By: Dalton Dubetz
Green roofs, often referred to as living or eco-roofs, have been around for as long as history can remember, dating back to the Hanging Gardens of Babylon.
Today’s green roofs were mainly inspired by sod roofs, which have been used in Iceland and Scandinavia for hundreds of years.
While Europe is still the leader in green roofs, the technology and science has been evolving and becoming much more commonplace in North America thanks to certain cities' initiatives.
One such initiative is Toronto’s “Green Roof Bylaw” which was signed into effect in 2009 and made Toronto the first city in North America to have a bylaw to require and govern the construction of green roofs on new developments.
The bylaw requires that any new development (commercial, institutional, residential), including additions, with a minimum gross floor area of 21,527 square feet have anywhere from 20-60% of the available roof space be a green roof.
Today, green roofs are much more complex than the birch bark and sod setups of the vikings.
Modern green roofs consist of a waterproofing membrane, a root repellent barrier, a growing medium, a drainage layer, and even irrigation systems depending on the intensity of the green roof.
Although a green roof comes with a much higher cost than a standard roof ($10-25/square foot vs $4-8/square foot), the positive effects of a green roof should result in the roof paying for itself over time.
Green roofs can improve the energy efficiency of your home, help to control and contain stormwater runoff, reduce the urban heat island effect, and even extend the life of your roof.
There are two types of green roofs; extensive and intensive.
Extensive green roofs are much more common for residential properties whether it is a new development or a retro-fit upgrade.
This is because extensive roofs have a shallow growing membrane (less than 6 inches) which reduces the weight added to the roof, though you still may need some structural upgrades to support the weight.
Because the growing membrane is more shallow, the available selection for plants on extensive roofs is more limited; however, this natural approach means less maintenance because the plants become self-sufficient once they are established.
Utilizing grass, plant, and sedum species that are native to the area can also help negate the need for irrigation because these species have evolved to survive in the surrounding conditions.
Intensive green roofs, as the name suggests, are much more involved and require more maintenance and some sort of irrigation as there is virtually no limit to the types of greenery that can be used, essentially turning your roof into a full fledged garden or park.
An intensive green roof will require a greater depth of growing medium (more than 6 inches), meaning they are much heavier and not as suitable for a retro-fit.
Because of the diversity that an intensive green roof offers they are often accessible to the public whether it be as a park to spend time relaxing in or as a garden to grow food for the building, its tenants, and/or the surrounding community. Or both!
While aesthetically and functionally different, these two separate types of green roofs share almost all of their benefits.
One of the benefits is the potential savings on energy costs.
Green roofs can help improve a home's energy efficiency because the growing membrane and vegetation act as additional insulation.
This means that in the cooler months less warm air escapes through the roof, and in the warmer months less hot air gets into the house.
This lessens the workload of your HVAC system which can lower energy costs and extend the life of your HVAC system.
The University of Michigan did a study in 2012 that concluded a green roof of 20,000 square feet would save $200,000 over its lifespan.
The additional insulation from the growing membrane and vegetation also help to reduce the amount of outside noise that passes into your home from above, providing sound insulation as well as heat/cold insulation.
On top of additional insulation that can lower energy costs, green roofs help to reduce the “urban heat island” effect.
The large paved surfaces in a city environment absorb solar radiation and release it back as heat. In city centres this can result in a temperature difference of 1-3℃ during the day and up to 12℃ at night between the city centre and surrounding suburbs/fields.
Green roofs help to combat the urban heat island effect because instead of releasing the absorbed solar radiation as heat, it is used for evapotranspiration.
Evapotranspiration is a process in which plants use the heat from solar radiation to evaporate water from their roots and the ground out through their leaves.
As water is evaporated through the leaves it helps to cool the surrounding air which in turn helps to keep a lower temperature on the roof, aiding in the reduction of the urban heat island effect.
Because evapotranspiration uses water from the plants roots as well as the soil underneath, it is important for green roofs to have good water retention, which they do.
Green roofs capture up to 80% of rainwater whereas a standard roof captures roughly 24%.
Capturing and retaining this amount of rainwater is an important benefit of green roofs because it not only contributes to evapotranspiration and keeping a cool roof, but it also means less runoff is entering the community’s stormwater and sewer system.
This lightens the load on water management systems which can reduce flooding in the event of a storm and even come with lower water management fees in some areas.
The growing medium and vegetation also act as a filter for acid rain and storm water, meaning the runoff that does come off your green roof is easier to treat, further lightening the load of water management systems.
The vegetation on a green roof also helps to filter the air by trapping dust and airborne particles, meaning cleaner air around your home.
Finally, the layers of a green roof protect your underlying roofing materials from extreme temperatures and UV radiation, which helps slow the aging process meaning a green roof can last over 40 years while a conventional roof would need to be replaced after 15-20 years.
With science, technology, and the data catching up to the theory of green roofs, now seems like the perfect time to transform your home into a personal Hanging Gardens and realize the benefits for yourself by contacting Shield Roofing & Exteriors for your free inspection and estimate.