Steep Challenge: A Case Study of a Sloped Residential Green Roof

In this guest post, David Aquilina, a business communications professional who represents Xero Flor America*, outlines a green roof project at Lake Toxaway, North Carolina. He provides details behind the design and installation. At the end of the article, you can watch a short video of the green roof project.

WHO SHOULD READ THIS

• HOMEOWNERS looking to learn more about green roofs
• CONTRACTORS, ARCHITECTS & ENGINEERS interested in designing and installing green roofs

Green Roofs for Healthy Cities will be offering their first online Green Roof Professional (GRP) Accreditation Training starting in February 2015. Their online training will serve as a comprehensive reference for individuals who design, install, and maintain green roofs. It provides tools and techniques needed to meet green roof project objectives on schedule, to specification, and within budget, and presents the latest industry developments, technical standards, and research findings. The course includes everything needed to take and pass the GRP Accreditation Exam.

LAKE TOXAWAY

With 646 acres of surface area and a 14-mile shoreline, Lake Toxaway is the largest private lake in North Carolina. Situated at an elevation of about 3,000 feet in the Appalachian Mountains in the western part of the state, Lake Toxaway is adjacent to more than 10,000 acres of wilderness areas.

The couple from Nashville knew what they wanted (and what they didn’t) for their new seasonal home on the lake. “Many other homes on the lake look like they belong on a broad expanse of ocean beachfront, not nestled on a serene lake in the mountains,” said the homeowner. In short, they wanted a home that would be modern yet fit into the natural surroundings.

The 6,700 square foot (sf) home features natural materials that complement its setting and makes full use of its unique location. Windows across the length of the home face the southern vista out over the lake. To provide enjoyment throughout the day and into the evening hours, the home also offers views to the west toward the adjacent rock face and sunsets over the mountains.

AN ARCHITECT’S INSPIRATION

The lot on which the home is situated is narrow and steep, dropping down from the road to the lake. The floor of the house is 20 feet below the road. That makes the top of the roof level with the roadway. With the dimensions of the home, a long expanse of roof is visible from the road and driveway entrance. The architect had an inspiration: a view of a green roof, rather than a plain rooftop, from the road would help visually integrate the home into its setting.

ROOF SLOPE ANGLES AND A SIX-SPECIES PLANT PALETTE 

Roof slope angle is a significant design factor in any green roof project. Roofs sloped from 10 to 45 degrees require green roofs with more depth and specialized components to retain and stabilize the growing medium layer against erosion and slippage. Thus, the 3:12 slope of the roof on the Lake Toxaway required a stabilization solution.

“Before the growing medium was added, a supplemental stabilization grid was integrated within the green roof assembly. It was installed over the water retention fleece and anchored on the aluminum edging at the perimeter of the roof,” said Clayton Rugh, Ph.D., general manager and technical director, Xero Flor America. “The cells of the grid hold the growing medium on the slope.”

After considerable discussion, a six-species plant palette was selected for the 3,959 sf green roof project. The six species provide sufficient diversity, and the *Sedum spurium* and *Sedum album* give the roof reddish colors in the fall. The green roof also enhances the lake’s water quality: it annually prevents an estimated 99,850 gallons of stormwater from running off into Lake Toxaway. The green roof was completed in April 2012 and has been through three growing seasons. “We cannot imagine our wonderful home without the green roof,” said the homeowner. “It just wouldn’t be the same.”

This article originally appeared in the Living Architecture Monitor, Fall 2014 issue. 

*Founded in 2002, XFA is the official and exclusive distributor in the U.S. for the Xero Flor Green Roof System engineered in Germany. All of Xero Flor’s system components are 100 percent American made.

The online Green Roof Professional (GRP) Accreditation Training starts on February 23 and runs until May 1, 2015. The course is capped at 50 with 30 discounted seats.