The Fine Points of Building Rooftop Landscapes

In our modern world, we’ve rethought a great many things. We used to throw our soda cans in the trash; now we recycle them. Automobiles used to run exclusively on fossil fuels. Now, in 2017, we can buy cars with electric, hybrid, and even hydrogen-powered motors, and our world is better for it.

Rooftops are being reimagined as well. They’re not mere rain bonnets for buildings anymore. Gardens are increasingly being planted on top of them, and aesthetic, therapeutic and environmental benefits are the result. Green roof installation represents a tremendous opportunity for landscape contractors who want to expand their businesses skyward.

Lou Horvath founded Intrinsic Landscaping, Inc., Glenview, Illinois, in 1962. “We were predominantly a medium- to high-end residential landscape company, known for my dad’s naturalistic designs,” said company president and current owner Kurt Horvath.

“Around 1998, we were approached by a waterproof membrane company, and they asked us if we wanted to partner with them in installing green roofing. I looked at it and I thought, ‘People are really starting to care about the environment, so there could be great potential here.’” There was. It’s now the major concentration for Intrinsic. Ninety-five percent of its work is done on rooftops. The company also does design/build, installs living walls, and maintains landscapes.

Back in 1997, when Charlie Miller, P.E., was a municipal civil engineer working on methodologies for controlling municipal stormwater, he was tasked with preparing a new manual for stormwater management practices for the state of Pennsylvania. One of the missions was to include as many new ideas and thinking as possible.

A professor friend from Germany suggested that Miller include a chapter on green roofs. He accepted the invitation to accompany the professor to Europe and was introduced to some of the major players.

“I was shocked at how well developed this industry was over there,” he said. “I came back convinced that it would be a great thing to start doing here.” He founded Roofscapes (now Roofmeadow) in Philadelphia, Pennsylvania, not long afterward.

Rose Seeger, LEED AP, ARCSA AP, GRP, owner of Green City Resources in Cincinnati, Ohio, was raised on the family farm she now owns. “My background was agriculture, but I went to college for engineering, so I understand structural loading and can read blueprints. On the ag side, I can grow anything. Installing green roofing put those two together for me, and I’ve loved it ever since.”

The company she founded in 2008 also designs, installs and maintains native landscapes and bioretention swales, creates rain and healing gardens, and rainwater harvesting systems.

Types of green roofs

Think of a multi-layer parfait, with different ingredients in each distinct layer, and you’ll have the basic concept of a green roof. The bottom layer consists of a high-quality waterproof membrane, which also can function as a root repellent system.

The next layer up is for drainage, consisting of aggregate. Over that goes a filtering cloth that keeps the drainage layer from becoming clogged with silt. Next is the growth medium, consisting of engineered soil, which includes compost and aggregate. This is what sustains the plant material that goes on top.

There are two major types of green roofs: ‘extensive’ and ‘intensive.’ An extensive green roof typically has a layer of growth medium that is six or more inches deep, while an intensive roof has six inches or less. There are also tray systems that allow greater flexibility, particularly in retrofitting applications on existing buildings.

Why the different types? “Ultimately, it’s the end use,” said Horvath. “If it’s an amenity level on, say, a residential multi-use building such as a condo development, that would call for an intensive roof garden, because there’s more human interaction.”

Intensive green roofs can also be found on some commercial buildings, where the spaces are used as outdoor employee lounges or therapeutic gardens. An extensive green roof is more utilitarian, mainly put there for its environmental or climate-control benefits.

The systems may be modular, with drainage layers, filter cloth, growing media and plants already prepared in movable, often interlocking grids, or loosely laid and built up, with each component installed separately.

A modular tray system is a way of creating an almost ‘instant’ green roof. “For residential customers, such a system is a safety net,” said Seeger. “Because it’s their home, they’re more afraid that their personal things might be damaged, in case of a leak.” Should something happen, the trays are easy to take up.

She described the tray system she’s used on several of her installations. “Each plastic bin is one foot long and two feet wide, with little egg-carton-type wells in the bottom, so that the water doesn’t collect there and just sit. There are little pinholes in the bottom of those wells for drainage, and usually a filter fabric over that, so that the soil doesn’t go down into the holes. The soil is put into the tray, and the plants, into the soil. The tray is laid down right on the roof, on top of the waterproof membrane.”

