Radon Rough-In Under a Slab: Plan, Seal, and Prepare for Testing

The rough-in preserves a practical route from the soil beneath the slab to a future vent system. Installing it before concrete is placed avoids later drilling, trenching, and guesswork. It also makes it possible to add a fan with less disruption if post-construction testing calls for active mitigation.

When to rough-in: new builds vs retrofit

New construction and major slab replacement are the right times to install a rough-in because the sub-base, membrane, pipe route, and penetrations are visible. After a slab is poured, a professional may need to drill one or more suction holes and diagnose pressure-field extension; do not assume one drilled point will serve every area.

Benefits and limits of a rough-in vs full mitigation

A rough-in is preparatory infrastructure. A complete passive system has a continuous gas-tight path to a compliant discharge location. An active soil-depressurization system also has a correctly selected fan, electrical provision, and performance verification. Test the completed home before deciding whether passive provisions are sufficient.

Prerequisites: Have an approved foundation and slab plan, utility layout, drainage and waterproofing details, and the local inspection sequence. Mark interior footings, grade beams, thickened slab areas, construction joints, sleeves, plumbing, electrical runs, and any area that will be inaccessible after the pour. If these features divide the slab into separate compartments, obtain a collection design rather than relying on a single central pipe.

Tools, materials, and PPE: Use the specified gas-tight pipe and fittings; compatible joining cement or couplings; a temporary cap; the specified collection material or collection network; the specified soil-gas retarder; compatible seam tape, membrane sealant, and penetration collars; approved elastomeric sealant; pipe supports and labels; a tape measure, marker, level, and camera. Wear gloves, eye protection, safety footwear, hearing protection for cutting or drilling, and task-appropriate dust protection. A fan and manometer are not rough-in materials unless the system is being activated now.

Sub-slab collector layout and preparation

Collector geometry is a design decision, not a universal circle or spacing rule. Sub-slab permeability, compacted fill, drainage details, interior footings, grade beams, utility trenches, and separate slab sections determine whether one suction point can communicate across the slab. Poor communication may require multiple points or a collection network. Do not intentionally use control joints as gas pathways unless the approved design calls for it.

Installing the rough-in pipe and through-slab routing

Install the pipe before the slab is poured whenever possible; a preplanned sleeve or riser avoids unnecessary core drilling. Protect the pipe where it crosses the slab and do not leave an unjoined fitting buried in concrete. Route and support it so water cannot collect in a low point or future fan. Do not connect radon piping to sanitary or storm drainage unless an expressly approved design permits it.

Temporary capping, labeling and leaving access for future fan

A future fan connection must not become an open soil-gas route into the home. Leave a continuous gas-tight riser or a deliberately designed connection point, cap it securely, and identify it on the as-built sketch. Plan the fan location, service clearance, and any required electrical work outside conditioned and occupied space. The eventual discharge location must be checked against the applicable code, openings, outdoor-air intakes, roof configuration, snow conditions, and fan instructions.

Choose the riser route before framing closes: an interior chase, garage-side route, or other approved path can preserve access while avoiding structural and utility conflicts. A route that looks short on plan may be a poor choice if it crosses a footing, a fire-rated assembly, or a future finished wall without service access.

Routing layouts: single suction, multiple suction and manifold options

A single suction point may suit a slab with a continuous, demonstrably permeable collection layer. Multiple collection points or a manifold may be needed where footings, grade beams, compacted soil, or separate pours isolate areas. A manifold and any later fan selection should be designed or reviewed by a qualified radon professional.

Pipe sizing, joints, slope and fitting best practices

Use the pipe material, diameter, fittings, and joining method required by the adopted code or project standard. PVC or ABS may be accepted in some jurisdictions; do not assume CPVC, mixed fittings, or solvent cements are interchangeable. For example, the 2015 IRC Appendix F calls for a minimum 3-inch gas-tight ABS, PVC, or equivalent pipe where that appendix is adopted; review the applicable IRC radon-control provisions rather than treating that example as your local rule.

Keep the route direct and support it according to the pipe manufacturer and applicable code. Avoid unnecessary restrictions, sags, and low points. Where condensation or drainage is possible, obtain direction from the applicable radon standard and the selected fan manufacturer; a fixed pipe-slope rule is not universal.

Penetrations through walls, rim joists and floors

Use sleeves or penetration details that protect the pipe and allow the specified seal. Air-seal with a durable product compatible with the pipe, concrete, membrane, and substrate. Where a penetration crosses a fire-rated wall or floor, use the listed firestop system required for that assembly; ordinary spray foam is not automatically an approved firestop. Do not add insulation, grout, or sealant that prevents future access where service access is required. Related guidance: Foamed Concrete for Insulation: Common Failures, Protection Details, and When to Avoid It.

Sealing does not replace a properly designed collection path, but it limits soil-gas entry routes and helps an active system work efficiently. Use only products that are compatible with the membrane, concrete, pipe, and intended exposure. Follow the product instructions for substrate preparation, joint configuration, backing material, cure conditions, and repair.

Sealing around pipes, control joints and perimeter gaps

Seal the slab-to-wall interface, construction and isolation joints, pipe penetrations, utility sleeves, and sump covers as specified. Use a flexible, durable sealant where movement is expected. Do not state a fixed sealant depth or concrete cover unless it comes from the approved detail or product instructions. Recheck seals after concrete placement and after other trades add penetrations.

Sub-slab membranes and vapor barriers: selection & placement

Do not assume any plastic sheet is a radon barrier. Use the soil-gas retarder specified by the adopted code, project design, or program requirement. Confirm the material, thickness or performance designation, lap method, seam tape or sealant, puncture-repair method, and compatibility with insulation, drainage, termite treatment, and waterproofing. Install it continuously where specified and detail it carefully at walls, columns, footings, and penetrations.

This work is outside a basic rough-in, but the rough-in should make activation possible. An active system uses a fan to draw soil gas through the collection path and discharge it at an approved exterior