

• Algal blooms: Fertilizers, pesticides and even waste are harmful to us, but they act as food and growth stimulants to algae. These tiny plants suck up those nutrients like there’s no tomorrow, causing their populations to explode. They then use up all the oxygen in the water, creating anoxic (oxygen-free) dead zones and choking out other plant and animal life.

Both intensive (deep and garden-like) and extensive (shallow and low-profile) green roofs do much to fight this. Instead of letting water run off surfaces, green roof systems grab it and hang on tight. In so doing, they keep chemicals and disease out of our streets and waterways, reduce erosion and inhibit algae growth.

Huzzah.

Green Roofs Are Really Good at What They Do

Have we mentioned yet that green roofs rule?

“Research at Penn State has shown that green roofs provide considerable reduction in the volume of stormwater leaving a developed site and they reduce the peak rates of runoff,” explains Penn State Extension. (2)

This is true in as little as 2-3 inches of media. It’s not just the growing media – which is usually engineered to retain as much water as possible – that’s doing the job, either. Retention and drainage underneath soak up and reroute water was well, while on top, plant leaves, stems and roots absorb water through their stomata (tiny pores that facilitate gas exchange). They then hold onto it for a while, breathing it back out as oxygen at a later time.

Boom: air purification and water retention all in one. Those are far from the only benefits, though.

What Other Benefits Accrue to Buildings with Green Roofs?

First, can we just say: green roofs look darn good. Aesthetically, there’s a huge difference between a barren rooftop, adorned only with pipes and vents, and a well-tended green space. After all, which you would prefer outside your window?

We know, we know. The vents. Obviously.

In all seriousness, though, we shouldn’t underestimate the value of transforming barren city spaces into lush green landscapes. No scientific evidence needed; we all know we’d rather look at a thriving natural ecosystem than cement – a truth that has repercussive mental health benefits, as we’ll discuss below.

In addition to stormwater and aesthetics, such rooftops also:

Provide Animal Habitat

Moreover, green roofs create amazing habitat for animals of all feathers (or stripes, tails, wings, what have you). Birds, bees, butterflies, flies, squirrels … our cities and world can’t live without these critical components of our ecosystem, yet increasingly, they have nowhere to go.

Over the centuries of urban development in America and abroad, humans have displaced countless animal species. Rooftop gardens and green roofs create desperately needed habitat for creatures that still want to live here, but simply can’t find homes.

Support Pollinators

Rooftop habitats also provide food and habitat for pollinators. Not only is this a good deed for our flower-loving friends, it’s critical to our own health and security. According to some estimates, pollinators are directly or indirectly involved in the production of 75 percent of the world's food crops. (3)

That includes wild species as well as the well-known honeybee, and others besides. The Food and Agriculture Organization of the United Nations found that “There are more than 20,000 species of wild bees alone, plus many species of butterflies, flies, moths, wasps, beetles, birds, bats and other animals that contribute to pollination.” (4)

That is a lot of animals we need to look out for if we care about our children’s future.

Which obviously we do, because we’re not monsters.

Mitigate the Urban Heat Island Effect

The urban heat island effect is an unfortunate consequence of our cities’ construction. Because we use so many materials that retain heat well – brick, cement, concrete, asphalt – our urban areas are significantly hotter by the end of the day than the surrounding countryside.

According to the EPA, “The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C).” (5)

That is a crazy big difference, with crazy big consequences. This increase in heat is unnatural to the organisms that evolved in Earth’s cool climate – at least, the climate of its Icehouse incarnation, a fascinating concept that is the subject of another article. (6)

(Suffice it to say that previous eras have involved much higher temperatures and levels of greenhouse gases, but the species currently on Earth are not equipped to survive in them. At all.)

Even today’s relatively moderate (compared to Greenhouse Earth) increase in heat is Super Not Great for humans, who suffer from higher levels of disease when cities get too hot. Ditto plants and animals.

Luckily, green roofs do a great job helping cool our blazin’ hot cities. They do this in two ways:

1. Reflecting more heat from vegetation than is reflected off impermeable surfaces like concrete and cement

2. Cooling the air further through evapotranspiration, which is the release of water vapor into the air through plant stomata

The results are impressive, studies show:

“If, for example, a group of buildings in Riyadh, Saudi Arabia, is entirely clad in vegetation, the gap between the buildings will become 9.1°C cooler during the day, according to the researchers’ model,” says New Scientist. “The gap’s peak temperature of the day is brought down by 11.3°C. And in London and Montreal, the peak temperature drops by just over 4°C.” (7)

Extend the Roof Lifespan

When designing or construction a building, we don’t consider the full life cycle of that building often enough. As with anything else, a roof only has a certain predicted lifespan – but the good news is, you can substantially increase it with a green roof.

