green roof in a city with lounge chairs and plants on it

The benefits of green roofs

LIVE greener | 7 MIN READ | 2019-07-31

Next time you’re in an urban area (especially New York or a European city) – look up – you’ll likely see trees and plants growing from rooftops, creating what may look like a mini-forest on top of a building. Why? Well, other than the fact that it creates the perfect, natural oasis to escape the city or have a picnic with friends, there are lots of benefits to green roofs. We’re diving in to what green roofs are, how long they typically last, and why builders everywhere are adding them to their plans.

Green roofs 101

green roof on apartment building

So what is a green roof (AKA living roof, rooftop garden, or green roof system)? Here’s the 411:

A green roof is a layer of vegetation grown on a rooftop. Simple as that!  

According to Green Roofs for Healthy Cities, they’re an extension of the existing roof that at least involves high-quality water proofing, a root repellent and drainage system, a filter cloth and a lightweight growing medium. Oh! And plants, of course.

Green roofs can be below, at, or above the ground, but in all cases they exist separate from the ground (so patios and backyards don’t count).

Green roof benefits

green roof with yellow flowers on condo building

Image: Green roof on Minto Communities’ 30Roe LEED Gold Certified condo in Toronto 

The benefits of green roofs aren’t super obvious, however as they’re getting incorporated to more and more new builds around the world, it seems the range of public and private benefits are starting to become more understood. LEED Certifications (an award given to buildings designed, built and operated sustainably) are impacted by sustainability initiatives such as green roofs – and buildings like Minto Place in Ottawa and 30Roe in Toronto (both LEED Certified Gold+) were designed with green roofs in mind – but why?

What do green roofs do (aside from being beautiful), and what are the associated benefits?

Reduced energy costs: Rooftops have a strong impact on the temperature of a building – being a place where the greatest heat loss happens in the winter and hottest temperatures enter in the summer. 

o Green roofs reduce energy costs by absorbing heat, rather than attracting it – like traditional black tar rooftops do. By absorbing heat, green roofs become like natural insulation for buildings. 

o 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%. That’s a lot of savings!

Waste diversion: Green roofs lead to less landfill waste by prolonging the service life of heating, ventilation and HVAC systems through decreased use. 

Storm water management: Green roofs store rainwater within the plants, where it’s returned to the atmosphere through transpiration and evaporation.

o They also moderate the temperature of the water and act as natural filters of the water that runs off the roof. 

o Because plants absorb rainwater, less runs off buildings and at a slower pace, meaning less stress on sewer systems at peak flow periods (during long rain storms, as an example). 

o Green roofs can retain 70-90% of precipitation in the summer and 25-40% in the winter.

o Check out this website for more details on the above facts.

storm water green roof diagram

Image Credit: BCIT

Combat the urban heat island (UHI) effect: With dew and evaporation processes, plants are able to cool cities down during hot summer months and reduce the UHI effect. 

o What’s an urban heat island? According to the National Geographic, it’s a metropolitan area that’s a lot warmer than rural areas that surround it. Heat gets created by energy from all the busyness in the city which makes the area hotter than the temperature outside.

Improved air quality: We’ve talked a lot about how plants can improve indoor air quality. They do the same for the outdoors too, by capturing airborne pollutants, atmospheric deposition and filtering noxious gases.

New spaces for the community: A green roof can also be a new amenity space within a city. Whether for wildlife or people, that’s up to the person who controls it. They can be turned into:

o Community gardens (like the rooftop garden coming to The Annex condos in Calgary) 

o Commercial spaces (like rooftop restaurants or office buildings)

o Recreational spaces (like the rooftop at Niagara West apartments in Toronto that has a dog run)

o Wildlife habitats within cities for birds and natural pollinators like honey bees or butterflies

5 tops questions about green roofs

little red cabin with a green living roof

There are tons of questions and fun facts that go along with green roofs – so here are 5 common questions (and answers) for you, below.

1. How much money do green roofs save?

While green roof savings depend on the size, type of plants, use of irrigation, and lots of other factors, you’ll save money by decreased utility costs overall. This includes things like:

o Roof life increased by 2-3 times (less cost for repair)

o Reduced air conditioning costs

o Reduced winter heating costs

2. How long do green roofs typically last?

Apparently by protecting the roof membrane, a green roof can extend the life of a roof by 2-3 times its average lifespan! 

In Europe (where green roofs have been a thing since the 1960s), green roofs have been known to last from 30-60 years.

3. What are the different types of green roofs, and what’s the difference between intensive, semi-intensive and extensive green roofs?

According to Gardenista, the category of green roof depends on how deep the planting medium is and how much maintenance is required. What types of plants you choose for your green roof impact the category, too. Here are the three main types:

o Intensive green roof: These recreate the conditions of a traditional, above-ground garden and need a lot of maintenance. 

o Semi-intensive green roof: Let’s assume these are somewhere in the middle of intensive and extensive green roofs. 

o Extensive green roof: This type of green roof is self-sustaining with tough and drought-resistant plants. Extensive green roofs are pretty much maintenance-free!

4. How much do green roofs cost?

According to Green Roofs for Healthy Cities, the cost of a green roof can vary depending on size, plant types, use of irrigation, and whether they’re to be accessible to people or not. 

o Intensive green roofs usually cost more upfront but are beneficial due to their accessibility.

o An extensive green roof may be installed for $10-$30 per square foot. 

o Basically, it sounds like the best bet for finding out costs is to consult a Green Roof Professional as it changes based on needs.

It’s important to note that while green roofs can cost 2-3 more than a non-green roof, their benefits far outweigh those of a traditional rooftop, including that they last twice as long (among many of their environmental and reduced utility cost benefits).

5. Can you DIY a green roof?

green roof DIY diagram

Image Credit: Gardenista

While there are lots of things to consider before you begin – like what category you want, if your structure is strong enough to support the weight, the costs involved, what plant types work with your climate and many other things – it is definitely possible to DIY your own green roof.

There are a ton of resources online to DIY a green roof. For starters, check out this Green Roof Guide by Instructables Workshop, these 4 easy steps to creating your own green roof, or these great tips by This Old House.

One thing to think about before DIYing a green roof (or getting one installed) is the maintenance. This website takes you through general green roof maintenance and care steps, summarized by these seven tips:

Weed out unwanted plants

Keep the drainage areas clear of plants

Add compost bi-annually

Weed out unwanted plants (again)

Water as little as possible

Watch for pests and diseases

Keep a detailed maintenance log or diary

Whether you have one, want one, or are just interested in how green roofs do amazing things for the environment and beautifying cities everywhere, there you have it - the 411 on all things green roofs!