Sub-Slab Depressurization Explained for Toronto Homes

# Sub-Slab Depressurization Explained for Toronto Homes

Sub-slab depressurization (SSD) is the workhorse of Canadian residential radon mitigation. Roughly 90% of Toronto mitigations use it, and for good reason — it is mature technology, predictable to design, and routinely cuts radon by 80-99% in a single intervention. This post explains how SSD actually works in a Toronto home, what the design decisions are, and what a homeowner should look at when reading a C-NRPP-certified mitigation proposal.

The Physics in One Paragraph

Radon is a heavy noble gas produced by the decay of uranium in soil. It seeps into homes wherever the pressure inside the home is lower than the pressure in the soil beneath the slab — through cracks, slab seams, the perimeter where the slab meets the foundation wall, sump pits, plumbing penetrations, and the porous concrete itself. Houses are routinely depressurized by their own stack effect (warm air rising) and by mechanical ventilation (bath fans, dryer vents, furnace combustion). SSD reverses the pressure relationship under the slab: by pulling air out from beneath the slab and exhausting it outside, the under-slab pressure drops below interior pressure, and soil-gas radon is intercepted before it enters the home.

It is, in essence, a chimney for the soil under your house.

The Components of an SSD System

Five physical components and one diagnostic.

1. The Suction Point

A 4-inch hole is core-drilled through the basement slab, typically near a corner or at an existing sump pit. A short cavity is excavated into the gravel sub-base beneath the slab to allow air movement. A 4-inch PVC pipe is sealed into the slab penetration with concrete or polyurethane, and the pipe extends a few inches into the gravel cavity.

Suction-point placement is the single most important design decision. A C-NRPP Mitigation Specialist runs diagnostic suction tests before installation: a small vacuum is applied at a candidate location, and pressure differential is measured at probe points across the slab. If pressure can be propagated across the full slab footprint from one suction point, one is enough. If the pressure does not reach the far corners, a second suction point is needed.

2. The Pipe

4-inch Schedule 40 PVC pipe runs from the suction point vertically up through the home to the attic, and from the attic out through the roof. The Health Canada and C-NRPP protocol specifies:

Discharge above the eave line.
Discharge at least 6 feet horizontally from any window, door, or fresh-air intake.
Continuous slope back to the suction point so condensate drains to the soil cavity.
All joints sealed and structurally supported every 6-8 feet.

3. The Fan

Mounted in the attic or on the exterior of the home, the fan runs continuously 24/7. It must be located outside conditioned space so any leakage at the fan housing exhausts outdoors, never indoors. Common fan sizes for Toronto residential:

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60-90 W for typical detached or semi-detached homes with a single suction point.
150-200 W for larger basements, multi-point systems, or hard-to-treat slabs.
200-300 W for very large or commercial-residential applications.

Lifespan is 10-15 years. Replacement is straightforward and the fan is the only major service item over the system's life.

4. The Roof Penetration

A standard plumbing-vent-style penetration with weather flashing and a discharge cap. The exhaust does not need to extend high above the roof — Health Canada specifies above the eave and away from openings, not above the roof peak.

5. The Manometer

A simple U-tube water-filled pressure gauge installed on the pipe inside the basement. It costs $20 and is visible at a glance. If the fluid columns are unequal, the fan is moving air and the system is working. If the columns equalize, the fan has failed and you need service. A working manometer is the homeowner's daily verification. Many Toronto homeowners overlook the manometer; it is the most informative, cheapest component of the whole system.

6. The Post-Mitigation Test (Diagnostic)

Not a physical component, but essential. A 91+ day long-term radon test is conducted after the system runs, ideally during the next heating season, to verify the system has reduced indoor radon below 200 Bq/m3. Without this test, you have a fan running but no documented proof of effectiveness.

Toronto-Specific Design Considerations

Three factors that shape SSD design in GTA homes specifically.

Foundation Type

Slab-on-grade single-pour basements (common in postwar bungalows in Etobicoke, Scarborough, North York): straightforward single-point SSD almost always works.
Walk-out basements with multiple slab elevations (common in Don Valley ravine-edge homes): often require multi-point systems because pressure does not propagate across slab seams.
Hollow concrete-block foundation walls (common in 1950s-1960s GTA stock): SSD alone may not address radon entering through block-wall cores; consider block-wall depressurization as an add-on.
Crawl spaces (Scarborough postwar, Etobicoke creek-side): require sub-membrane depressurization rather than SSD.

Existing Sump Pits

A pre-existing sealed sump pit can sometimes serve as the suction point, eliminating the slab core hole and reducing install cost. Many Toronto basements with active weeping-tile drainage have suitable sumps. The C-NRPP specialist evaluates whether the sump's drainage role and the radon role are compatible — typically yes, with a sealed lid and appropriate fittings.

Pipe Routing

Toronto homes vary widely in available routing paths. Common choices:

Through a basement utility closet up through the main-floor mechanical chase to the attic: cleanest interior path, minimal aesthetic impact.
Up an exterior wall on the outside of the house: best for finished basements where interior routing would require demo.
Through an interior closet and an open second-floor closet to the attic: middle ground.

The right answer is house-specific. A C-NRPP designer walks the home before quoting.

Common SSD Failure Modes

Three reasons SSD systems sometimes fail to bring a home below 200 Bq/m3:

1. Insufficient suction-point coverage. A single point cannot pressurize a slab with poor sub-base communication. Diagnosed by post-mitigation testing showing under-target results; fixed by adding a second suction point.
2. Block-wall radon entry not addressed. SSD treats the slab pathway; block walls are a parallel pathway. Fixed by adding block-wall depressurization.
3. Fan undersized. A 60 W fan in a home that needed 150 W. Fixed by upsizing.

A reputable C-NRPP firm warrants the result, not just the install. Ask the contractor explicitly: "Will you bring this below 200 Bq/m3 in the post-mitigation test, or do you fix it at no charge?" A confident installer says yes.

SSD vs Alternatives

For most Toronto homes, SSD is the right answer. Other methods exist:

Sub-membrane depressurization for crawl spaces (no slab to work with).
Drain-tile depressurization in homes with active perimeter weeping-tile loops.
Block-wall depressurization as an add-on for hollow-block foundation walls.
Basement pressurization as a second-stage measure when SSD alone cannot reach below 200 Bq/m3.
HRV-based dilution as a complement, not a substitute. See [Radon Resealing Cracks vs Active Mitigation](/blog/radon-resealing-cracks-vs-active-mitigation).

The C-NRPP designer selects the right method for the specific home; trust the diagnostic, not the marketing.

How RenoHouse Coordinates SSD Installation

RenoHouse does not install SSD systems directly — that is C-NRPP-certified specialist work. What we do is coordinate the SSD install with the surrounding renovation: roughing in pipe before basement drywall closes the chase, planning the manometer location to be visible but discreet, and scheduling the post-mitigation re-test once the basement is closed up. For coordination during a basement build-out, see [Radon During Basement Finishing in Toronto](/blog/radon-during-basement-finishing-toronto). For coordination with a slab-rebuild project, see [Basement Underpinning Toronto: 2026 Complete Guide](/blog/basement-underpinning-toronto-2026-complete-guide).