A slab-on-grade home presents the most straightforward radon mitigation scenario — and also the most varied, because slab construction covers everything from a simple single-story ranch to a multi-story home with no basement at all. Understanding what the installation looks like specifically for your foundation type removes the guesswork before a mitigator ever arrives.
What Is a Slab-on-Grade Foundation?
A slab-on-grade foundation is a concrete slab poured directly on the ground, with no basement or crawl space below. The home’s floor is the slab itself, or a flooring material installed directly over it. Common configurations:
- Simple slab: Single concrete pour, entire home footprint
- Monolithic slab: Slab and foundation wall poured as one continuous unit
- Post-tension slab: Slab reinforced with tensioned steel cables — requires special drilling protocols
- Slab with interior footings: Load-bearing interior columns or walls with separate footings that may interrupt sub-slab communication
Radon enters slab homes through control joints, expansion joints, floor penetrations (plumbing, conduit), the floor-wall joint perimeter, and any cracks that develop over time. The mitigation approach targets the pressure differential: create enough negative pressure under the slab to prevent soil gases from finding their way through these pathways.
Why Slab Homes Are Sometimes Harder to Mitigate
Basement homes have a natural advantage: the mitigator has significant slab surface area to work with, usually good sub-slab aggregate, and easy interior access for pipe routing. Slab homes can be more challenging for several reasons:
- Sub-slab fill quality varies enormously. Older slab homes, particularly those built before 1975, may have been poured directly on compacted clay or sandy soil with no gravel layer. Poor aggregate dramatically reduces the suction field radius from one entry point.
- Interior pipe routing is constrained. With no basement, the pipe must route through finished interior walls or up through a garage, exterior closet, or utility room — or take the exterior route along the outside of the home.
- Post-tension slabs require specialized drilling. Hitting a tensioned cable is a structural emergency. Any mitigator working on a post-tension slab must locate cables (via GPR or plans) before drilling.
The Diagnostic Phase for Slab Homes
Before any drilling, the mitigator performs a sub-slab diagnostic test to assess aggregate communication. This is more critical in slab homes than in basements because the consequences of a wrong assumption are more expensive to correct (additional core holes in finished flooring).
What the Mitigator Looks For
- Garage access: Most slab homes have an attached garage. The garage slab is often the preferred drilling location — unfinished, easy interior pipe routing, lower finish consequence if a second hole is needed
- Utility closet: Indoor mechanical room or HVAC closet usually offers a direct path to the attic
- Floor plan layout: Central location for maximum suction field coverage
- Post-tension identification: Builder records, sticker on electrical panel, or visual inspection for the cable-end “pockets” visible on the slab edge
Sub-Slab Communication Test
A 2″ diagnostic hole is drilled in the proposed suction point location. A shop vacuum is applied and the mitigator measures airflow at locations across the slab — near walls, at the opposite end of the home, in adjacent rooms. Good communication: airflow is detectable 20+ feet from the suction point. Poor communication: minimal airflow beyond 5–10 feet.
For a 2,000 sq ft slab home with good aggregate, a single suction point is typically sufficient. For a home with clay or sand fill, two or three points may be needed — each requiring its own core hole, pipe run, and connection.
Standard Slab Installation: Single Suction Point
Garage Entry Point (Most Common)
The garage offers the cleanest installation pathway in most slab homes:
- Core hole drilled through the garage slab (3.5″–4″ diameter)
- Riser pipe runs up the garage interior wall
- Fan is mounted in the garage attic or on the garage exterior wall
- Discharge pipe terminates above the garage roofline
- Pipe is painted to match the exterior where visible
This routing keeps all equipment in the garage and minimizes penetration into finished living space. The garage slab is often contiguous with the home slab (monolithic pour) or connected through communication gaps at the step-down between garage and home interior — both create adequate sub-slab connection.
Interior Utility Room Entry Point
For slab homes without garages, or when the garage slab is post-tensioned or isolated from the main slab, the mitigator identifies an interior utility closet or hallway with access to the attic above:
- Core hole drilled in the utility closet or hallway floor
- Pipe runs through the wall cavity from the closet up to the attic
- Fan mounts in the attic above
- Discharge runs out through the gable end or roof
This is the most aesthetically hidden installation — the pipe disappears into the wall and the only visible components are the manometer at the base of the riser and the labeled pipe section in the closet.
Exterior Routing for Slab Homes
When interior routing is impractical — fully finished walls, no accessible attic, or complex multi-zone slab with framing complications — the pipe runs on the exterior of the home. This is common in slab homes in warmer climates where garages are detached and interior mechanical rooms are uncommon.