According to some estimates, a vegetated roof can extend the building membrane’s longevity by two or even three times. That’s because it adds significant additional protection to the roof membrane, the layer responsible for keeping moisture and the elements out of a building.

Who doesn’t want less roof replacement? No one, that’s who.

Reduce Energy Consumption

Green roofs provide exceptional insulating abilities. Roofs covered with greenery stay warmer in winter and cooler in summer. That means less energy needed to maintain a comfortable ambient temperature year round, to the tune of about 15 percent. In a single-story structure, the benefits accrue to the entire building. In a tall building, those benefits accrue to the top floor.

This is one of the reasons that green roofs are so integral to the design of net zero buildings, which seek to use no more energy than they can produce.

Act as a Fire Retardant

Because well-tended plants are resistant to fire, it stands to reason that roofs with a layer of vegetation on top stand less of a chance of going up in flames. Given the state of California and Australia, among other places, that’s a definite benefit.

Let’s bear in mind, of course, that dead plant matter can constitute a danger, which is why green roof stewardship is such an important aspect of the green roof life cycle. While studies are limited, it’s reasonable to assume that a green roof kept moist is safe. Keeping other design aspects in mind can make it even safer. (8)

Improve Air Quality

Remember the part about how green roofs can clean the air? They do this through some of the same mechanisms by which they glug down stormwater: through their stomata and through their roots. Some of the compounds that are toxic to us and contribute to the greenhouse effect, such as carbon dioxide, are necessary to plant survival. They obligingly take it in and produce lovely O2 in its place.

They also absorb many other chemical compounds, such as soot and jet fuel, pesticides and greenhouse gases. They fix such compounds with their roots or metabolize them into healthier molecules for the other organisms in our cities.

Thanks, plants!

Extend Useable Outdoor Space

Cities are cramped. As anyone who has ever rented a New York City apartment ... or walked along a New York City street ... or seen a magazine picture of New York City ... cities are large places made up of small spaces.

Anytime we can make a bit more elbow room in these confined conditions, why wouldn’t we? That’s probably why NYC is among the frontrunners for putting its rooftops to work, creating farms and human hangouts and animal habitats.

And it’s probably why, according to Green Roofs for Healthy Cities, “In 2019, 14 respondents recorded 763 projects in 34 US states and three Canadian provinces across North America, installing 3,112,818 square feet of green roofing.” That’s a downtick from 2018, but it’s still a pretty mind-boggling amount of space we’re adding to our outdoor spaces on an annual basis. (9)

To which we say: Yay!

Reduce Electromagnetic Radiation into and out of Buildings

Electromagnetic or EM radiation and its effects are human health is an open case, but why not protect ourselves against it if we can? Cities are awash in radiation generated by cell phone towers, cell phone themselves, WiFi hubs and other signal-producing devices.

Green roofs can help mitigate that. According to some research, a green roof can cut radiation by nearly 100 percent, keeping all that extra EM noise out of buildings where people live and work. It also means preventing radiation from escaping from buildings, which can in turn protect us against those structures that produce particularly high amounts.

Reduce Noise

Turns out, green roofs are also a fabu noise barrier.

“Green roofs are able to reduce the sound exposure near or inside a building by mitigating diffracting sound waves over (parts of) roofs and by reducing sound transmission through the roof system,” one study explains. This even held true when there wasn’t a bunch of water (an excellent noise insulator) on the roof: “Commonly used porous growing substrates were shown to have good sound absorbing properties when dry.” (10)

This is a big deal, especially when it comes to dense urban living, where noise pollution is a common issue. With the growing population rate, we will need to learn to live more closely together without driving each other crazy – not historically a strength of humanity. Green roofs can help keep the sound of your neighbor’s band practice where it belongs ... inside his garage.

