Here at Rise, we write a multitude of sustainable home features. However, only one offers year-round beauty that changes with the seasons, decreases your home’s heating and cooling needs, filters and reduces stormwater runoff, and attracts pollinators and enhances the durability of your home: a green roof.
What is a Green Roof?
Green roofs are “green” because they are living: they have plants on them. There are two types of green roofs: intensive and extensive.
What is an Intensive Green Roof?
An intensive green roof means the plantings require more intensive maintenance, soil, and support, like gardens, trees, or park-like terraces often seen on partial roofs of commercial buildings.
What is an Extensive Green Roof?
An extensive green roof, the focus of this article, requires less soil, making it lighter in weight, lower maintenance, and is just covering a roof with small plants.
How Do I Build an Extensive Green Roof?
Extensive green roofs are typically planted with small succulents that are drought tolerant and adapt well to your climate. The do-it-yourself option can be daunting, as it requires a minimum of five layers:
- The roof deck itself, which is the existing structure.
- A waterproof and root-proof membrane. This is a thicker and more protective membrane to protect the roof from water accumulation and wandering roots.
- A Drainage board is a superior filter layer to allow the excess water to make its way to the waterproof membrane and then flow towards the drainage pipes.
- The substrate is the growing medium. Plants need something to grow in, and the substrate is perhaps the essential component because it needs to be just enough for the plants to grow, but not too heavy.
- Finally, the plants. Sedums are the most commonly used plants, as they are drought-tolerant, hearty, and come in a wide variety of species that bloom and change color at different times of the year.
Another option is to purchase and install pre-planted tray systems, such as Live Roof. Live Roof partners with local nurseries throughout North America to pre-grow plants that work well for your particular climate. The benefit of this type of system, in addition to being lighter weight and easy to install, is that it is beautiful on day one!
Pros of Green Roofs
The benefits of a green roof are many.
Financial Benefits of Green Roofs
Increased Roof Longevity
Green roofs extend the roof’s life by two to three times. Why? Because they protect the roofing material from the elements: hot and cold, rain, sleet, snow, and hail. Any financial analysis should include the cost savings of not having to replace a roof in 15–20 years — so that is the minimum payback period.
Reduced Energy Usage
Green roofs reduce the home’s energy usage, for two reasons. First, they add extra insulation to the living space underneath, which means lower heating bills. Second, the plants are cooler than other roofing material, reducing the urban heat island effect, which typically leads to lower cooling costs. The generally accepted rule is that green roofs can reduce heating and cooling loads by 10 percent. But, note that it would only be for the adjacent rooms that it insulates.
Environmental Benefits of Green Roofs
Reduced Stormwater Runoff
Stormwater can be a problem for many municipalities because all of the water running off hard surfaces gathers quite quickly. It then runs into stormwater drains, absorbing contaminants along the way and overflowing the system. Some municipalities, like Kitchener, Ontario, and Portland, Maine, issue a credit if you install stormwater management features at home. If your city or town offers this type of program, it can be another financial benefit.
Biodiversity and Pollinators
Plants add biodiversity, and flowering plants attract pollinators. Imagine a bird flying above a city. Where does it want to land? Green roofs provide safer havens and more natural habitat. Flowering green roofs attract butterflies and bees.
Improved Air Quality
Green roofs absorb carbon dioxide and reduce greenhouse gases.
Health Benefits of Green Roofs: Beauty
If there is an area of your home where you can see the roof, that is the perfect place for a green roof. It is better for your psyche to look out at a pretty garden, instead of a rubber roof membrane. What is the financial benefit of that? Priceless.
Cons of Green Roofs
As with many added home features, the cons are the additional cost and maintenance.
Cost of Green Roofs
Green roofs typically cost $15-$20 per square foot. They need to go on top of an existing roof, which may need reinforcement, so the cost is entirely incremental.
Maintenance of Green Roofs
While extensive green roofs are low maintenance, they do require some TLC.
Keys to Getting Started With a Green Roof
Four things need to be considered before building a green roof or renovating an existing roof into a living roof alternative.
- Roof Slope. Green roofs can be planted on flat or slightly sloped roofs. In general, the slope needs to be between 2 and 10 degrees to provide adequate drainage, without making it so steep that the roofing medium may slide or skid or the water collects at the bottom.
- Extra Weight. Most people understand that soil is heavy, especially when weighed down with rain or snow. In general, a green roof can weigh anywhere between 13 and 30 pounds per square foot. However, that is variable. The Live Roof tray system has a saturated weight ranging from 15 to 65 pounds, depending on the chosen system and soil depth. A building contractor (or the engineer on the building team) will be able to estimate the load-bearing capacity of your roof. If you are building new, the design team should know as early as possible about a plan to have a green roof (and which type of system). This way, they can include any additional reinforcement that the structure may need.
- Building Code Restrictions. Most municipalities recognize the benefit of green roofs, and some even provide financial incentives. But, it is always wise to check with your local building council regarding potential code restrictions.
- Drainage. Lastly, your living roof needs drainage options, such as scuppers or downpipes, so that the excess water can leave your roof and not drown the plants.
Bottom line
Few sustainable home products can provide this many financial, environmental, and health benefits as a green roof. If your budget and roof slope allows, a green roof is a worthwhile investment.
