Slab-on-Grade Radon Mitigation

What the problem is

A slab-on-grade home sits directly on the ground with no basement or crawlspace between soil and living space. Radon enters through shrinkage cracks, the perimeter cold joint where slab meets stem wall, and plumbing penetrations. Because there is no buffer level, soil gas that gets through the slab arrives directly in the rooms you occupy.

Slab homes are often assumed to be low risk. They are not. What matters is the soil below and how well suction can be distributed through it.

How to know if you have it

• A 48-hour test on the ground floor came back at or above 4.0 pCi/L.
• Visible slab cracking, especially near the perimeter or at plumbing chases.
• Post-tension or heavily reinforced slabs — these change where holes can be cut, not whether radon enters.
• A garage slab or patio slab poured against the house creating an untreated joint.

How the slab is diagnosed

Sub-slab communication testing is the single most important diagnostic for this foundation type. A test hole is drilled, vacuum applied, and pressure measured at additional points several feet away. Strong communication means one suction point can serve a large area. Poor communication — common where a slab was poured over compacted native soil or caliche rather than clean aggregate — means multiple points or a higher-static-pressure fan.

What the mitigation process looks like

1. 1Suction point locations are chosen based on the communication results and on interior routing options.
2. 2A core hole is cut and soil is excavated below to form a suction pit.
3. 3PVC is run — often through a closet, mechanical chase, or the garage — to a fan mounted in the attic or on an exterior wall.
4. 4The exhaust terminates above the roofline, away from windows and intakes.
5. 5Slab cracks, the perimeter joint, and penetrations are sealed with polyurethane sealant.
6. 6The core hole is patched flush and the manometer is mounted at eye level.

Expected timeline

What affects the cost

• Number of suction points the communication test justifies.
• Interior versus exterior pipe routing and how much finished space is involved.
• Attic access and whether a new electrical circuit is required.
• Amount of crack and joint sealing.
• Fan capacity needed for the soil conditions.

What happens after installation

Verification testing confirms the drop, the manometer gives you an ongoing at-a-glance check, and the sealed cracks stay serviceable for years. If you later add a room, enclose a patio, or cut the slab for plumbing, retest — you have changed the very surface the system depends on.