Green Arc: Highlight Barrel Roof

St. Lawrence Market district, which comprises the St. Lawrence Hall and the South and North Buildings, has been a historic landmark in Toronto for over 200 years. Integrated into this heritage-rich context, the new five-storey St. Lawrence Market North commands attention with its glass atrium and the striking greened barrel roofs, covering an area of 1,500 m². This extraordinary green roof sets a new benchmark for architecture and sustainability. Installing a green roof on a 45° pitched surface pushes the boundaries of sloped green roof construction. This is where Zinco, experienced green roof system manufacturers, are ideally suited. They provided custom system build-ups, including the “Pitched Green Roof” with Floraset® FS 75 and the “Steep Pitched Green Roof” with Georaster®.

St. Lawrence Market is one of Toronto’s oldest and most significant public spaces. Founded in 1803, the market has historically served as a trading hub, the location of Toronto’s first town hall, and the site of early law courts. Today, it remains a central part of urban life — hosting farmers’ and vintage markets, retail shops, offices, events, lectures, concerts, and exhibitions. The spectacular new St. Lawrence Market North, designed by RSHP (formerly Rogers Stirk Harbour + Partners) in collaboration with Adamson Associates Architects, replaces the former 1968 single-storey structure. It now includes a market hall at ground level, with offices and courtrooms on the upper floors. Mezzanine levels feature a gallery and spaces for receptions and weddings. A pedestrian bridge links the St. Lawrence Market North building to the adjacent St. Lawrence Hall.

Design is Making a Statement

The architects created a bold visual language through the building’s symmetry, its central glass atrium, and the two prominent greened barrel roofs. The atrium can be opened seasonally, transforming into an arcade that allows market activities to spill into the adjacent streets.

The building maximizes natural light and ventilation and is designed for low energy consumption. The barrel roof greenery reduces energy demands —cooling the interior during summer and insulating it in winter. The plants also retain and evaporate rainwater, enhancing the microclimate and helping mitigate the urban heat island effect. Moreover, they filter dust and airborne pollutants, improving air quality.

The green roofs support Toronto’s sustainability goals while offering a lush, visible expanse of urban greenery that provides habitat for insects and plants and contributes to an improved cityscape.

Whatever’s Possible

Like all steep pitched roofs, barrel roofs with slopes exceeding 35° require building-specific feasibility assessments and enhanced shear and erosion control. The roof area of St. Lawrence Market North totals 2,000 m², of which 1,500 m² were available for greening after accounting for ventilation systems and other roof structures.

The gently curved areas (0–25° pitch), totaling 1,100 m², were fitted with Zinco’s “Pitched Green Roof” system using Floraset® FS 75. The steeper edge areas (up to 45° pitch) added another 400 m² and were completed with Zinco’s “Steep Pitched Green Roof” using Georaster®, along with specialized shear protection measures.

Both build-ups were installed over a double-layer modified bituminous waterproofing system, itself laid atop 150 mm of thermal insulation, with Root Barrier WSF 40 and Protection Mat BSM 64. The latter is especially effective for pitched roofs due to its high water storage capacity.

Transferring Shear Forces

Shear forces can typically be transferred via a sufficiently high and strong roof edge. However, to meet the aesthetic goal of a narrow roof edge on this project, a custom green roof edging with eaves profiles was installed, based on precise structural calculations.

Support brackets were integrated into the roof substructure and sealed. These brackets anchor the eaves profiles, forming a stable abutment that transfers shear forces into the building structure, preventing any slippage of the green roof layers. Slots in the profiles ensure proper drainage. Angle profiles were also used to frame the sides and define the transition between the Georaster® and Floraset® FS 75 zones, ensuring stability and shear control.

A Stable, Continuous Unit

In steep areas (25°–45°), the Georaster® elements from Zinco’s “Steep Pitched Green Roof” system form a secure and interlocking framework. Made from recycled HD-PE, these 54 x 54 cm, 10 cm-high units provide ample root space while maintaining structural stability. Well-rooted vegetation reduces erosion risk significantly.