Create Jobs

It’s hard to say exactly how many jobs green roofs create, mostly because measurement of the green jobs sector – a Bureau of Labor Statistics initiative from 2010 – was canceled in 2013 as a spending reduction measure. While unfortunately that limits our hard figures, we can anecdotally affirm the rise in green jobs, hand-in-hand with the rise in green roof installations.

Despite the cancellation of the data collection program, however, it is still interesting to cast your eyes over the BLS’s planned collection measures, and especially its definitions of green jobs. See the sources section. (11)

Provide Arable Land for Urban Agriculture

Vertical and rooftop farming has slowly amassed interest as a viable way to feed the billions now crammed into cities across the globe. This is true not just in the largest metropolises like Chicago or Hong Kong, but also in smaller cities.

Think about it: rooftops get plenty of sunlight. They are quite literally closer to the sun, and while that means nil in terms of the pure surface-to-sun distance, it means a lot when we consider the shade factor. On the ground, tall buildings often shade out ground-level lots all day long, whereas sunlight is abundantly available on sky-high terraces and rooftops – which frequently go unused anyway.

This is a true “every little bit counts” scenario. Consider the victory gardens that people planted during the world wars. When more people took ownership of their own food supply, right there in the crowded city, more people got fed. It’s a simple equation, folks.

Speaking of grub, rooftop farms covered with growing veggies offer most of the same benefits as green roofs covered with non-edible vegetation. Plus: dinner.

Offer LEED Credits

LEED certification, a widely recognized environmental stamp of approval administered by the United States Green Building Council, is a coveted gold star in the building industry. LEED stands for Leadership in Energy and Environmental Design, which are exactly the qualities it looks for in new projects and retrofits.

Good news for green roof proponents, explains Green Roof Technology: “Overall, Green Roof systems installed on 50% or more of the roof surface virtually guarantees 2 LEED points and can contribute an additional 7+ points toward LEED-certification, almost 20% of the total needed for a project to be LEED-certified.” (12)

Either on their own or in conjunction with other sustainable building elements, green roofs can contribute mightily to earning this certification. What’s not to like?

Provide Mental Health Benefits

Green roofs are also a great way to up the mental health of the average citizen. Studies have shown clearly that natural settings put the mind at ease, help you think better, boost creativity and help people remain in the moment. (13)

Plus, they can also improve physical health, at least for those who actively work on them. (14) Think urban farmers, pleasure gardeners, or even owners of rooftop patios that have a lot of ornamental vegetation. In addition to lower body mass index, those studied demonstrated greater quality of life measures, higher satisfaction and a stronger sense of community and social wellbeing, all of which are in turn linked to greater health.

We all have a desire to engage with the natural world, and green roofs are an excellent antidote to the monochrome colors and harsh lines that are a reality in every city.

Turn Water Into Wine

… okay, no. But basically, right? We had you, admit it.

As Green Roofs for Healthy Cities sums up, “Green roof technologies not only provide the owners of buildings with a proven return on investment, but also represent opportunities for significant social, economic, and environmental benefits, particularly in cities.”

Great, you’re thinking. I’m jazzed on all these benefits and all. But let’s talk turkey. Is a green roof nothing more than a money suck that will prevent my kids from going to college in the name of cleaner air?

Um, no. Head to the next section for an attitude adjustment.

Are Green Roofs Profitable?

In order to answer the question of whether or not green roofs are profitable, let’s first assess how much green roofs actually cost.

Granted, figuring out the cost of a green roof sight unseen is a bit like figuring out how much surgery will cost, when you have no idea who the surgery is for, what part of the body is to be operated on and if the patient has preexisting health conditions.

Which is to say ... you will not get a hard and fast number here. (Though you can certainly get one if you contact us and set up a consultation.)

What we can do is take a look at the basic factors involved. Green roofs will cost more as the following factors increase:

• The size of the roof to be covered, though this is mitigated by lower costs per square foot as you increase the area

• The weight of the green roof to be installed, as an extensive roof will require fewer resources and less time to put in than an intensive or garden roof

• The difficulty of the project

• The amount of retrofitting or specialty engineering required to make the rooftop structurally sound

In some cases the amount of rainfall to be captured can also drive the price up, but not necessarily. Typically the ultimate goal is to maximize stormwater absorption with the least amount of depth and weight.

If we’re strictly using media as the mechanism to capture stormwater, then it would be a simple positive correlation: more stormwater = higher price. However, some products such as mineral wool come in bigger modular components, and are therefore easier to install than heavy bags of soil. Plus they make up a significant amount of volume, which takes less labor to install.

