Benefits of Green Roofs

More frequently, we are receiving inquiries regarding the installation of green roofs.

For those people that have never heard of such a thing, it is best to define what a green roof is, what it is intended to do, and what are the most important elements required to construct a green roof assembly.

What Is A Green Roof?

In Toronto, a number of years back; a bylaw was passed to require the installation of so-called green roofs on new construction related commercial flat roofs. The bylaw discussed that such new builds either had to have a green roof or cool roof assembly, which was designed to help curb the “urban heat island effect”.

(The urban heat island effect refers to the phenomenon of pavement and hot tar roofs as examples; which served to absorb heat on mass and contribute to raising the temperature in cities and urban environments. As a result the energy required for cooling such buildings, and residential structures due to increased temperatures; could be reduced by implementing various methods. Employing such methods serve to reduce our carbon footprint, reduce energy use, and mitigate stormwater run-off).

Cities like New York and Chicago had been predecessors that mandated the requirements to utilize green roof and cool roof technologies.

During that year when the bylaw was passed in Toronto, AVENUE ROAD ROOFING® had been nominated and received an award for installing a green roof assembly on the Drum Artz Centre in downtown Toronto.  Brown & Beattie Engineering conducted the structural assessment of the subject building and supplied specifications for the project. XeroFlor was the supplier of the green growing medium. The project was ultimately featured on HGTV with host, Carson Arthur.

As a result, we realized that green and cool roofing would become the future of the commercial roofing industry. Since that episode, we have seen the retro-fit commercial roofing market (as opposed to new construction), slowly adapt to requesting such work.

Slowly; because investing in a green roof costs considerably more initially, than simply installing a cool roofing system. (Cool roofing systems can be defined as waterproofing membrane systems which have a white reflective surface). It is possible to have a cool roof system with vegetative (green roofing) installed on top of it.

What Does A Green Roof Do?

The term “green roofing” does not refer to a system in of itself that is responsible to keep a flat roof dry. Rather the vegetation serves to absorb some of the rain water to sustain the plant life. Water that is absorbed in the growing medium would otherwise end up in storm sewers which can contribute to over-taxing the GTA water system.

A green growing medium can contribute to extending the life of the waterproofing assembly beneath; much like how gravel helps a built up roof system to reflect the heat of the sun and prolong membrane life.

Toronto City Hall – source:

Types Of Green Roof Systems

There are essentially two types of green roofing systems available. One is referred to as “extensive”, and the other is termed “intensive”. 

Extensive systems are most popular because they are less expensive initially to install and are highly drought resistant. They are most typically the lightest type of green roofing assembly.

Essentially; the system involves the installation of a polyethylene root barrier sheet which is designed as a separation to protect the underlying waterproofing membrane. A fleece layer is then installed which is designed to help retain water resulting from rain or snowfall.  The next installation step involves laying a sod-like roll which contains a variety of sedums (typically 13-20), which are predesigned for the climactic region of the installation site.

Large gravel is installed at roof drain locations in order to help filter earth from washing into drains.

Designing A Green Roof

When designing such a green roof, it is important to incorporate such things as pathways to permit servicing of large garden areas. This is most typically achieved by installing rigid insulation and pavers over top, in a grid pattern. Access is required as fertilizing the garden should be done twice annually.

It is important to design a green roof in an urban setting with access to a water supply. Although the garden relies on rain primarily; there can be the need to augment watering manually if going through a long period without rainfall, or if necessary to combat fire risk. This can be very important for green roof assemblies which are located high up, or in awkward access locations. 

The variety of sedums can supply a continuous bloom from various plant materials from spring right into the fall season.

Intensive green roof systems are essentially designed with trays of planted species , and can also include vegetable planting if so desired. Such systems represent a way to grow fresh produce in an urban environment. Costs for intensive systems can vary based on plant life selected. There are services which can deliver earth to the roof top via high pressure hoses, in order to fill large volumes of containers and planter boxes.

Dr. Joy Schmidt is a Canadian pioneer in the field of green roofing systems. She was instrumental in helping to get the Toronto by-law passed, and originally was a specifier and designer of such assemblies both in Europe, the U.S., and Canada. In fact, she holds the title in the Guinness ® World Book of Records for the largest green roof, which she designed and installed on the Ford™ Motor Plant in Michigan.

Related Content: The Benefits Of Converting A Commercial Roof To Green Space

Installation Of A Green Roof

When contemplating the installation of a green roof; there are some key steps necessary to achieve a superior and safe result. 