- Core hole near an exterior wall (inside a closet or laundry room adjacent to an exterior wall)
- Pipe penetrates through the exterior wall, typically near the bottom of the wall framing
- Pipe runs up the exterior wall, typically in a conduit or with a protective sleeve for aesthetics
- Fan mounts on the exterior wall at an accessible height (not ground level — fan needs airflow around it)
- Discharge pipe continues up the wall above the eave line
Exterior installations cost less in labor (no interior routing work) but require additional time for cosmetic finishing — painting the pipe to match the exterior, sealing the wall penetration with weatherproof materials. In regions with freeze-thaw cycles, fan life is slightly shorter than attic-mounted installations.
Post-Tension Slab Protocol
Post-tension slabs require mandatory pre-drill cable location. Options:
- Ground-penetrating radar (GPR): Most accurate. A GPR technician maps cable positions before drilling. Cost: $150–$400 for residential.
- Original construction documents: Some builders keep PT cable layout plans. Available through the building department or original builder if the home is not too old.
- Cable anchor locations: PT cables are anchored at the slab perimeter — the visible “pockets” at the slab edge show cable spacing (typically 18″–24″ on center) and can indicate probable cable locations in the interior.
Core holes in post-tension slabs must be located in the center of the space between cables — never within 6 inches of a cable. Mitigators without GPR experience or access to PT plans should not drill post-tension slabs. The consequences of a severed PT cable include immediate structural failure and require emergency engineering repair.
Sealing Strategy for Slab Homes
Sealing is critical for slab homes because the entire slab is the radon entry surface. After the suction point is installed, the mitigator identifies and seals all secondary entry pathways:
- Control joints and saw cuts: These run in a grid across the slab and are common radon entry points. Polyurethane backer rod + caulk is the correct treatment.
- Floor-wall perimeter joint: The gap between slab edge and drywall/baseboards, if accessible, should be sealed with polyurethane caulk.
- Plumbing penetrations: Every pipe through the slab (toilet flanges, drain pipes, conduit) should have the annular gap sealed with hydraulic cement.
- Visible cracks: Fill with low-viscosity polyurethane or epoxy injection for structural cracks.
In slab homes, sealing effort often has a proportionally larger impact on results than fan size, because the slab surface area available for uncontrolled entry is larger relative to the sub-slab cavity volume.
Expected Results for Slab Homes
Slab homes with good aggregate communication and thorough sealing achieve results consistent with basement homes: 85–95% reduction in radon levels is the typical outcome for a properly installed single-suction-point system. A home testing at 12.0 pCi/L pre-mitigation typically achieves 0.8–1.5 pCi/L post-mitigation.
Homes with poor aggregate (clay, sand fill) may require two or three suction points to achieve the same reduction. Each additional suction point adds $150–$400 to the installation cost but can be the difference between achieving 2.0 pCi/L and remaining at 5.0 pCi/L.
Frequently Asked Questions
How is radon mitigation different for a slab home versus a basement?
The fundamental method (Active Sub-Slab Depressurization) is identical. The differences are in access and routing: slab homes have no exposed basement slab to drill from, so the entry point must be in the garage, a utility closet, or an interior floor — and interior pipe routing to the attic is more constrained than in a basement with open ceiling.
How many suction points does a slab home need?
Most slab homes with standard gravel aggregate need one suction point. Homes with clay or sand sub-slab fill, or large footprints over 3,000 sq ft, may need two or three. The diagnostic test performed before drilling determines this — do not agree to a multi-point system without seeing the diagnostic results that justify it.
Can radon mitigation be installed in a post-tension slab home?
Yes, but it requires mandatory pre-drill cable location using ground-penetrating radar or original construction documents. An experienced mitigator familiar with post-tension protocols can safely install a system by drilling in the spaces between cables. This is not a job for a mitigator without specific PT slab experience.
What does radon mitigation look like from inside a slab home?
The most visible components are: (1) a 3″ white PVC pipe rising from the floor in a utility closet, garage, or along an exterior wall; (2) a U-tube manometer (liquid-filled gauge) mounted on the pipe; and (3) a labeled warning sticker. The fan is in the attic or on the exterior wall — not visible inside the home.
Does the pipe have to go through the roof on a slab home?
No. Discharge can exit through the gable end of the attic (preferred — avoids roof penetration) or through the roof via a standard plumbing pipe boot flashing. Exterior routing exits through an exterior wall below the roofline and the discharge pipe runs up to above-eave height along the exterior.
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