Other factors, such as your specific climate, what types of plants you want to use and the right media for your project can also impact it. Here are some basic assumptions on which most green roof professionals operate:

• An intensive green roof usually costs more than an extensive one, though again, depending on whether you’re using media or geotextiles, this can vary

• A highly accessible roof will cost less than one that’s hard to get to (e.g. high up)

• Roofs that are already strong enough to support the intended green roof won’t cost as much

• If the client already has a design, that will cost less than if one is needed

• Plants, growing media and amenity components all impact price

That said, green roof installation does fall into a basic cost range:

• $35-$45 per square foot for an extensive green roof smaller than 5,000 square feet

• $15-$25 per square foot for an extensive green roof larger than 5,000 square feet

• $75 and up per square foot for an intensive green roof smaller than 5,000 square feet

• $55 an up per square foot for an intensive green roof larger than 5,000 square feet

So yes, that’s a lot of money. But there are a number of ways to make your green roof pay itself back, both in the short term and over its long lifespan.

How to Make Your Green Roof Pay

Green roofs do require significant investment, but it’s worth it in many ways.

Reduced Energy and Stormwater Costs

Stormwater management is very, very expensive. Consider the fact that polluted runoff “is the number one source of contamination to California’s renowned waters,” for instance. Or that this costs the state’s economy  an estimated $21 to $51 in healthcare expenditures. (15)

Now consider the fact that, according to a New York state study, every cubic foot of unmanaged stormwater costs downstream residents $1.20. (16)

Also, there are those reduced energy costs we talked about. If your building costs thousands of dollars to heat or cool, then 15 percent is a pretty significant savings.

Lastly, recent research indicates that green infrastructure such as rooftop ecosystems may cost significantly less than traditional grey infrastructure projects such as culverts or dikes. Says CityLab of green infrastructure installations: “About a quarter of projects raised costs, 31 percent, kept costs the same and more than 44 percent actually brought costs down.” (17)

Increased Property Values

Perhaps unsurprisingly, the beauty of green roofs also offers a significant economic benefit. According to one study, installing a green roof can raise property values 7 to 11 percent. If you plan to sell the building in the next few years, that’s certainly worth thinking about.

Oh, and did we mention that those property value hikes are conferred on surrounding buildings as well at a rate of up to 5 percent? All for one and one for all, we say. (Thank you, Alexandre Dumas.)

And let’s not forget that rentals with green roofs can charge around 7 percent more than those without. Considering all this, a green roof can pay itself back in a surprisingly short period of time, especially if it’s on a building shared by multiple people, such as an apartment or condominium.

Credits, Grants, Discounts and Greater Construction Allowances

LEED certification isn’t the only perk that accrues to green building. State and federal governments, municipalities, nonprofits and private institutions all offer financial incentives to build green roofs as well. This can dramatically undercut the cost and make a green roof more profitable more quickly.

Revenue-Generating Amenity Space

Then there’s the actually-making-money-honey route. Coffee shops, restaurants or small convention spaces in the midst of green roofs attract lots of visitors. Once you pay off the overhead, those profits will roll in.

So before you swallow hook, line and sinker that green roofs are too expensive, consider what we’ve just said, and make a smarter choice.

Now let’s turn our attention to another important green roof factor to consider: the intricate layers that make up the efficient whole.

What Are the Layers of a Green Roof?

We like to think of a green roof like a cake. Mostly because we really like cake, but also because it’s an effective comparison.

Like a cake, a green roof has distinct layers. Also like a cake, those layers depend on a specific order to function properly. Who wants frosting on the bottom, after all? So, with cake metaphor firmly in hand, we’d like to introduce you to the specific layers of a green roof, from the bottom up:

Layer 1: Waterproofing Membrane

This critical layer protects the roof. It makes sure that all that water getting sucked up by vegetation and media stays out of building materials, insulation, HVAC systems and more.

Layer 2: Root Barrier

No green roof system is complete without a means of keeping roots, as well as water, out of the building. As anyone who’s ever seen Virginia creeper knows, roots are adept at clinging to and digging into building surfaces.

A root barrier ensures that doesn’t happen from above. These barriers are formulated from a flexible material that keeps roots out, typically Linear Low Density Polyethylene (aka LLDPE), but avoid chemicals or compounds that might react negatively with the rooftop, soil or plants.

Layer 3: Drainage Layer

The goal of a drainage layer is to move excess water – that which can’t be absorbed by plants or media – to a drain or scupper. Drainage is a tough engineering question, because we must create balance between what plants need and what the roof can hold. Typically we take into account slope, rainfall quantity and load-bearing capacity of the roof.

Layer 3.5: Optional Water Retention Layer

Water retention layers come in many different forms, from mineral wools to fabrics to foams and more.

While a water retention layer is optional, it is an increasingly popular component in green roof design. Adding one in allows the system to detain and retain substantially more liquid than otherwise. This can help engineers address the ever-growing demand for roofs to manage increasing amounts of stormwater – which becomes a heavier reality with climate change.

Counterintuitively, the water retention is also used to keep the roof wet enough so that it can continue to absorb stormwater. When roofs dry out completely, they have a harder time taking up water when it falls. Water retention layers can maintain the power of capillary action for longer throughout a dry season.

Confused? Cake mix is a good comparison. (We swear, it is. We’re not just obsessed with cake. Well, okay ... we are. But it’s ALSO a good comparison.)

When you pour boxed cake mix into a bowl and then add water, at first the water slides right off. It picks up a bit of cake dust and pools around the edge of the bowl, right?

But as you stir and the dry mix moistens, it starts to take water in more and more easily. By the time you add other liquids – the oil, the eggs – they mix in quite readily.

It’s the same on a green roof. An already-damp roof bridges the gap between pieces of aggregate through surface tension, and therefore attracts water more easily, inhibiting its flow off the roof by moving it laterally. Contrast that with a completely dry roof, where capillary action isn’t present and water just gushes downward through the dry particles like water running off cake mix.

It’s important not to go too far, though. Some mineral wools, for example, are exceptional at wetting quickly and retaining water from the get-go. However, they do not dry efficiently, and in fact are so good at sucking it up that they might take it from plants who need it. Therefore it’s important to consider every material in conjunction with every other.

Layer 4: Filter Fabric

Remember that the goal of a green roof is to manage stormwater and make sure that it’s cleaner when it enters the street. Water that runs off a roof full of chemicals, soil and gunk isn’t so good for the world.

Enter the filter fabric, which captures nutrients, chemicals, soil and debris from the media and plant layers and traps them on the roof. This also keeps the drainage layer, drains and weirs clear of debris for longer.

Layer 5: Growing Medium

We’re about to blow your mind, so better sit down. Seated? Okay, check it out: green roofs also require something for plants to grow in. Crazy, we know!

Surprisingly, it’s not usually soil. Lightweight media made from a variety of materials, such as expanded clay or shale, are usually better bets – though this varies widely roof by roof. At its most basic, growing media needs to retain water, hold nutrients and keep plants healthy over the long haul.

Layer 6: Plants

Tada! We’re at the plant layer! Plants are the frosting of a green roof, both because they manage a huge percentage of the stormwater that falls on it, and because they offer the most room for creativity and design.

Depending on your aesthetic and needs, you can use a huge variety of native plants. While we don’t typically recommend a sedum-a-thon, there are local varieties native to each part of the US (including one here in Illinois) and throughout the world. And if your goal is only to manage stormwater on a roof, not to create habitat or amenity spaces, then a variety of sedums does do the job well.

If you want to go the native route – and you should – we invite you to check out some of our favorite supplies:

• Pizzo Native Plant Nursery

• Prairie Moon Nursery

• North Creek Nurseries

So now that you know what goes into each green roof, it’s time to learn all about putting those layers together.

How Do You Build a Green Roof?

Building a green roof is a collaborative process between stakeholders and professionals. It generally involves three main phases:

Design

This stage typically requires the coordinated efforts of engineers, architects and green roof professionals, or some combination thereof. Together, they ensure that the green roof respects the weight-bearing abilities of the structure, that the roof will operate as intended and that it looks good.

An excellent design involves taking many factors into account, including:

• The local climate

• Average rain and snowfall patterns

• Existing roof conditions and load-bearing capacity

• The intended use of the roof, ranging from stormwater management to amenity spaces for people

• Desired plant palette and aesthetic

• Other goals: pollinator habitat, solar energy, urban farm

Once the stakeholder’s needs have been expressed, they participate with the pros to ensure they are happy with the design as a whole.