In the gentler pitched areas (0°–25°), Floraset® FS 75 drainage and water storage elements made of EPS hard foam were used. These 75 mm-high components have large internal cavities, promoting interlocking of the substrate and forming a stable, continuous surface. These sections are fully framed with angle profiles for additional structural coherence.

Water Management

A barrel roof is an exposed environment for vegetation. High wind, direct sunlight, and steep slopes lead to rapid water runoff and potential drought stress.

Water is retained in the Protection Mat BSM 64 and in the Zinco system substrate, which was installed to a depth of 10 cm. Dripline irrigation pipes are laid directly on the substrate to supply water during dry periods, drawing from a storage tank that collects surplus rainwater.

Pre-Cultivated Vegetation Mats

Early irrigation is crucial during the establishment phase. The pre-cultivated sedum mats used here contained well-developed root systems, helping them quickly integrate with the substrate layer and resist erosion.

Today, a biodiverse carpet of 17 sedum species covers the roof — including Sedum forsterianum 'Silver Stone', Sedum reflexum 'Blue Spruce', and the vibrant Sedum spurium varieties ‘Dragon’s Blood’ and ‘Voodoo’.

Maintenance is minimal — mainly occasional fertilization and weed removal, thanks to the dense and competitive sedum coverage.

Perfect Synergy

The barrel roofs rise above two of Toronto’s busiest intersections — Jarvis Street and Front Street. As such, the green roof installation posed logistical and organizational challenges.

Leaf Maintenance, a Toronto-based landscaping company, completed the green roof in just a few weeks during the summer of 2023. Materials were stored in tight quarters and lifted to the roof by crane, necessitating temporary street closures. Due to frequent high winds, the installation was carried out in phases, with substrate applied immediately after the drainage layer to weigh it down. A blower truck was used to deliver the substrate directly onto the roof.

The Arc Comes Full Circle

St. Lawrence Market North exemplifies how historic spaces can be reimagined with sustainability, biodiversity, and modern green infrastructure. Its iconic green barrel roofs have created ecological and aesthetic value for Toronto’s public realm.

Undoubtedly, the architectural and environmental resonance of this project will echo far beyond Toronto — establishing a powerful precedent for the green roof designs of the future.

Author: Heidrun Eckert, Business Unit Manager Zinco GmbH

Plant list

Sedum Acre “Oktoberfest”
Sedum Ellacombanium
Sedum Foresterianum “Silver Stone”
Sedum Sexangulare
Sedum Sediforme
Sedum Kamtschaticum
Sedum Album
Sedum Floriferum “Weihenstephaner Gold”
Sedum Hybridum “Czar’s Gold”
Sedum Reflexum “Blue Spruce”
Sedum Spurium “Coccineum”
Sedum Spurium “Dragons Blood”
Sedum Stoloniferum
Sedum Spurium “Voodoo”
Sedum Spurium “Summer Glory”
Sedum Spurium “Fuldaglut”
Sedum Spurium “Red Carpet”

Site board

Construction project: St. Lawrence Market North, Toronto, ON M5E 1C4, Canada

Client: City of Toronto

Construction year: 2023

Roof area: approx. 2,000 m²  

Green roof build-up: Zinco system build-ups “Pitched Green Roof” with Floraset® FS 75 and “Steep Pitched Green Roof” with Georaster®

Contractor: Buttcon Limited, Woodbridge, ON L4H 4G3, Canada

Architects: RSHP, The Leadenhall Building, London, EC3V 4AB, Great Britain and Adamson Associates Architects, Toronto, ON M5V 2H8, Canada

Wind uplift protection: Motus Consult, Calgary, AB T2H 2K6, Canada

Landscape architect: Quinn Design Associates Inc., Toronto, ON M5T 1X5, Canada

Roof installation: Dufferin Roofing, Toronto, ON M9L 1Y8, Canada

Green roof installation: Leaf Garden Maintenance, Toronto, ON M4K 1N7, Canada

System supplier: Zinco Canada Inc., Carlisle, ON L0R 1H0, Canada

System manufacturer: Zinco GmbH, 72622 Nuertingen, Germany