Radon Rough-In Under a Slab: Pipe Routing, Sealing, and Must-Have Details

The sub-slab rough-in is meant to provide a ready path for a future radon mitigation system. It also reduces disruption later by giving you a clean, planned interface for the stack and vents. This approach aligns with best practices for homes with slab-on-grade or crawlspace transitions.

You should plan this during a new slab pour or a major remodel, when retrofit potential is feasible and when local guidance supports it. Design essentials cover pipe size, intake and exhaust lines, exterior or attic routing, cleanouts, and how the rough-in will interface with future foundation details. Consider corridor routes that avoid footings, rebar, and utilities, and think about sleeves, depth, and frost protection. During the pour, sleeves and seals should integrate to prevent concrete infiltration while preserving airtight connections for future connections. Finally, document the as-built layout and note inspection requirements so future contractors can pick up where you left off or perform testing after the full system is installed.

When to rough-in (new builds vs retrofit)

A sub-slab rough-in is most cost-effective during new construction or major slab work. Here’s why:

New Construction: It’s like building a highway before you need it. You’re pouring the slab anyway, so adding pipes now saves digging later.

Major Remodel/Retrofit: If you’re already tearing up the slab, it’s wise to add pipes then. Retrofitting after the fact can be messy and expensive.

Benefits and limits of a rough-in vs full mitigation

A sub-slab rough-in is like preparing your home for a future guest (radon system). Here’s what it does and doesn’t do:

Benefits: It creates passive pathways for future radon mitigation. No digging, less disruption, and you’re ready when you need it.

Limits: A rough-in alone won’t mitigate radon. It’s a preparation step. For active reduction, you’ll still need to install fans, seals, and other components later.

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Start by defining the scope, applicable codes, and permit needs. Identify required inspections and the documentation you’ll collect, such as plans and material specs. This keeps the project aligned with local expectations from the outset.

Next, prep the site and layout, examining slab type, sub-slab conditions, and any obstructions. Decide on the collector location, vent pathway, and accessible cleanouts, while planning for moisture barriers and cleanable access. Choose pipe diameter and material, along with seals, grommets, gaskets, and dampers, and document airtight transition details and backflow prevention. Map routes under the slab with minimal bends, plan vertical risers to the attic or exterior, and coordinate penetrations with utilities. Finally, establish pre-pour and post-pour sealing methods, determine an air-tightness test approach, and outline inspection milestones so inspectors can verify the rough-in at key points.

Sub-slab collector layout and preparation

Locating and preparing the gravel/collector zone is crucial for effective radon mitigation. Here’s how:

Gravel bed: Excavate a 4-6 inch deep layer of clean, coarse gravel under the slab area. This provides a pathway for radon gas to move towards the collector.

Collector layout: Position the collector in the center or near the middle of the gravel bed. Ensure it’s large enough (at least 16 inches in diameter) and deep enough (at least 4 inches below the slab) to capture sufficient radon gas.

Suction points: Place suction points around the collector, ensuring they’re at least 2 feet away from any walls or obstructions. This helps maximize sub-slab communication and prevents short-circuiting of the system.

Installing the rough-in pipe and through-slab routing

Properly positioning the riser and routing the pipe is vital for a successful rough-in. Here’s how:

Riser position: Place the riser vertically at the marked location, ensuring it’s centered and plumb. This will allow for easy connection to the fan later on.

Create core holes: Use a core drill bit to create clean, circular holes through the slab at the marked locations. Keep them dry to prevent clogging with concrete residue.

Seating the pipe: Insert the pipe through the slab, ensuring it’s properly seated and doesn’t wobble. Use grommets or other sealing materials as needed to create an airtight seal.

Temporary termination: Above the slab, temporarily cap the pipe with a suitable cap. Ensure the cap is secure but can be easily removed when installing the fan later on.

Temporary capping, labeling and leaving access for future fan

Properly capping and labeling your rough-in pipe ensures easy identification and access when installing the fan later on.

Capping: Use a temporary cap that’s secure but can be easily removed. Ensure it creates an airtight seal to prevent any backflow of gas.

Labeling: Clearly label the pipe above the slab, including the date of installation and any relevant notes. This helps future contractors understand your system’s history.

Access: Ensure there’s enough space around the capped pipe for easy access when installing the fan later on. Keep this area clear of obstructions to facilitate future maintenance or diagnostics.

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Outline the main routing choices, such as toward exterior walls, into an attic chase, or to a dedicated stack, and weigh trade-offs based on slab type and home layout. Use simple visual cues or diagrams to aid decision-making. Consider the comfort of future access when choosing a pattern.

