Transforming a Leaside Semi-Detached: A Complete Basement Lowering & Underpinning Project in Toronto
When maximizing living space in Toronto’s highly competitive real estate market, converting a shallow, unusable basement into a fully functional living area is one of the smartest investments a property owner can make.
Strong Basements Inc. recently partnered with a local realtor and a team of investors to completely transform a property in the Leaside neighbourhood. The goal was to convert a cramped basement into a premium 8-foot-high living space featuring a recreation area, a 3-piece washroom, a laundry room, and a mechanical room.
Because the property is a semi-detached home with an older natural stone foundation, this project required highly technical basement underpinning and meticulous project management. Here is a comprehensive behind-the-scenes look at how our team executed this complex structural transformation.
The Scope of Work
To deliver the maximum possible square footage and ensure the permanent structural integrity of the home, Strong Basements Inc. provided a full suite of foundation, mechanical, and waterproofing services:
Sequential Excavation and Underpinning: Digging down in carefully engineered stages and extending the foundation walls to safely lower the basement floor, completely eliminating the traditional concrete bench.
Complete Interior Basement Waterproofing: Installing a continuous dimpled membrane, a weeping tile system, and a heavy-duty sump pump housed in a dedicated tank with a 1.5-inch ABS drainage outlet.
Advanced Underground Drainage: Roughing in the underground ABS plumbing for a new 3-piece washroom and laundry, sloping it efficiently toward a premium sewage ejector pump.
Engineered Concrete Floor Construction: Applying and vibratory-compacting a 4-inch subfloor gravel bed, laying a 6-mil poly moisture barrier, installing heavy-duty steel welded wire mesh, and pouring a 32mPa concrete floor finished with a power trowel.
Exterior Plumbing Modernization: Excavating a 7-foot deep trench to replace a high-risk clay building trap with modern PVC piping and highly accessible back-to-back cleanouts.
Overcoming Unique Leaside underpinning Project Challenges
Every older Toronto home has its quirks. Our foundation underpinning contractors navigated several major hurdles on this Leaside property, working closely with the structural engineer to ensure a flawless execution.
1. The Party Wall & Neighbour Relations
In a semi-detached home, your foundation is shared. The elderly neighbours living behind the party wall run a podcast, meaning excessive noise would directly disrupt their daily routine.
The Solution: We handled the legal and administrative heavy lifting, obtaining a separate building permit for the neighbour’s property, securing their letter of consent, and adding them to our underpinning liability insurance as a separate insured party. We also coordinated a strict noise schedule so they could continue streaming their podcasts without interruption. The result? Zero complaints from the neighbours throughout the entire project.
To learn more about the party wall agreement for underpinning and lowering the basement, visit Underpinning Your Semi-Detached Home
2. Tight Access and Logistics
Like many Toronto properties, the home had extremely limited space on the front driveway for heavy machinery and massive piles of excavated dirt.
The Solution: We engineered a custom soil-removal setup, safely feeding a mechanical conveyor belt through a side basement window. We then utilized smaller, highly maneuverable 6-yard soil disposal bins placed carefully in the laneway. This kept the site clean, efficient, and operational without blocking neighbourhood traffic.
3. An Aging, Unpredictable Natural Stone Foundation
The original foundation was constructed from century-old natural stone, which can be highly unpredictable, irregular, and delicate during heavy excavation.
The Solution: To ensure absolute safety, the structural engineer designed the underpinning in four strict stages rather than the standard three. While this added one extra week to our timeline for excavation, municipal inspections, concrete pouring, and grout application, it guaranteed the structural stability of the fragile stone above our workspace.
4. Maximizing Space with Flush L-Shape Underpinning
Traditional underpinning often leaves a concrete “bench” around the interior perimeter, eating into valuable square footage and limiting layout options. The investors wanted every possible inch of space for the future 8-foot-high living area.
The Solution: We utilized an advanced L-Shape underpinning method, installing custom L-shaped concrete formwork sequentially along the excavated sections. This specialized technique creates a continuous L-shaped footing at the bottom, allowing us to pour a perfectly flush foundation wall from top to bottom. This completely eliminates the space-wasting concrete bench, providing maximum usable square footage.
5. Custom Liquid Grout for an Uneven Stone Footing
Because the bottom of the original natural stone foundation was incredibly jagged and uneven, applying a standard troweled grout between the new concrete underpinning and the old stone would have left dangerous structural voids.
The Solution: With the engineer’s approval, we engineered a custom containment system. We built specialized wood forms on top of the newly poured concrete underpinning sections and sealed all edges with expanding foam to create watertight “wells.” We then poured a high-flowability liquid non-shrink grout into these forms.
To guarantee a perfect structural seal, we deliberately built the top of the well higher than the original footing. This ensured the liquid completely filled every single crevice, overlapped the old stone for maximum load transfer, and cured into a rock-solid, 2-inch thick support layer.
6. Upgrading a "Time Bomb" Plumbing System
During excavation, we discovered two major plumbing issues threatening the home’s future. Inside, the main sanitary line at the exit point was sitting too high, making standard gravity drainage for the newly lowered basement impossible. Outside, we spotted a rusted cast-iron breather and cup sticking out of the front lawn—the telltale sign of an outdated underground clay building trap, notorious for clogging and collapsing.