The first step involves verifying that the flat roof structure can sustain the weight of the green system in addition to the weight of snow /water load, and roof traffic. If supporting rafters are too small, it may cause the roof to bow, or even cave in, if the structure has not been designed or strengthened to accept such weight.

Therefore ; first steps include determining the weight of products used in the overall assembly, and retaining an experienced engineer to inspect and calculate load requirements. The engineer may then specify any additional framing or reinforcement strategies required to make the roof sound.

It is advised to start with a new waterproofing membrane to be installed on the roof. Certain waterproofing assemblies are preferred for green roof installations. 

White surfaced TPO, white EPDM, (which are single ply systems), and three ply modified bitumen systems with a grey or white granular finished surface are suggested options when installing a green roof system.

Utilizing a white roof system reflects the heat of the sun (certainly on any remaining exposed roof surfaces); and even covered, tends to remain cooler. TPO as an example will not rot when exposed to  prolonged moisture.

Single ply assemblies also offer an advantage in that they are light weight and have a high tensile strength. Three ply modified bitumen systems are hearty and virtually “bullet proof”; as they have two base plies fully adhered, and a final granular surfaced cap sheet fully adhered over the initial two plies. 

Roof substrates should incorporate a minimum of two percent drainage as recommended by waterproofing manufacturers. 

The key is that the waterproofing work should be completed to manufacturer standards, and then tested to be watertight before installation of any green garden materials. In fact many large roofing assets will incorporate an electronic grid system beneath the waterproofing assembly to signal if there ever is water ingress through the system.  This can help isolate a specific leak area if service is ever required in the future, and helps to avoid the removal of unaffected garden areas when completing service work.

When replacing the roof, it also makes sense to consider upgrading the level of thermal insulation used in the roof assembly. By adding additional insulation; the building owner may also become entitled to further grants or tax credits for doing so. Building owners should check for federal and or provincial programs that become available from time to time, that can offset the cost of this important thermal upgrade.

Related Content: University of Toronto Researchers Offer Unique Perspective on Green Roofing

Green Roof Benefits

Green roofs added to waterproofing assemblies that are well installed, can potentially extend the useable roof life by 50%. In Europe, where green roofs are not a newer phenomenon, flat roofs have been reported to last in excess of thirty years. 

Some additional advantages resulting from the installation of green roofs include the fact that they create oxygen, and attract insects and birds to the urban environment, which are also critical in maintaining the balance of nature, pollination, and other life essential activities.

One can also consider that having a green roof acts as a blanket over the roof, further helping to resist heat loss and reducing the heating and cooling costs of the building.

When one considers the thousands of hectares of low rise commercial building stock in North America, converting to green roofs (or minimally to cool roofs), could have a very substantial impact on reducing the use of fossil fuels and natural gas to heat and air condition such facilities. 

It has already been proven that they have been able to lower the temperature of a city like Chicago by four degrees, as a direct result of their green roof and cool roof initiative, during hot summer months.

For commercial flat roof stakeholders; the City of Toronto had provided a grant for those building owners who would invest in green roof or cool roof technology. The program pays $5 per square meter to a maximum of $ 50,000; to offset the initial investment. Building owners must apply in advance of completing such work. To learn more, visit The City Of Toronto website and click on the ECO Incentive Grant.

Related Content: Examining the Benefits of Green Roofing in Summer and Winter

The Future Of Green Roofs

In the near future, as the Federal Government discusses carbon credits and taxation. It could be based on carbon emissions, and rating buildings based on their energy efficiency (think similar to Energy Star Ratings on appliances); it may make greater economic sense for building owners to invest in the greening of their buildings.

In the context of residential flat roof owners, one must consider in addition to structural requirements, the ability to access the roof. It will be necessary to service the roof garden, so installing such an assembly on a “widows peak “ flat roof  may not make sense if surrounded by steep slopes beneath the flat roof area. 

One must also consider having a barrier to the exterior perimeters of the flat roof. By code, to protect one who is servicing the garden; there must be a proper fence or rail in order that one cannot fall off the roof within four feet of the roof’s edge.

Where it makes sense to have a residential green roof, may be over a flat garage or a second storey walkout flat roof. The result can be beautiful adding pleasant views from an adjoining bedroom or den.

The average cost to install a green assembly, in addition to any roof waterproofing costs or design cost elements, runs from $25-$35 per square foot.

To learn more about green roofs, cool roofs or commercial. Industrial or residential flat roofing; contact: