Good for the Environment, Good for You: Show Your Home’s True Colors with a Green Roof 

The grass is always greener on the other side of the. . . roof? Green roofs are becoming a common sight in the D.C. area, and for good reason. 

While we usually think about kitchens or decks when we hear “home renovations, here’s a look at why a green roof may be an excellent choice for your next home improvement project. 

Figure 1: Green roof on the American Society for Landscape Architecture, Washington DC Photo credit: DOEE

What is a green roof?

A green roof is a roof partially or entirely covered with plants and soil. It called green because, well, grass is green. But the name also implies all the ecological benefits of a green roof, as we’ll explore below.

While all you see from above is the vegetation and soil, a green roof is actually a complex, multi-layered system. Different structures may require additional components, but the basics are a:

  • high quality waterproofing
  • root repellent system
  • drainage system
  • filter membrane
  • lightweight growing medium
  • plants

Figure 2- Cross-section of a green roof. Illustration credit: Blank Space LLC.

There are two main types of green roofs: extensive and intensive. Extensive green roofs have less than six inches of soil and plants between 2’ and 3’. Intensive green roofs are much thicker. Plant heights are between 3’ and 15’, and the soil is at least 1’ deep.

Why consider a green roof? According to Touchstone owner, Ben Srigley, “green roofs can benefit a home in many ways, including energy efficiency, ecological conservation, and aesthetics.”

Green roofs are energy efficient

Green roofs can both lower your air conditioning bill in the summer and your heating bill in the winter. Let’s dive into a short science lesson to see how it works.

Think about walking outside barefoot in the summer. Step on the grass, and your feet will be comfortable. But step on the pavement and OUCH. The concrete is much hotter than the plants.

Like the concrete, roofing materials trap heat, but not moisture. The roof gets hot, and so does the building underneath it and air around it. This creates a phenomenon called “urban island heat effect.” Cities, full of concrete, steel, and asphalt, are considerably hotter than rural areas.

 “The annual mean air temperature of a city with 1 million people
or more can be 1.8–5.4°F warmer than its surroundings.
In the evening, the difference can be as high as 22°F.”

Plants, on the other hand, have a natural air conditioning mechanism called “transpiration.”

Plants take in water through their roots. They use a small percentage of the water for their own growth and metabolism and store the rest in their leaves and stems. When the plant absorbs or creates heat (through photosynthesis), the water evaporates out of small holes in the leaves, cooling off the surrounding air.

Figure 3: Green roof on the US Coast Guard Headquarters, Washington DC Photo credit: US GSA

Covering your roof with plants cools off the surrounding air and the entire building. You pay less for air conditioning. And in the winter, the extra bulk of the green roof system insulates the building, lowering your heating costs.

“Research published by the National Research Council of Canada found
that an extensive green roof reduced the daily energy demand
for air conditioning in the summer by over 75% (Liu 2003).
Green Roofs for Healthy Cities

Green roofs are good for the environment

Green roofs are generally good for conservation. They prolong the life of your roof and reduce the load on your HVAC system.

But their most significant effect on the environment is stormwater management. When rain falls on a conventional roof, it spills off, running through the streets into the gutters. As it travels back to the main waterways it collects “gasoline, fertilizers, nitrogen, bacteria, pesticides, sediment, trash, metals, all kinds of harmful things and deposits them into the water,” explains Chase Coard, CEO of Ecospaces, a construction firm that specializes in green roofs.

And the run-off water overloads the sewer system. “As it rains in DC, as little as a half an inch of rain can overwhelm our combined sewer system, causing it to spew RAW sewage into our waters!” continues Coard.

But plants retain water. They use some of it, and then slowly, without overwhelming the sewer system, return the rest to the environment through transpiration or evaporation. According to Green Roofs for Healthy Cities, a non-profit that promotes the green roof industry, plants also act as natural filters, removing impurities and pollution from the water.

In summer, green roofs can retain 70-90% of precipitation.
In winter, green roofs can retain between 25-40% of precipitation.
Green Roofs for Healthy Cities

From the Potomac River to the Tidal Basin to Rock Creek, all the water in Washington benefits from green roofs.

Green roofs improve the aesthetics of your home

A green roof isn’t only good for the environment. It’s good for you, too. Green roofs create a more beautiful, pleasing home by:

  • Creating a beautiful space
    This may be the most obvious benefit. A home with a green roof enjoys the beauty of the plants and flowers, of course. But you’ll also see incredible biodiversity, with birds, butterflies, bees, and more visiting your roof.
  • Extending the lifetime of a roof by 2-3 times
    A traditional roof takes a beating from rain, hail, snow, ultraviolet radiation, and extreme temperatures. The green roof system shields and protects your roof.
  • Reducing outside noise in your home
    A 1999 study found that a green roof can reduce outside sound by 40 – 50 decibels. (Peck et al. 1999)
  • Fireproofing your roof
    Many common green roof plants, such as sedums and succulents, are naturally fire-resistant. And the waterproofing layers of a green rood often contain fireproof materials. Plus, many green roofs have irrigation systems that can help stop fires.

Rebates and reductions

Since green roofs benefit the city, the government offers a variety of incentives for homeowners who install green roofs.

In fact, thanks to its rebates, grants, and incentives, D.C. leads the nation in green roof installation!

For example:

  • Under the RiverSmart Rooftops Green Roof Rebate Program, the DOEE offers a rebate of $10 – $15 per square foot for installing a green roof in D.C.
  • Under the RiverSmart Rewards program, homeowners with green roofs may be eligible for significant discounts on their stormwater fees. (Check out this calculator to see your possible discount.)
  • Under the worldwide LEED program, your house can earn LEED credits for a green roof. Those credits can help you qualify for discounted homeowner’s insurance, tax breaks, and other incentives.

Figure 4: Drone view of an urban green roof. Photo credit: Martin Reisch from Burst

Considering a green roof?

A green roof is definitely NOT a DIY project.

Coard cautions that the, “biggest factor in considering a green roof is your home’s structural load capacity. Roofs are designed to carry a certain load that green roofs can many times exceed. Make sure to check with a structural engineer before moving forward with your green roof.”

Related to structural load is professional advice on the type of green roof to install. Sedum roofs are an excellent choice for a shallow green roof with minimal maintenance.

On the other hand, if your roof has space and structural load capacity, an intensive green roof can grow a rich variety of plants and flowers. Just keep in mind the extra maintenance. (Yes, you can be the only family on the block to mow your roof!)

A professional contractor can help you evaluate your options to create the most beautiful and functional green roof possible for your home.

Most of us only think about our roofs if they leak. But installing a green roof can change your roof from an overlooked rain barrier to your home’s showpiece.

Want to hear more? Contact us, and let’s discuss an incredible new green roof for your home.