The Solution:
Interior Upgrade: We installed a Liberty Sewaje Ejector System Pro380-Series, widely recognized as the most reliable system in Ontario, directly next to the newly installed sump pump system. All new underground ABS drainage rough-ins for the washroom and laundry were expertly sloped and connected to this ejector pump before the concrete was poured.
Exterior Upgrade: We excavated a 7-foot deep trench in the front yard, completely removing the rusted cast-iron breather and fragile clay trap. We replaced it with a modern, straight 4-inch PVC pipe featuring two back-to-back cleanouts. When we backfilled the trench, we intentionally left the PVC cleanout cups slightly protruding above the soil grade, giving future plumbers easy, dig-free access.
The Anatomy of a Perfect Concrete Floor
The final stage of the structural work was the concrete slab, which requires meticulous preparation to ensure a dry, crack-resistant finish.
Subfloor Preparation: We applied a 4-inch subfloor gravel bedding, evenly distributing and tightly compacting it with a vibratory plate to create a highly stable base.
Moisture and Structural Reinforcement: A continuous 6-mil poly vapor barrier was laid over the compacted gravel to permanently block ground moisture. Heavy-duty steel welded wire mesh was then installed on top to maximize the slab’s structural integrity.
The Pour and Finish: We poured a 4-inch thick concrete slab using a premium, high-strength 32mPa concrete mix. To achieve a flawless surface, a concrete professional finished the freshly poured slab with a powered trowel, creating a perfectly smooth, polished, and level floor.
The Final Result
The project was a resounding success. The structural engineer specifically praised our complex L-shape formwork and our deliberate attention to detail with the liquid non-shrink grout application.
When the client and investors walked the site, they were greeted by a fully cured, dry concrete slab and a robust mechanical corner fully equipped with a working sump pump, sewage ejector, and floor drain. They were thrilled with the true 8-foot ceiling height, the flawless concrete finish, and the massive increase in usable space created by the benchless underpinning. They were so impressed with the structural canvas we delivered that Strong Basements Inc. is already slated to handle the underpinning on their next investment property.
We successfully passed this dry, structurally sound, and fully prepped basement over to the finishing renovation team to complete the drywall, framing, and final fixtures.
Planning a Basement Renovation in Toronto?
If you are looking to increase your basement ceiling height, add a legal basement apartment, or reinforce an aging foundation, you need a contractor who understands the complexities of Toronto’s soil and older homes.Contact Strong Basements Inc. today to discuss your basement lowering and foundation underpinning needs.
Make your basement a legal unit
Frequently Asked Questions
Do I need my neighbour's permission to underpin a semi-detached home in Toronto?
Yes. Because semi-detached homes share a central foundation wall (known as a party wall), any structural work beneath it directly affects your neighbour’s property. You cannot legally excavate or underpin this shared wall without their consent. The process requires a formal Party Wall Agreement, securing a separate building permit for their side of the property, and providing proof of specialized underpinning liability insurance to guarantee their home remains completely safe and protected during the project.
What is L-Shape underpinning, and why is it better than traditional benching?
Traditional foundation benching leaves a wide border of concrete (a “bench”) around the interior perimeter of the basement, which reduces your usable square footage and limits your floor plan. L-Shape underpinning involves pouring the new concrete foundation perfectly flush with the existing wall, using an L-shaped footing at the base for support. While it requires more complex formwork and precision engineering, it completely eliminates the bench, giving you maximum wall-to-wall living space.
Why use liquid non-shrink grout instead of standard grout for an old stone foundation?
Century-old natural stone foundations usually have highly irregular, jagged bottoms. If a contractor tries to pack standard, thick grout into that gap by hand, it can leave hidden voids that compromise the structural load transfer. By building watertight wood forms and pouring a high-flow liquid non-shrink grout, the liquid naturally rises to penetrate every single crevice and gap. Once cured, it creates a rock-solid, perfectly sealed 2-inch structural bridge between the old stone and the new concrete.
What is an underground clay building trap, and why must it be replaced?
A clay building trap is an obsolete, U-shaped plumbing fixture found buried in the front yards of many older Toronto homes. Originally designed to block sewer gases, these fragile clay traps are notorious “time bombs.” Over the decades, they crack, attract tree roots, and create severe bottlenecks that lead to basement sewage backups. Proactively replacing the trap with a straight 4-inch PVC pipe and highly accessible ground-level cleanouts permanently removes the risk of a collapsed drain.
How thick is a new basement concrete floor, and what materials are used?
A premium residential basement floor is poured to a thickness of 4 inches. To ensure it is strong, level, and completely dry, it requires several engineered layers:
The Base: A 4-inch layer of tightly compacted gravel for stability and drainage.
The Barrier: A continuous 6-mil poly vapour barrier to block rising ground moisture.
The Reinforcement: Heavy-duty steel welded wire mesh to prevent structural cracking.
The Concrete: A high-strength 32mPa concrete mix, which is denser and more durable than standard mixes, finished with a power trowel for a perfectly smooth surface.
