Case study: How to install a green roof on a private home

The site: A more than 900-square-foot expanse atop a historic building in the Prospect Lefferts Gardens neighborhood of Brooklyn. The homeowner recently re-roofed the building, in anticipation of installing a green space, she had the roofer raise the skylights several inches and bring the waterproofing material up the sides of the roof’s walls.

The plan: The homeowner envisioned a wide carpet of greenery in addition to a small space for growing some fruits and vegetables, as well as an area to hang out with friends. She hired architect and green roof pro Inger Staggs Yancey of Brooklyn Greenroof to bring her idea to life.

According to Yancey, installing a green roof is like putting together a complicated puzzle, and because of this, it's not something you should DIY. During the initial design discussions—for this site, there were a lot—Yancey brings on a structural engineer. "The first thing you want to do is to find out how much weight the roof can support, whether it’s a lot or a little. That will change the kinds of designs you can entertain."

The plants: The engineer’s assessment showed that the roof could hold 55 pounds per square foot—enough strength to support a full rooftop farm. "The average green roof needs a minimum of 2.5 to 3 inches of growing media," Yancey explains. "If you’re going to grow vegetables, you need media to be 7 inches deep. But the more media you have, the more it will weigh."

For a time, the homeowner entertained the idea of installing a full rooftop farm but eventually decided to return to her initial concept of a low-maintenance green space with a few planters for edibles. The majority of the roof would be covered with a flowering sedum.

"Sedum is the standard green roof plant," Yancey says. "It’s extremely drought tolerant, low maintenance, and never needs to be reseeded or watered. It’s a plant adapted to growing on mountaintops in low-nutrient soil. Any flowers that fall off decompose and form the fertilizer the plant needs, so it does its own little composting in a way."

The layers: Green roofs are made of layers designed to protect the building structure from water while holding and sustaining the plants. The bottom layer is a root barrier, typically made of very thick plastic sheeting that will prevent any strong roots from digging into the building.

"Roots are drawn to carbon," explains Yancey. "And some of the more affordable roofing materials are made primarily of carbon. It’s one of those unlucky coincidences. You have to work hard to keep them separate."

Several inches of edging material—typically stones or gravel—will then be used to form a root-free border separating the growing area from any vertical elements like walls or, in this case, the sides of the skylights. Metal slats placed between the rocks and the growth media help ensure separation while retaining drainage.

Next is a moisture-retention layer that looks something like an egg-crate mattress. Enough water to sustain the plants will collect in the divots of the egg crate, while excess water can drain out through holes atop the raised bumps.

"One question I always get from homeowners is ‘What happens to the water when it rains? Where does the water go?’" Yancey says. "The answer is that it goes the same place it would if you didn’t have a green roof. The roof is designed so the water percolates through the green roof and can flow to the gutters and downspouts in the same pattern as a normal roof."

Over the moisture layer goes a filter fabric specially engineered to hold in the dirt while draining out any water. There are a range of filter fabrics and growing media types, and it’s important for them to be compatible. Matching them incorrectly could result in clogged fabric or media that washes away.

Then it’s time for the growing media—a.k.a. dirt—and putting in the plants. In this case, Yancey chose pre-grown sedum that comes in large tiles.

The timeline: Once Yancey and the homeowner finalized the plans for the roof, Brooklyn Greenroof applied for a permit with the city’s Department of Buildings. In New York City, green roofs with dirt less than four inches deep do not require a permit. However, the city strongly encourages permitting for safety reasons, and the permit qualifies homeowners for a tax abatement. While most green roof permits will come through in about a month, the process took a bit longer in this particular case.

After that, the company began ordering the necessary materials and coordinating deliveries. Installing a green roof usually takes about a week, but because of the layered nature of the installation, it’s important for materials to arrive in sequence. So planning the installation takes a significant amount of time.

"It’s kind of like putting together an intricate jigsaw," Yancey says. "I have a system where I weigh down the first layer with the next layer in the same day, otherwise the materials or the growth media could blow away. It’s another reason why no one should DIY a green roof."

With the help of a crane and a block-and-tackle pulley system above the roof hatch, the materials made it up onto the roof in nearly correct succession: First the rolls of root barrier, then the 1x2-meter tiles of the moisture layer and rolls of filter fabric. The final day was supposed to be laying out the growth media and large square tiles of sedum, but the rock edging and metal border arrived several days late and had to be installed the following week—after the plants were already in place.

The entire installation took about five days in total, with a weekend between planting and finally putting in the edging. The homeowner also built a set of copper-covered planter boxes to sit on the raised cornice area near the front of the building, with Wooly Pockets planted with strawberries hanging down nearby.

The money: The average cost for a bare-bones green roof—including the design, permitting, and installation—will typically run between $18 and $22 per square foot. A deeper or more specialized roof, like this one, can cost more, between $30-50 per square foot. The tax abatement for this project came out to $5.23 per square foot of green roof.