Category: Technical
High Performance. Low Emissions. Future-Proofed
The Toronto Green Standard version 4 (TGS v4) raises the bar for all new development in the city. Green roofs play an important role especially on tight, urban sites. TGS v4 applies to all new planning applications in the City of Toronto received on or after May 1, 2022. Below is a review of the requirements and how green roofs help meet them.
Five performance categories:
- Air Quality
- Building Energy, Emissions & Resilience
- Water Quality & Efficiency
- Ecology & Biodiversity
- Waste and the Circular Economy
Importance of Green Roofs
Green roofs, with their multiple benefits, play an important role in TGS v4 via 2 categories Water Quality & Efficiency (WQ) by Managing Stormwater and Ecology & Biodiversity (EC) through Landscape & Biodiversity.
WQ 1.1 Water Balance, Quality & Quantity Control
Tier 1 Water Balance requires the site to retain a minimum of 50% of the total average annual rainfall volume (or equivalent 5mm from each rainfall event) generated from all site surfaces through infiltration, evapotranspiration, water harvesting and/or reuse, in accordance with the Wet Weather Flow Management Guidelines (WWFMG).
Green roofs can retain rainfall through initial abstraction (IA). According to the WWFMG, the IA are 5mm and 7mm for extensive and intensive systems, respectively. Furthermore, stored water in the cistern can evapotranspirate on the green roof through irrigation water reuse. These mechanisms are key to achieve water balance on site. NLSM offers optional irrigation reports to support water balance calculations.
TGS v4 defines an Intensive green roof as having a minimum growing medium depth of 150 mm with a diverse mix of plants appropriate to the growing medium depth and roof height. On the other hand, extensive green roofs contain hardy, low-profile plants growing in less than 150 mm of growing medium.
WQ 1.3 On-site Green Infrastructure
Tier 1 requires the total landscaped site area, located at and above grade. It includes at least 1 of the following six green infrastructure features:
- A Green Roof covering at least 80% of Available Roof Space
- An Intensive Green Roof for 80% of the Green Roof Area provided
- A Biodiverse green roof to support pollinator species covering a minimum of 50% Green Roof Area
- 25% of the Lot Area at or above-grade, planted with native flower/pollinator species
- At-grade Bioretention facilities provided to capture and control 75% of runoff from on-site hardscape surfaces, or
- Reforestation of a portion of the site (beyond the limit of a stewardship plan)
Note that three of the six options above involve green roofs. Putting green infrastructure on the roof is particularly attractive for development in tight urban sites with perhaps zero lot lines where there is little or no space to accommodate green infrastructures at grade.
The TGS v4 defines a Biodiverse green roof as an Intensive green roof that supports pollinator species. It is located at or below the 8th storey of the building to maximize pollinator access and must follow the recommended plant species found in Appendix A of the Design Guidelines for Biodiverse Green Roofs, with at least two species in bloom at all periods over the growing season. The design must also address two or more applicable Design Strategies from section 4.2 of the guide as listed below:
- Increase depth of growing medium
- Vary composition (structure) of growing media
- Provide topographic variety
- Provide microclimates
- Diversity plant species
- Provide perching habitat
- Provide nesting opportunities
- Provide water source (s)
A permanent irrigation system is required on both intensive and biodiverse green roofs to provide supplemental watering as necessary. Direct access to the roof should be provided for maintenance through a door or hatch located away from the edges to enable safe maintenance. A maintenance contract is recommended for at least the first 5 years of the green roof to ensure it receives a good start.
EC 2.3 Green and Cool Roofs
Landscape & Biodiversity Tier 1 requires buildings, where the Green Roof By-law is applied, to install a green roof to meet the minimum requirements as stated in the bylaw, or where the bylaw does not apply, provides 100% of the Available Roof Space with one of a combination of the following:
- Green Roof
- Solar PV, or
- Cool roof
NSLM Green Roof Systems Comply with TGS v4
All NLSM green roof systems meet the TGS v4. NLSM offers optional professional engineer-stamped wind letter for submission to the City. This chart summarizes the compliance path and helps you select the system that best meets your project’s needs:
| Categories* | NLSM Systems | WQ 1.3 Compliance Options | WQ 1.1 Initial Abstraction** |
| Extensive | LiteN’LessTM II | 80% of available roof space | 5 mm |
| LiteN’LessTM III | 80% of available roof space | 5 mm | |
| StormCapTM | 80% of available roof space | 5 mm | |
| StormCapTM+Detention | 80% of available roof space | 5 mm | |
| TerraExt Sedum Standard | 80% of available roof space | 5 mm | |
| TerraExt with BioBerm | 80% of available roof space | 5 mm | |
| Intensive | Flora Garden | 80% of green roof area provided | 7 mm |
| Biodiverse | Alpine Meadow | 50% of green roof area provided | 7 mm |
| Alpine Meadow+ | 50% of green roof area provided | 7 mm |
** Based on Toronto Water’s Wet Weather Flow Management Guidelines (WWFMG)
Supplier of more than 2.5 M SQFT Coast-to-Coast
NLSM is Canada’s leading supplier of pre-vegetated roofs such as LiteN’Less™, Xeroflor® and StormCap™ systems. We customize to local requirements. Email or call (416) 637-5772 Ext 1.