Build

Remember all those layers we talked about above? This is where they go in, one on top of another, each layer checked and rechecked before putting the next one down.

Or at least, they should be. We have definitely seen instances in which the waterproofing membrane was installed above the drainage layer. That’s just a bathtub, is what that is. (Luckily, Ecogardens has never been guilty of such a crime. *ahem* Professionalism! *ahem*

Once everything is installed, some companies will tell you that your roof is “finished.” And while it’s true that green roofs are often low-maintenance, they are never no-maintenance. That’s just a recipe for insect takeover, plant death and wasted dollars.

Say hellooooooo to the stewardship phase.

Steward

Studies are clear: A properly tended green roof can last up to 50 years, or even longer in some cases. (18)

In short, they are very resilient ecosystems. They have to be, because rooftop environments are not easy ones. Wind, rain, snow, ice, brutal sunlight and general exposure place a lot of stress on them, and they have to stand up to it if they’re going to last.

We have to work in partnership with each roof to ensure they’re getting what they need. For roofs on which Ecogardens has the maintenance contract, we visit three to six times a year to check on plants, take readings on media and materials, supply nutrients, and adjust the environment to meet the needs of plants and people.

Of course, it’s much easier to steward a roof properly when the installer uses the right materials in the first place.

One of the most important of these is media.

What Is the Best Green Roof Media Type?

The best green roof media type is a question with many answers, depending on what you want out of your ecosystem. To paint with a broad brush, you need a material that can retain and detain stormwater with maximum efficiency, hold onto water and nutrients that plants need to survive, and stand up to many freeze-thaw cycles (at least in the Midwest).

While soil is sometimes appropriate for the green roof environment, many industry professionals are moving away from it for a number of reasons.

For one, it compacts easily, which is a problem in thinner profiles where you’re only working with a few inches to start with.

For another, it doesn’t hold onto organic compounds very well. They tend to run off quickly, giving plants a quick flush of growth when they’re added and then exiting the system, leaving hungry plant babies behind.

So what types of media do work well? Here are some of our favorites lately:

• Expanded shale

• Expanded clay

• Pumice

• All-fabric systems

Not all materials are appropriate for all roofs, especially for intensive ones where plants need richness and depth in their growing media. Cutting-edge engineering, however, has led to a far greater variety of options than in decades past, which means we can:

• Install green roofs on a greater variety of surfaces

• Make green roofs lighter so they can go on more buildings, and therefore ...

• Respond to stormwater in a larger area

• Steward plants more easily

In some cases, you’ll need custom media to get the job done. If so, we recommend working with a good regional blender who understands the local materials and climate. Here in the Midwest, our favorites include:

• Rooflite

• Midwest Trading

• Renewed Earth

There’s one more component of the design and build process that we should discuss in greater depth, and that is weight.

Okay, So What About Green Roof Weight?

Like any engineering project, green roofs contain a lot of moving parts.

... Figuratively, anyway. In reality, the goal is to ensure parts don’t move, and that goes double when it comes to the roof. Because movement is, like, a really bad sign. It’s mostly downward. And collapse-y. No Bueno.

If we want to prevent such “movement,” we have to keep green roof weight constraints in mind from the very beginning. Any number of factors play into this, including:

• Project materials

• Plant selection

• Labor costs

• Climate and stormwater needs

• Roof design and materials

• Roof size, slope and orientation

• Preexisting waterproofing

• Aesthetics and visibility

• Other green systems, such as rainwater harvesting or solar panels

• Intended use of the green roof

Notably, there is a big difference between wet and dry weight. A green roof after a heavy rainstorm, with plants, media, water retention and drainage layers all working overtime, clocks in a significantly more than a dry roof – between 1.5 and 2 times the dry weight. It’s important that even when wet your system is not heavier than what the roof can handle structurally (measured in pounds per square foot).

What Types of Plants Should You Use on a Green Roof?

If we were betting people, we’d bet your conception of a green roof involves succulents.

Specifically, those low-growing, fleshy-leaved succulents known as sedums (also called stonecrops). Sedum has earned its reputation as green roofing’s workhorse for a good reason: It rocks at trapping moisture as it comes from the sky, then releasing it back into the air at a later date – reducing the impact of stormwater on cities and ecosystems.