Irrigation and leakproofing

A high-quality waterproof membrane is the most mission-critical component of a green roof. Ordinary rooftops are designed to dispose of water quickly. Putting plants up there means doing just the opposite—encouraging water to stay around. And sometimes, an irrigation system is installed, at least until the vegetation gets established. So, leak prevention is vital.

Membrane materials vary by manufacturer, but the most common ones include polyvinyl chloride (PVC), modified bitumen (MB), thermoplastic olefin (TPO), ethylene-propylene-diene terpolymer (EPDM), or hot fluid rubberized asphalt. TPO is often used on retrofits.

“PVC isn’t usually well-received, as it can become brittle over time,” said Julie Leavitt, vegetative roofing assembly manager for The Henry Company in El Segundo, California. “Modified bitumen used to be utilized quite a bit, but not so much anymore.”

The advantage of a hot-fluid-applied membrane is that it’s seamless.

There’s no way for water to penetrate beneath it, as it could with the sheeted materials, such as EPDM or TPO, which have seams that can leak.

Leaks are also caused by root intrusion. Adding a layer of laminated copper will discourage them. “A PVC or thermal polyolefin membrane is both waterproof and root-resistant,” said Miller.

You’d think that a water-stingy drip system would be the best method for irrigating a green roof. Surprisingly, it’s not. “The engineered soil in the growth media is so porous that water from a drip emitter would just go straight down,” Seeger explains. “It wouldn’t spread out into the soil as it normally would.” She uses pressure-regulating four- or twelve-inch spray heads combined with smart controllers.

Many green roofs integrate rain water-harvesting cisterns into their designs. The stored rainwater is used to sustain the plants.

Many, many good things

What are the main benefits of green roofs? “It depends on what type you’re talking about,” said Horvath. “Generally speaking, the main advantage of green roofs is for managing stormwater. But they also help keep buildings cooler. Then, there are the psychological and environmental benefits, like creation of habitat.”

They drastically reduce stormwater runoff, with all of its heavy metals and other attendant pollutants, that flows off the tops of buildings and into streams, rivers, lakes and oceans. This problem is even more acute in some of our older cities that have infrastructures built at a time when their populations were much smaller.

Many of them have combined sewer systems that mix runoff with raw sewage. When big storm events happen, these systems get overloaded. Vegetation helps relieve that overflow by slowing it down.

“A green roof can hold a large volume of water within its growth medium—and some of the retention areas below it—for a long enough time so that the peak flow period can pass,” said landscape architect Steven L. Cantor, RLA, ASLA, of the Elizabeth Kennedy Landscape Architect firm in New York City, author of the book Green Roofs and Sustainable Landscape Design, published in 2008. “Therefore, a storm does much less damage.”

These installations don’t just delay stormwater, they also cleanse it, removing pollutants before they can be washed back out into receiving waterways. For instance, certain plants are known for their ability to uptake heavy metals.

Given these facts, it’s not surprising that an environmentally-conscious city such as San Francisco would pass an ordinance requiring green roofs on all new commercial construction, as it recently did. Other cities have done the same, increasing the demand for them.

Of course, the oldest North American cities are mere babes compared to Europe’s. Not surprisingly, the Continent is way ahead of us in this arena. “They got started before we did, in the late 1970s,” said Steven W. Peck, founder and president of Green Roofs for Healthy Cities (GRHC), the North American green roof and wall industry association.

“We didn’t really get going in the U.S. and Canada until the late ’90s. The Europeans have also been quick to develop policies that provide financial or regulatory incentives or requirements for green roofs.”

Green roofs do cut energy use, but how much can be hard to quantify on new construction, as there’s no baseline for comparison. “But we have some big clients who have noted internal temperatures that are markedly different in the buildings with green roofs, compared to the ones without them,” said Horvath.

“One of our clients has a large number of electrical motors constantly running in its facility. With the building now being that much cooler, they’re getting longer lives out of those motors. Their ROI increased tenfold, way beyond their initial expectations, which was to make the building more comfortable for people to work in.”

