Underpinning Cost in Markham

Structural engineer designs the underpinning sequence, specifies concrete strength (25-32 MPa), rebar (#15M at 400 mm on center is typical), and footing width (depends on soil bearing capacity — Toronto clay at 75 kPa needs wider footings than bedrock at 500+ kPa). Drawings go to the city for a building permit. Engineer will specify geotechnical testing if soil conditions are unknown.

Install temporary support beams inside the basement to carry floor loads while you remove foundation sections. If you're lowering the floor, this also involves demolishing the existing slab, relocating mechanical systems, and capping plumbing that's embedded in the concrete. HVAC ducts, electrical panels, and gas lines often need temporary relocation.

Dig every other 4-ft section around the perimeter, going down to the new footing depth (usually 7-8 feet below grade for basement lowering). Hand-dig the last 12 inches to avoid disturbing the soil bearing surface. Each hole gets formed, rebar tied, and poured with 25-32 MPa concrete. You MUST let these cure for 7 days before digging the adjacent sections — concrete only reaches 75% of design strength at 7 days, but that's enough to carry the building load.

After phase 1 sections hit 7-day strength, excavate and pour the skipped sections. Now the entire perimeter has new footings at the target depth. The old foundation sits on top of the new concrete. If you're adding height, you'll pour a new wall section to connect the old foundation down to the new footing.

Excavate the interior to the new floor elevation (typically 6-8 inches below the bottom of the new footing). Install weeping tile around the interior perimeter, sloped to a sump pit. Lay 4-6 inches of clear gravel, then poly vapor barrier, then pour a 4-inch reinforced slab. This is also when you pour the new concrete walls if you're adding basement height.

Since you've excavated the exterior, this is the ideal time to install a rubberized waterproofing membrane and new weeping tile on the outside. Backfill with gravel near the foundation (for drainage), then clay fill near grade (to slope water away). Many homeowners skip this step to save money and regret it 5 years later when water seeps through the foundation.

Building inspector reviews the footing excavations (before you pour), rebar installation, and final slab. Engineer does site visits at key stages. Once approved, you can finish the basement — framing, insulation, drywall, flooring.