How to Maintain Your Green Roof

Green roofs can appear on conventional flat or slightly sloped roof surfaces. In urban areas, green roofs are an increasingly popular choice for homeowners because vegetation attracts pollinators and decreases stormwater runoff. A green roof protects homes from direct solar heat—reducing cooling loads by as much as 75% for homes with air conditioning. In winter, a green roof minimizes heat loss through added insulation on the roof (resulting in less money spent on energy bills). As a result, green roofs translate into fewer greenhouse gas emissions—lowering your home’s energy footprint.

Green roofs also reduce the urban heat-island effect. According to the Environmental Protection Agency (EPA), “Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution, and greenhouse gas emissions, heat-related illness and mortality, and water quality.” In other words, vegetation planted on a green or living roof helps cool the surrounding air.

A green roof typically lasts between 35 to 50 years—much longer than several other conventional alternatives. An “extensive” green roof (i.e., one with a shallow soil profile and low-growing, horizontally spreading plants)—which is the kind homeowners choose—is nearly maintenance-free. Still, a roof garden will always need a little care. Here’s a guide to maintaining a happy, green, healthy live roof.

Weeds and Water 

Autumn is the time to get up on the roof (late spring and mid-summer are good, too). It’s prime time for pulling weeds when they’re still small. It will also help your roof look its best next spring. Live roofs are continually exposed to weed seeds via wind and birds. Depending on your home’s location, common weeds on green roofs include poplar and maple seedlings, clover, spotted spurge, horseweed, dandelion, and foxtail. If allowed to spread, they’ll compete with your plantings for nutrients and water and eventually take over.

Kevin Flynn and Roxanne Nelson have a live roof on their St. Paul home. They “go up there every couple of weeks in the summer to pull out invasives and tree seeds,” Flynn says. While weeding, check for any insect damage or fungal diseases that might need to be addressed.

Weeding is one of the keys to ensuring a new green roof gets well established, which can take from two to five years. According to the Minnesota Stormwater Manual, “Once the vegetation is well established (i.e., once most of the green roof surface is covered with desired plants, invasive species are minimal, and there are very few areas of the bare ground), maintenance is still crucial, but maintenance needs typically decrease.”

Most homeowners choose water- or drought-tolerate plants for a live roof to minimize another part of maintenance: watering. Still, to thrive, especially during a drought period, roof plants may need supplemental irrigation. Not to worry: Complicated irrigation systems aren’t required; usually, a nice drenching with a hose will perk up your sedums and succulents.

And be careful not to overdo it. Water is heavy. Additional weight on a roof, not to mention a roof garden, can compromise the integrity of your home’s structure. EcoBrooklyn Living Walls suggests using plants suited to your climate and that require little moisture to become established and thrive. Don’t be afraid of underwatering. For slightly pitched roofs, they also recommend you start watering at the top so it can trickle down to the plants at the bottom.

To Fertilize or Not

Fertilizing a green roof isn’t always considered a best practice. Still, some installers suggest it. Plant Connection, Inc., for example, recommends fertilizing during green roofs during their first few years of establishment. This is true whether they are pre-planted or seeded. Over time, the soil medium will build up enough organic matter from the natural cycles of the plants' life. After a few years, they say, the roof should not require any more fertilization.