DESIGN CONSTRUCTION
November 10, 2025 228
Low-flow basements could reduce costs and jumpstart some stalled residential projects
A technological change beneath Toronto's streets has created a barrier to new housing. As of early 2022, the city's foundation drainage policy requires most new buildings to use waterproof (or “bathtub”) basements, banning the small groundwater drains that once kept basements dry. The rule is designed to protect sewers and has resulted in millions of dollars in additional construction costs and thousands of tons of embodied carbon in projects across the city, particularly mid-rise affordable and nonprofit housing. A team of engineers led by Grounded Engineering is now proposing “low-flow basements,” a new concept that could exempt some projects and help free up stalled housing without overwhelming city infrastructure.
Expanded foundations in an excavation, image by Bryan Crljenica
Toronto Water implemented the Foundation Drainage Policy on January 1, 2022 to maintain wastewater capacity even during more severe storms and flooding. The new regulation prohibits any new foundation drainage into municipal systems, meaning there can no longer be septic tanks or subfloor drains in basements to collect groundwater. Instead, any structure that comes into contact with the water table must be sealed to withstand the groundwater pressure. The change effectively replaced traditional spread foundations with solid slab foundations that acted as watertight tanks, a design once reserved for subway stations. Because the city views the new foundation drainage policy as merely an enforcement of the existing wastewater ordinance, it did not require or seek Council approval, even though it brought significant changes to the status quo. It applies citywide, regardless of conditions.
Spread out foundation formwork in a shallow excavation pit, image by Bryan Crljenica
Replacing shallow, spread-out foundations with raft foundations requires more concrete, steel, excavation and shoring, increasing both costs and carbon emissions. For a typical six-story mid-rise building with a basement, this represents an additional cost of $1.5 million to $3 million and an increase in embodied carbon of up to 25%. These premiums apply to every basement, not just high-rise towers. Affordable housing and nonprofit housing projects like the West Neighborhood House on Ossington Avenue and the Kindred Works church renovation are also billed, adding to the financial burden. The directive also states that financial distress cannot be used as an exception, so smaller projects (including low-risk, low-rise sites) are subject to the same technical requirements as a high-rise tower or subway station.
Detail of the waterproofed basement (left) and detail of the drained basement (right), image courtesy of Jablonsky, Ast and Partners
Grounded Engineering, along with a team of engineers including Jablonsky, RJC and Smith and Andersen, submitted a proposal to the city to exempt low-flow basements from the waterproofing rule. These are basements from which less than 50,000 liters per day (approximately the flow of a single garden hose) would enter the municipal sewer system. This threshold reflects the Ministry of Environment's limit for water withdrawals that do not require provincial permits, making it a reasonable starting point. In practice, many medium-height and single-story basements fall into this range.
The graphic shows that the cost of raft foundations increases sharply for smaller medium-sized buildings, image courtesy of RJC
The concept works within the existing framework. Section 5 of the Foundation Drainage Policy already allows exceptions, although the current criteria exclude cost as a factor. This low-flow approach provides a clear, measurable standard where infrastructure impact is minimal. Qualifying projects could use traditional drained foundations with edge countersinks and subfloor drains without incurring the financial burden of a fully waterproof structure.
In practice, low-flow basements would be regulated by engineering data. Each project would submit technical reports confirming discharge rates and local sewer capacity. Smart meters could then monitor flow rates in real time. City officials are considering whether thresholds should apply per capita, per square foot or be tied to affordability. As part of a motion approved by Councilor Josh Matlow in July 2025, Toronto Water is modeling these scenarios to determine how low water subsidies could be safely implemented.
Waterproof basements increase carbon content by up to a quarter compared to a typical middle floor and add thousands of tons of concrete and steel, running counter to the city's Toronto Green Standard. An exemption for low discharges would reduce material consumption while protecting the sewer system through measurable discharge limits.
While waterproof basements sound like the safer option, they often pose long-term risks for owners. Lashing anchors used to secure heavy raft panels penetrate the waterproofing membrane, which eventually becomes leaky. Once inhabited, it is nearly impossible to repair a foundation that is several stories underground, so condo boards require costly maintenance. The Deep Foundations Institute recommended that the city require emergency drainage systems as a safety measure, but the city only began approving these systems after extensive lobbying by the engineering team led by Grounded. Without built-in strainers or sump connections, even small leaks can flood the parking lot, and subsequent pump upgrades will only partially help.
Lashing straps, image by Bryan Crljenica
Momentum is building after Councilman Josh Matlow's motion directed staff to examine the technical and policy framework needed to allow exemptions for low-flow basements. Senior staff at Toronto Water and the City Manager's Office have expressed interest in the proposal. Toronto Water, meanwhile, is developing modeling tools to test potential thresholds and enforcement methods, but currently has no formal process for granting exemptions.
By treating all groundwater discharges as equally problematic, current policy has delayed many housing projects. A low-flow exemption could reopen these opportunities, particularly for mid-rise and community-based projects with only a basement level. The approach ensures that full waterproofing standards are met for high flow conditions such as: B. Scarborough's groundwater-rich soils are maintained, ensuring that environmental protections remain intact and the city's wastewater capacity is protected. For a city looking to meet housing and climate goals, this may be one of the simplest and most impactful solutions.
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