Enhance Biodiversity
BioBerm™s are rounded mounds of growing media on an otherwise level vegetated roof. They offer a natural, seamless way to integrate extensive and intensive green roof systems. This not only adds visual interest but supports more varieties of plants and a wider range of wildlife. Biodiverse roofs can contribute up to 2 points under LEED v4.1 “Sustainable Sites – Protect or Restore Habitat”. See Portfolio of Green Roofs with Bioberms™.
Manage Structural and Cost Limitations
While intensive systems support greater biodiversity, they can be too heavy and/or expensive for many projects. BioBerm™ is a smart way to creatively manage structural and cost limitations. Designers can strategically add depth to those parts of roof that can support the added load, such as over a load-bearing beam or near a structural wall, while keeping weight and cost low with an extensive system on the rest of the roof.
Create Microclimates and Microhabitats
In addition, the transition between extensive and intensive systems creates localized hills and valleys on the roof. The hills favour drought tolerant plants while the valleys support taller and denser vegetation, which attract different insects, birds and wildlife. As a result, BioBerm™s can create different microclimates and microhabitats to support potentially higher biodiversity than an intensive system.
In Toronto, stormwater is often stored in underground cisterns to meet site runoff requirements. This water is reused through irrigation or toilet flushing to create storage space for the next rain event. BioBerm™s enable designers to combine plants with high and low irrigation needs on one roof, so the green roof will evapotranspirate more when water is available but perverse moisture when the tank is empty.
Steps How to Create a BioBerm™?
Although not necessary, a good way to create a BioBerm™ is to cover the entire growing substrate surface with a pre-vegetated sedum mat. Simply cut slits and plant plugs through the openings into the growing medium underneath. The pre-vegetated mat has many benefits to offer.
Sedum Mat is an Instant Green “Cover Crop”
While the native plant plugs grow, the sedum mat prevents wind and water erosion of the growing medium during establishment. It also acts as a “living mulch” to preserve soil moisture, reduce irrigation needs and minimize airborne weed encroachment.
Many native and adapted vegetation will thrive in 100-200 mm of growing medium in the BioBerm™. Select plants whose natural habitats are similar to the rooftops and avoid invasive species. Design plant palettes that offer longer bloom time to provide nectars for bees and butterflies and produce seeds and berries for birds. City of Toronto’s Design Guidelines for Biodiverse Green Roofs is an excellent resource in creating a successful biodiverse green roof.
Supplier of more than 2.5 M SQFT Coast-to-Coast
Canada’s leading supplier of pre-vegetated roofs, NLSM customizes to local requirements, offers warranty & maintenance support, provides stormwater calculations specific to location and building/roof type, stormwater test data & related innovative hydrologic modeling. Click here to get started on your next green roof.
Read Rooftop Stormwater Management Technologies for Climate Change Adaptation and Resilience by Dr. Karen Liu and Sasha Aguilera in Construction Canada January 2022 Vol. 64 No. 1.
The article looks at the increased intensity and frequency of recent extreme weather events such as heatwaves, rainstorms and hurricanes and how green infrastructure helps urban centres become more resilient. It includes info about new flood maps, efforts on climate action and how green infrastructure, including green roofs, blue roofs and blue-green roofs are being used to manage stormwater and mitigate flooding of urban centres.
Supplier of Green Roof Solutions Nationwide
With over 2.5 million square feet installed, NLSM is Canada’s leading supplier of pre-vegetated systems and accessories to suit any design. Our systems are independently tested to CSA A123.24 standards. We provide P.Eng stamped letters as well as expert support in design, budgeting, documentation, warranties, and maintenance. Email or call (416) 637-5772 Ext 1.
A Tale of Two Assemblies…CRA versus PMRA
Conventional (CRA) and protected membrane roof assemblies (PMRA) are the most common low-slope commercial roofing systems in Canada. The key difference is the placement of the roof membrane relative to the thermal insulation.