Using the plantings to precool the ambient air going into an AC system’s intake helps maximize the energy savings. “Every degree Fahrenheit you reduce in the temperature of the intake air saves about one percent in energy costs,” said Peck, because the system doesn’t have to work as hard.

Wal-Mart did a study of 17 of its fully green-roofed stores. The company found a significant reduction in air conditioning costs, from seven to 15 percent, saving each store about $6,000 to $18,000 annually.

These installations can also play a big role in reducing the ‘urban heat island effect.’ The plants, through evapotranspiration, create a layer of cooler air above the roof’s surface. The air spills down the side of the building, cooling the surrounding area. It’s like air conditioning for the outdoors.

European studies have shown a one- to two-Centigrade improvement, said Peck. A reduction of just one degree Centigrade translates to a four percent reduction in peak electricity demand.

Rooftop landscapes can provide psychological benefits as well. Cincinnati’s Mercy West Hospital hired Seeger to create a two-and-a-half acre, prairie-like rooftop garden to provide a calming, therapeutic vista for 250 of its patients.

Greening a roof also extends its lifespan. “A typical roof lasts about ten to 20 years,” said Miller. “At this point, we don’t really know how long a green roof can last, but it’s clearly at least 40 years, and I’m banking on 100, if not more.”

That’s because soil and plants make a terrific sunblock, shielding the roof from the sun’s ultraviolet rays, the biggest destroyer of roofing materials. They’re pretty good insulators, too, making a roof less subject to expansion and contraction due to seasonal temperature fluctuations.

One of Miller’s most exciting recent projects is a large, elevated public park in Philadelphia. It illustrates how a green roof, combined with other green technologies, can accomplish several goals at once.

“The large parking area is covered with pervious pavers. Underneath them lies a 20,000-gallon cistern, which is only two inches thick. The water is released, via gravity, into weirs in the green roof areas. Those areas, in turn, have drain restrictors that hold water in the base.”

The whole design is intended to capture rainwater, release it slowly, and capture it again. It supports the vegetation, reduces runoff, and creates evapotranspiration.

Green roofs can even be educational. At the Rothenberg School, in Cincinnati’s inner city, Seeger installed a rooftop vegetable garden that teaches the pupils about healthy foods. “The garden administrator took the kids around the planters, and then asked them what they saw. One little girl said, ‘Green beans.’ Then one boy yelled, ‘You did not! There were no cans out there!’”

What to know before you start

Installing these rooftop landscapes is a good business, and it’s growing. It’s been extremely lucrative for Horvath and Miller, who build lots of them, all over the U.S. and Canada. Seeger is newer at it, and keeps afloat by creating living walls and therapeutic gardens as well.

Before you plunge in, Horvath advises learning all you can. Green Roofs for Healthy Cities has an educational program that will teach you all the integral elements.

You don’t have to become a roofing contractor, but it’s a good idea to learn some of that trade’s principles. “The National Roofing Contractor’s Association (NRCA) is a good resource for understanding different types of waterproofing and roofing materials,” said Horvath. “They have their own vegetative manual as to what’s appropriate, from a roofing standpoint.”

Seeger advises investing in safety training. “It will set your company apart from the competition. You don’t want to walk onto a jobsite and have people saying, ‘Oh, no, a bunch of landscapers; they don’t understand fall protection, or how to work with a crane.’ We’re all OSHA-certified in crane rigging and signaling, and have fall-protection training. That puts whoever hires us at ease.”

If an engineer isn’t already involved in your project, it’s a good idea to have one review your plans. Green roofs are heavy, and even heavier when wet. You need to know if the structure can support that weight.

Maintenance

Seeger maintains most of the green roofs she installs. Once a month, she and her work crew will spend an entire day on a two-and-a-half-acre site, pulling weeds and trimming, all by hand. Not surprisingly, her eco-friendly clients don’t like chemicals.

“It’s not difficult. The soil is so loose, the weeds pull right out.” Having the maintenance contract provides her company an ongoing revenue stream, long after the installation is done.

Green Roofs for Healthy Cities would like to see one billion square feet of green roofing installed across North America by 2023. Will your company be one to help achieve that goal?