1st Place, Exterior
Many of the buildings constructed to meet the post-World War II population boom in North America are now in deep need of a retrofit. Typical residential towers of the era, like 191 and 201 Sherbourne in Toronto, included exposed structural concrete walls and single-pane windows. As a result of the buildings’ long-term exposure in an urban environment, their concrete exteriors were in poor condition with cracks and loose spalls. To compete with newer rental properties, maintain structural safety, and align with Toronto’s sustainability goals, the owners decided to revitalize the buildings’ exterior and update the heating and cooling systems.
In collaboration with Zeidler Architecture Inc., Thornton Tomasetti’s Building Science, Façade Renewal and Engineering teams helped to develop a plan that would bring the early 1970s vintage towers into the modern era while preserving their original Brutalist architecture. The project scope was generally divided into two main categories: concrete restoration and overcladding and window replacement.
CONCRETE RESTORATION
Thornton Tomasetti initially performed a visual review of the exposed concrete from grade and a representative number of balconies. During this examination phase, the team selectively utilized an artificial intelligence algorithm developed in-house to identify and quantify spalled concrete areas. Upon identifying multiple falling hazards, a near-hand investigation and loose spall removal program encompassing the entire building façade was performed from swing stages.
With the information from this investigation, Thornton Tomasetti prepared drawings and estimated quantities for concrete repairs to the balconies, exposed walls, and architectural roof structures. Elastomeric coatings were specified to coat and protect the balcony slabs and exposed walls to increase the long-term durability of the structure. The team also checked all balconies for slope away from the building and adjusted the coating buildup as required.
The repair project began as COVID-19 restrictions were in full effect. The contracting team, led by Buttcon, followed strict health and safety protocols but also had to navigate a nearly full house because many residents were required to work from home. Early into construction, the owner sought ways to reduce the noise and vibration caused by the concrete chipping, particularly at the balcony edges. The concrete restoration contractor, SST Group, secured a German-made pneumatic crushing device to demolish the deteriorated balcony edges. This equipment significantly reduced noise to the tenants and accelerated the restoration schedule.
Although portions of the plaza between the buildings were slated for redevelopment, Thornton Tomasetti participated in several efforts to identify and resolve leaks. This involved performing limited test pits in the plaza overburden and tracing water ingress. Fixes included a variety of waterproofing systems, crack injection technologies, mastic asphalt overlay on parking ramps and concrete patching.
OVERCLADDING AND WINDOW REPLACEMENT
The envelope re-cladding consisted of a hybrid system with a curtainwall assembly in front of the existing masonry knee walls and a double-glazed, argon-filled window above it. This strategy improved the buildings’ thermal efficiency and protected the masonry knee wall and exposed slab edge from continuing deterioration. Overcladding and window replacements approximately halved the expected heat loss of the typical bedroom and living-room bays.
The two-part solution also considered that the building would be fully occupied during the renova- tion process and minimized the amount of time tenants would need to be displaced from their units. The curtainwall overcladding was installed from swing stages on the building exterior, reducing the need for entry into the tenant suites. Additionally, most of the window assembly was also hoisted into place using swing stages.
The existing windows were single-pane glass with operable sliders just above the knee wall. They had long since outlived their service life, exhibiting poor innate thermal performance, damaged seals and deteriorated sealant. Likewise, the original knee wall construction did not meet contemporary standards for cavity wall construction. The collar joint was filled with loose mortar, and there were no weeps above the slab edge. Upon removing the existing window, the team observed inconsistently applied mortar placed below the existing sill. In many locations, the knee wall masonry was badly deteriorated. However, reconstructing the knee walls would have been extremely costly and would provide little thermal benefit. Overcladding was a better solution.
The new curtainwall overcladding spans over the brick, protecting it from continued weathering without imparting any new loading and without creating negative changes in the hygrothermal performance of the assembly.
The interface between the new windows and the curtainwall required unique detailing. To dissuade pigeons from resting on the ledge without using spikes, the design called for 45-degree sloped exterior flashing. A high-performing insulating blanket ensured the continuity of the insulation line within the tight space available at this joint. A weather barrier laps over the re-surfaced knee wall ledge and continues to the new curtainwall extrusion, creating a continuous barrier below the sloped transition. The window blocking was slightly recessed to permit a continuous sealant bead to tie the sill to the waterproofing without blocking the weeps. On the back side of the windowsill, a backing angle assisted with field positioning and provided structural fastening into the prepared concrete masonry unit substrate.
The designers iterated these flashing details with the window fabricator, Stouffville Glass Inc., through multiple mock-ups to ensure a constructible design. The process included developing a two-component exterior flashing that would accommodate deviations in elevation and plane exhibited in the existing construction.
The successful execution of this project required a holistic approach to evaluating envelope performance, including heat-transfer analysis; hygrothermal analysis; evaluation of weighted thermal transmittance; and practical knowledge gained from coordination with the window fabricator, installer and product manufacturers.
The overcladding and concrete coating solutions significantly reduce future repair-cost exposure. The holistic design balances envelope performance with budget and constructability concerns to breathe new life in mid-century towers with long future life cycles. Overcladding Toronto’s post-war apartment towers is an efficient, carbon-sensitive way to reduce operational emissions from the city’s existing building stock. Extending the lifespan of aging multi-unit residential buildings, using high-performing overclads, reduces the need for extensive demolition; it can be scaled to ensure the long-term livability of dense vertical communities.
PHOTOS: Thornton Tomasetti unless otherwise noted
Retrofit Team
METAMORPHOSIS AWARD WINNER and STRUCTURAL ENGINEER AND BUILDING ENVELOPE CONSULTANT: Thornton Tomasetti
ARCHITECT: Zeidler Architecture Inc.
GENERAL CONTRACTOR: Buttcon
CONCRETE RESTORATION CONTRACTOR: SST Group
WINDOW SUPPLIER, INSTALLER: StouffvilleGlassInc.
Materials
ELASTOMERIC CONCRETE WALL COATING: SIKA 550W from Sika USA
ELASTOMERIC BALCONY COATING: SIKA Sikalastic 735 AL Topcoat and Sikalastic 710 NP Base Coat from Sika USA
ANTI-CORROSION COATING AND BONDING AGENT: SIKA Sikatop Armatec 110 EpoCem from Sika USA
CEMENTITIOUS REPAIR MORTAR: SIKASikacrete-211FlowPlus; MonoTop 623 (Overhead and Vertical Patch); and MonoTop 721 (Reprofiling Mortar) from Sika USA
PUNCHED WINDOWS: FeatureLine 990 from Alumicor
CURTAINWALL: VersaWall MidLine 2200 from Alumicor
GLAZING: SunGuard AG50 and Crystal Gray from Guardian Glass
SLIDING GLASS DOORS: 7000 Series from Clearview
STOREFRONT: FlushGlaze BF 3400 from Alumicor
EXTERIOR SEALANTS: DOWSIL 795 from Dow
WEATHER BARRIER: Blueskin SA from Henry and ExoAir 110 from Tremco
GLASS GUARDRAILS: Alugard