It doesn’t necessarily rock, however, at a lot of the other services green roofs at supposed to provide, making it Not The Best Bet for many green roofs here in the Midwest – and around the world.

Firstly, it’s not native. Or at least, not in our neck of the woods.

To be exact, the Midwest has exactly one native stonecrop: Sedum ternatum, or three-leaved stonecrop. While it’s great for green roofs, variety is called for. Without plant diversity, a susceptibility in one plant (blight, frost, what have you) can wipe out your entire green roof.

Then there are aesthetics, which dictate some variation. And any home to animals should involve a number of species, the better to offer them food and shelter.

Naturally, plant palette is limited when you only offer 3-6 inches of growing media, but there are plenty of native herbs, grasses, flowers and sedges that will do the trick nicely. We recommend looking for plants with the following qualities:

• Ability to withstand storms, sun, shade, biting wind, snow, rain and other elements

• Resilience without bullying or invading other plants

• Roughly 2 feet or less in height

• Works well with novel green roof components, such as foam aggregates or fabrics

Also, keep in mind that seasonal plantings usually aren’t the best choice for sustainability or green roofs. Broadly speaking, we like to use hardy perennials in a variety of categories. Here are some of our favorite alternatives for green roofs:

Trees and Shrubs

Trees and shrubs, while not appropriate to extensive environments, are perfectly amenable to intensive spaces. Cottonwoods, for instance, will grow 25 feet tall in only a few feet of media or soil, while a wide variety of shrubs happily adorn green roofs across the globe.

Grasses and Sedges

Both grasses and sedges can grow well in minimal planting depth, offering bushy green summer appeal, seasonal fall and winter color, and habitat for bugs and other critters. While you have to steward grasses carefully (to keep them a fire break rather than a fire hazard), they are among the most excellent of green roof choices, used well.

Creeping Plants and Groundcovers

The traditional idea of a green roof is one blanketed with a carpet of low-growing plants. While we try to avoid sedums, for the simple reason that they are not native to the Midwest and therefore not as well suited to dealing with the environment, we adore creeping plants and groundcovers in general. We have many local herbs, vines and herbaceous perennials from which to choose – as does every region.

Flowering Plants

Who doesn’t love flowers? Flowering plants offer excellent aesthetic appeal, especially to spaces made for human use. They also keep birds and bees fed and happy, which makes us happy too. Again, there are oodles of native perennials, so you can say goodbye to resource-intensive seasonal blooms.

Now let’s turn our attention away from plants and toward an important tech question: that of solar panels.

Can You Put Solar Panels on a Green Roof?

In the past several years, industry experts have increased the efficiency, effectiveness and breadth of green roof technologies by a considerable degree. The result are systems that work together seamlessly to achieve a simultaneous marriage of environmental goals and energy savings.

Prime among these happy partnerships are solar panels and green roofs. The most efficient model is when panels are raised into the air on legs, with vegetation growing underneath them. However, some systems employ a bank of solar panels flush against the roof surface, but surrounded on all sides by greenery. Known as biosolar, the combination helps each do its job better.

Solar panels shade green roof plants and protect them from the elements. This helps plants remain healthier, which means they do a better job of sucking down stormwater and moderating temperatures in the building.

Green roof plants, for their part, keep solar panels cooler and help them work more efficiently. Electronics (including photovoltaic systems) are limited by their ability to cool down, which vegetation helps them do naturally. 

Vegetation also protects the building envelope, because solar panel technicians aren’t walking directly on it. While foot traffic isn’t ideal for plants, low-growing varieties can bounce back fairly easily, and the rooftop surface itself avoids high impact.

But let’s talk dollars and cents ... is this combo an effective use of your moolah? According to a University of Illinois study, yes. As the study explains:

“The payback period of using just solar panels was calculated to be 13 years, the payback period of just a green roof was calculated to be approximately 73 years, and the payback period of the combination of the two was 13 years. Even though the initial cost of installing the combination of solar panels and a green roof was found to be the largest, the payback period was about the same as installing just solar panels.” (19)

So who should install solar panels along with a green roof, and when? In a nutshell, you should consider it if:

• You have a large roof, so you get the benefits of scale

• You get a lot of sunlight

• Your local government mandates green systems on your roof

• You will get tax credits

If none of those things describe you, it’s probably not the right fit, though even in rainy climates, solar panels can harvest a good degree of power.