While installing a vegetated or green roof system on a conventional roof is relatively straightforward, component configuration can be trickier with PMRA.
Here, we untangle the two so you can design and construct a vegetated system on either roof assembly with confidence.
What is a Green Roof’s Conventional Roof Assembly?
In a Conventional Roof Assembly, the roof membrane is installed on top of the thermal insulation. Although many insulation products are suitable, choose ones with sufficient compressive strength to support the weight of the vegetated system. A cover board is recommended to spread the load across the insulation.
The vegetated system is installed above the roof membrane, starting with the root barrier and followed by the rest of the buildup. Some roofing manufacturers recommend laying a protection fleece or a protection board to protect the roof membrane from foot traffic during the vegetated system installation.
What is a Protected Membrane Roof Assembly (PMRA)?
In a Protected Membrane Roof Assembly or PMRA, the thermal insulation is placed on top of the roof membrane. It is also called Protected Membrane Roof (PMR), Inverted Roof Membrane Assembly (IRMA), or upside-down roof, referring to reversed positions of the membrane and insulation compared to the conventional assembly.
The insulation is now exposed to the elements and must resist water absorption, thermal expansion and contraction, freeze-thaw cycling…etc. The only insulation that is suitable for PMRA is extruded polystyrene (XPS), which is ballasted by aggregates, pavers or concrete topping to prevent flotation and wind uplift.
The amount of ballast required depends on the thickness of the XPS used, starting from 60 kg/m2 (12 psf) for 50 mm (2”) of XPS, and increases by 24 kg/m2 (5 psf) for every additional 25 mm (1”) of XPS. A filter fabric is usually placed between the XPS and the ballast, which not only prevents dirt from settling in the joints but allows the XPS to “raft” or float together, thus reducing the amount of ballast required.
Can a Vegetated System Act as Ballast in a Green Roof?
A vegetated system can replace some or all of the ballast required depending on its dry weight. This is the lowest weight when there is no moisture present in the system. This can be measured using ASTM E2397. Let’s look at an example where 60 kg/m2 (12 psf) of ballast is needed to hold down the XPS on a PMRA.
Let’s say “Vegetated System A” weighs 70 kg/m2 (14 psf) when dry and 100 kg/m2 (20 psf) fully saturated. It can replace all the ballast required as its dry weight is greater than 60 kg/m2 (12 psf). Now, we have a lighter “Vegetated System B” that weighs 40 kg/m2 (8 psf) when dry and 70 kg/m2 (14 psf) fully saturated. We will need at least 20 kg/m2 (4 psf) of ballast to make up the 60 kg/m2 (12 psf) required.
The ballast aggregates are installed on top of the insulation/filter fabric and double the duty as a drainage layer for the vegetated system. Note that if the vegetated system is used to ballast the XPS either partly or entirely, it cannot be removed without substituting alternative ballast in the future.
Where Does the Drainage Go in a Green Roof?
DuPont, a major XPS manufacturer, recommends that the drainage should always be above the XPS. This allows the water to drain off the top surface of the insulation and creates a “diffusion open” assembly. The drainage provides a ventilating air space for topside venting to create open diffusion.
Although installing a drainage layer under the XPS can divert water to drains quicker, unfortunately, it can create a “convective loop” below the XPS. Air can move through the drainage layer and potentially reduce the overall insulation value.
As a result, DuPont recommends installing the drainage above the XPS to ensure the insulation value is maximized. A good positive slope to drain is an effective and better way to divert water to drains in PMRA.
Root Barrier Installation for a Green Roof
DuPont recommends the root barrier be installed above the roof membrane and under the XPS. Root barriers are flexible sheets made of polyethylene or polypropylene that physically block root ingress to the roof membrane.
These plastic sheets are vapour impermeable. If placed on top of the XPS, water vapour in the system cannot escape and can potentially be driven back into the XPS, which can reduce its insulation value. Thus, a root barrier should be placed below the XPS to achieve “open diffusion”.
Green Roof Best Practices for Specifications
As the root barrier often falls under “Vegetated System” instead of “Roofing System” in construction specifications, this can create a workflow/coordination issue. Therefore, we recommend the root barrier be specified under “Roofing System” so it can be installed by the roofing contractor. It should also be referenced under the “Vegetated System” so the green roof contractor is aware it exists but focuses on the rest of the vegetated system buildup above the PMRA.
Further Reading and Reference Material
DuPont’s “Vegetative Roof PMR – A Design Primer” is a good reference that offers practical building science advice for designers who are interested in vegetating PMRA. All NLSM’s systems can be installed on conventional and PMRA.
