Key point: Basement lowering in Toronto is not a single dig — it is a precisely sequenced series of small excavations, each poured and cured individually before the next begins. Understanding the sequence is the best way to know what to expect when a crew is working in your home.
Before the Crew Arrives
Two things happen before any physical work begins: engineering and permits. A licensed structural engineer visits the site, measures the existing foundation depth, evaluates the soil and wall conditions, and produces stamped drawings specifying the excavation sequence, section dimensions, concrete mix, and reinforcement requirements. These drawings go to the City of Toronto with a building permit application.
The permit review takes 4 to 8 weeks in most Toronto wards. The permit is posted on-site before work begins — this is a legal requirement and also signals to neighbours that the project is authorized. BaseWorks handles the engineering coordination and permit application as part of every project.
Phase-by-Phase: What Is Actually Happening
Site Preparation — Day 1
The crew clears the basement, installs temporary lighting, sets up ventilation fans to manage dust and concrete fumes, and places dust barriers at interior doorways to limit debris migration into living areas. Soil removal routes are established — ideally through an exterior access point, window opening, or walkout. The permit is posted and the superintendent reviews the engineer's drawings with the team.
Slab Demolition — Days 1–2
The existing concrete floor slab is broken with electric jackhammers and removed piece by piece. This is the noisiest phase of the entire project — typically one to two days. Broken concrete is carried out and hauled away. The exposed sub-base gravel (or soil, in older homes) is revealed and assessed. If the engineer specified soil testing, samples are taken at this stage.
Layout and Marking — Day 2
The engineer's section sequence is transferred to the perimeter walls with spray paint or chalk. Toronto building code and standard practice require that no more than a third of the perimeter is open at any one time — the alternating-section pattern prevents over-exposing the footing and keeps the building stable. Each section is typically 3 to 5 feet wide; the exact width is specified in the engineer's drawings.
Section Excavation — Repeating Daily Cycle
Workers hand-dig (or use a small machine excavator for larger sections) the soil beneath the existing footing in one section at a time, following the sequence. The existing footing is undercut to the target depth — the amount of headroom gain is determined by the engineered depth. Soil is moved out in buckets, wheelbarrows, or by conveyor, depending on site access. Each section is cleaned to the design dimension, squared up, and inspected before forming.
Forming and Pouring Each Section
Once excavated and cleaned, the section is formed with plywood or steel forms to contain the concrete pour. If the engineer's drawings specify reinforcement (rebar), it is placed before pouring. Concrete — typically 25 MPa or 30 MPa mix as specified — is poured and rodded to eliminate voids. The City of Toronto inspector may perform an interim inspection at this stage before concrete is placed in critical sections. After the pour, the concrete must reach adequate strength before the adjacent section is started.
Curing and Progression
Concrete needs time to cure. In warm weather (above 10°C), sections gain adequate strength quickly and the crew progresses to the next section within 24 to 48 hours. In cold weather, insulated blankets or heated enclosures may be required to prevent freezing during curing — this adds time. The sequence continues around the perimeter until all sections are poured and cured. This phase typically runs 3 to 5 weeks depending on home size and weather.
Interior Soil Excavation
With the perimeter footings at the new depth and fully cured, workers excavate the interior soil to the target depth. This is the bulk removal phase — large volumes of soil are removed to bring the entire floor area to the new elevation. Interior excavation is typically faster than the perimeter footing work because it does not need the same alternating sequence; it proceeds from one end to the other once the footings are stable.
Drainage, Gravel, and Vapour Barrier
After interior excavation, the floor is prepared for the new slab: a gravel sub-base (typically 100mm of compacted clear stone) is placed and levelled, a polyethylene vapour barrier is laid to manage moisture, and if the project includes drainage tile, weeping pipe is installed in this phase. Any waterproofing components (dimple mat on walls, sump pit crock) are installed before the slab is poured.
New Slab Pour
The new reinforced concrete floor slab is poured across the entire floor area. The slab is typically 4 inches thick, reinforced with welded wire mesh or rebar as specified, and finished smooth. The slab must cure a minimum of 7 days before foot traffic and 28 days before finishing work (framing, drywall, flooring) can begin.
Final Inspection and Permit Close
BaseWorks schedules the final City of Toronto building inspection. The inspector reviews the completed work — footings, drainage, slab — against the stamped engineer drawings. A pass closes the permit. You receive copies of the inspection record. The basement is now ready for finishing work.
What to Expect as a Homeowner
Phases 2 through 4 are the most disruptive: jackhammers, dust, and crews moving soil through your home. BaseWorks keeps dust barriers in place and cleans the access route at the end of each workday. During slab demolition (days 1–2) and interior soil removal, noise is significant — typically 8 AM to 5 PM weekdays per Toronto bylaws.
During the curing phases (phases 6 and 8), the crew may not be present every day. This is normal — concrete curing is passive and does not require on-site presence. Inspections may occur with short notice; BaseWorks coordinates these directly with the City and will notify you when an inspector is coming.
Utilities (water, electrical, gas) are typically not interrupted during underpinning unless there are basement-level connections that need to be moved for the new floor elevation. BaseWorks identifies these during the pre-construction assessment and plans around them.
Frequently Asked Questions
Underpinning is the method; basement lowering describes the outcome. Underpinning installs deeper mass concrete footings to support the foundation walls while soil is removed. The result is a lower floor — i.e., the basement is "lowered." The two terms are often used interchangeably in Toronto.
Significant dust and debris during active excavation phases. Workers seal interior doorways and use dust barriers, but the basement itself is a construction site. Soil is carried out by hand or through windows and exterior access. Most homeowners clear the basement entirely before work begins.
Yes — there is no structural reason to vacate. Most families stay home during the project but avoid the basement during active work hours. The noisiest phase is demolition of the existing concrete slab, typically one to two days. Excavation involves jackhammers and compactors but is confined to the basement level.
After permits are posted, workers begin with site setup: installing ventilation, dust barriers, temporary lighting, and access routes for soil removal. The existing concrete slab is then broken and removed. Only then does underpinning excavation begin, working in alternating sections.
A City of Toronto building inspector performs interim inspections throughout the project — typically before concrete is poured in each section and at the final completion stage. BaseWorks schedules and coordinates all inspections. The permit is not closed until the inspector signs off on the completed work.
Not when done properly. The alternating-section method ensures that no more than a third of the perimeter is open at any one time. Each section is poured and cured before the next is touched, so the building always has structural support. Cracks in first-floor finishes are uncommon and usually cosmetic if they occur.