How to Install a Green Roof
Cut energy bills and reduce the strain on the environment with a roof covered in sedums
A solidly engineered roof helps protect your house against the elements, of course, but the one installed over Sarah Jack and Scott Harris's eat-in kitchen does that and more. Planted with hundreds of low-growing succulents, the little flat-roofed extension off their 1925 Colonial Revival helps slow down and filter rainwater runoff. Residential green roofs such as theirs protect natural waterways from oily street residue by lessening the chances that municipal storm-drain systems will overflow. It also keeps the room below it warmer in winter and as much as 6 to 8 degrees cooler in summer, thereby reducing utility costs. The 140 square feet of plantings helps absorb air pollutants, too, and—at $13 to $45 per square foot installed—should last about twice as long as a conventional roof. For all these reasons, the Teaneck, New Jersey, couple felt good about springing for the eco-conscious home improvement. But they also value its aesthetic appeal. "The roof looks so lively and colorful," says Sarah. "It's definitely improved our view."
The LiveRoof modular system they chose is typical of residential models, consisting of plastic trays filled with a soilless engineered growing medium and fully mature sedum plants that can handle both deluge and drought. At 40 pounds each, the trays are heavy and meant to be installed by a certified contractor; R&S Landscaping did the job here. See how this roof went from bare to bountiful—plus the latest on living roof DIY options.
Shown: Hardy sedums are most popular for green roofs because they thrive in a soilless growing medium and are drought tolerant. They just need a yearly trim and occasional fertilizer to look their best. Homeowners can choose from a palette of suitable plants. Sedum spurium 'Tricolor' is one of the most ornamental.
Prep the Roof
Even a flat roof should have some pitch so that it sheds water toward gutters; for a green-roof installation, that pitch should be at least ¼ inch per foot of run. Before the plants go up, a structural engineer should check to ensure that the roof can support the weight of the engineered growing medium and plants. When saturated, each tray in this system adds up to 30 pounds of load per square foot, which can stress a traditional roof built to support about 25 pounds. For this project, the roof was built to withstand the added load and then covered with a waterproofing membrane. Here, green-roof installer Rob Gaffney adds a second layer of rubber roofing membrane as a root barrier to prevent damage to the roof.
"Get a structural engineer to tell you what your existing roof needs to support the weight of a green roof. You might have to add more joists, strengthen existing ones, or add bracing."
—Tom Silva, This Old House general contractor