Supplier of more than 2.5 M SQFT Coast-to-Coast
NLSM is Canada’s leading supplier of pre-vegetated roofs that are customized to local requirements. NLSM offers warranty & maintenance support, provides stormwater calculations specific to location and building/roof type, stormwater test data & related innovative hydrologic modelling. Contact NLSM for the best green roof option to meet your design goals.
Read Wind Design Process for Modular Vegetated Roofing Systems by Dr. Karen Liu and Sasha Aguilera in Construction Canada January 2021 Vol. 63 No. 1.
This article was written by Next Level Stormwater Management’s Green Roof Specialist Dr. Karen Liu and Design Ambassador Sasha Aguilera – key participants in the research consortium that developed the first national wind testing standard for vegetated roofing. It explains the Canadian Standards Association (CSA) A123.24-15 Standard Test Method for Wind Resistance of Modular Vegetated Roof Assembly and explains the three simple steps that help determine a vegetated roof’s ability to withstand and protect against wind forces.
NLSM Provides P. Eng Stamped Wind Resistance Letters. Get Started! Contact us
Watch Video of NRC and UL Testing
More Than 2.5 Million Square Feet Coast-to-Coast
NLSM is Canada’s leading supplier of pre-vegetated roofs such as Xeroflor, LiteN’Less™ and StormCap™ systems. We are your one-stop-shop. We customize to local requirements, provide wind and stormwater calculations specific to location and building/roof type, P.Eng letters, and other documentation. We offer various products & services including warranty & maintenance support. Click here to start your design.
Reinventing the Traditional Green Roof for Detention by Sasha Aguilera and Brad Garner in Construction Canada January 2019 Vol. 61 No. 7.
Reinventing the Traditional Green Roof for Detention discusses how detention green roof systems are a reliable and quantifiable rooftop stormwater management tool. The article examines how to use detention technology in dense urban areas to help owners achieve ROI by saving valuable real estate while meeting municipal requirements.
Read Designing Vegetated Roofs to Survive Winter by Cristina Senjug in Construction Canada May 2018 Vol. 60 No. 3
The article discusses how in cold climates such as Canada, properly designed vegetated roofs, installed with hardy and mature plants that are well hydrated before winter, have a good probability of survival. If in the following spring, there is evidence of damage—caused by extreme temperature fluctuations, erosion due to high winds, or plant desiccation because of foot traffic—a basic repair is possible to help these sections quickly recover.
Supplier of Green Roof Solutions Nationwide
With over 2.5 million square feet installed, NLSM is Canada’s leading supplier of pre-vegetated systems and accessories to suit any design. Our systems are independently tested to CSA A123.24 standards. We provide P.Eng stamped letters as well as expert support in design, budgeting, documentation, warranties, and maintenance. Email or call (416) 637-5772 Ext 1.
Read Designing Extensive and Semi-Extensive Vegetated Roofs for Long-term Performance by Sasha Aguilera and Kees Govers in Construction Canada July 2016 Vol. 58 No. 5.
Awarded the 2017 F. Ross Browne Award for “Article of the Year”, this article co-authored by Sasha Aguilera and Kees Govers encourages consultants and contractors to think about their designs and implementations related to vegetated roofs in such a way that systems can perform as long as the building is in use. The article discusses everything from budgeting and design, to access, irrigation, wind uplift, designing for problem areas, care during construction, post-installation, maintenance, vegetation and overburden warranties. To ensure excellence in performance, it’s important the details in design and installation are considered as critically as the systems themselves.
Supplier of Green Roof Solutions Nationwide
With over 2.5 million square feet installed, NLSM is Canada’s leading supplier of pre-vegetated systems and accessories to suit any design. Our systems are independently tested to CSA A123.24 standards. We provide P.Eng stamped letters as well as expert support in design, budgeting, documentation, warranties, and maintenance. Email or call (416) 637-5772 Ext 1.
Read Waterproofing Considerations for Green Roofs by Dr. Karen Liu in Construction Canada March 2012 Vol. 54 No. 2
The article discusses the necessity of a robust and durable roofing system to provide a solid foundation for green roofs and tries to help answer, “What is the best roofing/waterproofing system for green roofs?
Supplier of Green Roof Solutions Nationwide
With over 2.5 million square feet installed, NLSM is Canada’s leading supplier of pre-vegetated systems and accessories to suit any design. Our systems are independently tested to CSA A123.24 standards. We provide P.Eng stamped letters as well as expert support in design, budgeting, documentation, warranties, and maintenance. Email or call (416) 637-5772 Ext 1.