Also note that it’s possible to retrofit existing green roofs or solar panel arrays to accommodate the other system. That depends on the slope of your roof, its weight-bearing capacity, and other factors.

No matter what type of roof you have, one thing is for certain: You must maintain it.

How Do You Maintain a Green Roof?

Throwing vegetation up on an exposed environment and dusting off your hands is a little like having a baby and exclaiming “Look how pretty he is! Well, our job here is done. Onward!”

Um ... no. Like any other system (or baby), green roofs require upkeep. Just as a tot is not equipped to face the harsh reality of life outside the womb on its own, a green roof is not able to “take care of itself” over the long haul.

Don’t get us wrong: Green roofs aren’t babies. They’re resilient, flexible and good-natured, provided they get the right care 3-6 times a year. This all comes back to stewardship, which we talk a lot about here at Ecogardens, as well as proper monitoring. Let’s take a look at both.

The Definition of Good Stewardship

Stewardship means partnership between you and the roof. It’s not as simple as making a maintenance plan and then carrying forward without regard to weather, animal life, plant behavior or microclimate.

Instead, a proper green roof stewardship approach means checking what your plant needs, monitoring its progress as it fills in (if it was planted from seeds and plugs), and adjusting your actions accordingly. Only then will your green roof operate at peak performance – and live.

You know. If that kind of thing matters to you.

Maintaining Peak Performance

If you want to maintain a green roof properly, the most important step you can take is to work with a green roof professional who knows what to look for when it comes to plant health, weather patterns, the state of growing media, and the efficiency of stormwater components.

When these factors are ignored, several things can happen:

• Green roofs stop draining effectively and start holding water

• Components stop working efficiently or break

• Plant diversity dwindles to only a few of the most aggressive species, which makes the roof less effective and resilient

• The roof may die

Sadly, many green roofs are consigned to exactly this fate through neglect after the handoff. That’s why it’s so important that people in the green roof industry, as well as project stakeholders, make an effort to enforce green roof programs after the installation – and for the next 50 to 70 years of their viable lifespan.

We do have a bit of good news, though: If you have a green roof that’s been neglected, you can still save it.

Does Your Green Roof Need First Aid?

Green roofs are living systems, and that means they can get sick.

Not so much in the thermometer and cough syrup sense; more in the plants ailing and bare spots appearing sense, due to the reasons listed in the previous section.

Bad design or inept installation may also play in – and both are more common than you think. The unfortunate truth is, not every green roof “professional” really deserves the epithet.

So what’s a roof-lover to do in this situation?

Enter green roof first aid.

What’s the First Step in Administering Green Roof First Aid?

If you want to get your roof back on track as quickly as possible – and trust us, you do – then you need to bring in a professional pronto. They will:

• Give the roof a thorough visual examination

• Take samples of green roof media

• Look for key indicator species, such as moss, weeds or insects, that can offer clues about what’s wrong with the roof

• Check the basic structures and components of the roof

• Assess plant diversity

Once they’ve diagnosed the problem(s), they will recommend a course of action. That usually means a) short-term action to heal the roof and b) a long-term stewardship plan to prevent it from getting sick again.

Now all that’s left to discuss is ... how can you get started with healing that existing green roof or putting a new one in place?

Is It Time to Get Started with Green Roofs?

The best reasons and intentions in the world don’t necessarily overcome the natural anxiety about putting such a daunting system in place.

To which we say: Fear not, gentle environmental friend. (Did that rhyme set you at ease? Yes? Good.)

Getting started with your own green roof isn’t as hard as you might think. If you ...

• Want to save energy and pay less on your utility bills

• Understand the necessity to manage stormwater more effectively

• Like the thought of making habitat for native plants and animals

• Hope to inspire others to do more to help the environment

• Would love a great outdoor hangout

... then you’re probably ready for a roof. Which means it’s time to get in touch.

At Ecogardens, we have more than 30 years of experience creating environmental solutions to common urban problems, and green roofs are one of our most reliable stormwater management tools. We’d love to bring that experience to you next.

Call or email today, and let’s get started!

And remember, you can feel free to download that report at any time to take with you, read on the airplane, show your family and friends, or print out and cover your office wall à la A Beautiful Mind. I mean, it’s an option. Just putting it out there.

That said, thanks for reading, and good luck fighting the good fight!

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