For sizing and materials, plan to use common options like PVC or CPVC, and keep diameter guidance practical for your layout. Verify that fittings, solvents, and sealing methods are compatible with your chosen pipe. Plan penetrations through slabs and walls with appropriately sized sleeves, and coordinate fire stopping, grout, and insulation to minimize heat loss and condensation. Ensure slopes support gravity-driven venting and maintain cleanouts and vent terminations within code limits. Avoid long, exposed exterior runs and sharp bends by securing proper supports and planning for future upgrades as needed.

Routing layouts: single suction, multiple suction and manifold options

When routing your radon pipe under the slab, you’ve got a few layout choices. Each has its pros and cons.

Single Suction Point: This is simplest for small slabs. One collector covers the whole area. But if soil communication’s poor, it might not be enough.

Multiple Collectors: For larger slabs or poor soil communication, multiple collectors can do the job. Each has its own pipe run to the stack. More complex but better coverage.

Manifold System: This is for big slabs with varied soil conditions. Pipes from several collectors meet at a manifold before going to the stack. It’s the most complex but offers best coverage.

Pipe sizing, joints, slope and fitting best practices

Size your pipe right for smooth airflow. Most use 3″ or 4″ PVC Schedule 40/80, ABS, or CPVC.

Glue joints with the right solvent. Keep it airtight. Slope pipes consistently at least 1/4″ per foot away from the slab to ensure condensate drains out.

Use fittings sparingly. Too many can restrict airflow. Keep angles gentle, no sharp 90-degree bends.

Secure pipe with straps or hangers every 3-4 feet. No sagging, no stress on joints.

Penetrations through walls, rim joists and floors

Make clean, airtight penetrations. Use a core bit to drill through the slab, wall, or floor.

Insert a sleeve for pipe protection. Seal around it with fire-stop caulk or foam. This stops air leaks and prevents heat loss.

At rim joists, use expandable foam sealant. It expands to fill gaps, keeping your penetration airtight. Insulate the pipe to prevent condensation.

Always check local code for specific penetration requirements. Safety first.

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Explain how sealing and membranes reduce leakage paths under the slab and how they interact with the routing. Prioritize air gaps, seam integrity, and long-term durability during rough-in. Think ahead to future connections and serviceability.

Choose membrane types that are gas-impermeable and compatible with concrete and moisture. Document thickness, permeability, and exposure considerations for UV and moisture. Detail proper placement beneath the slab, at edges, and around penetrations, with overlaps and taped seams that wrap around pipes. Include transitions to walls, piers, and footing pours, and ensure movable joints are accommodated. Use elastic, non-shrinking sealants and appropriate gaskets where pipes and wires pass through, and verify your system remains accessible for maintenance. Plan field checks and records to confirm seam integrity before the pour.

Sealing around pipes, control joints and perimeter gaps

Proper sealing at pipe origins, construction joints, and perimeters is crucial to prevent radon gas leaks. Use gas-tight sealants designed for concrete and compatible with your mit system.

Around pipes: Apply sealant generously on the pipe before insertion into the slab. Ensure it’s covered by at least 2 inches of concrete. Use mechanical seals or gaskets where necessary, especially around larger pipes.

At control joints: Seal them with a flexible, non-shrinking sealant to accommodate movement and prevent gas leaks. Apply in a continuous bead along the joint before concreting.

At perimeters: Seal the edge of the slab using a similar process as control joints. Ensure the sealant extends at least 2 inches into the concrete.

Sub-slab membranes and vapor barriers: selection & placement

Use gas-impermeable, multilayer membranes as sub-slab vapor barriers. They reduce leakage paths and enhance mitigation performance.

Placement: Lay the membrane continuously beneath the slab, overlapping seams by at least 6 inches. Tape seams with a compatible, gas-tight tape to prevent leaks.

Around penetrations: Ensure membranes wrap around pipes and wires tightly, using gaskets or sealants if necessary. Seal any gaps between the membrane and footings or walls.

Transition to walls/piers: Extend the membrane up the wall at least 6 inches, taping it securely. For piers, wrap them with the membrane before pouring concrete.

Describe how passive rough-ins can be upgraded to active systems, including component choices and control schemes required to meet code and manufacturer specs. Outline typical steps to bring a rough-in to active status without reworking the entire slab. This helps ensure future performance with minimal disruption.

Discuss fan siting and electrical considerations, balancing noise, weather exposure, and service clearances. Decide indoor vs. outdoor locations, ensure GFCI protection where required, and plan for a robust wiring path. Route under the slab with short, direct runs, maintaining airtight joints and proper slope. Include condensate handling, trap seals, and anti-backflow features, while sealing bulkhead penetrations and wall penetrations to keep the system durable in daily use.

SSD system types: passive, passive-stack, and active SSD

Passive systems rely on natural ventilation to draw radon out. They’re simple but less effective in some conditions.

Passive-Stack systems add a vertical pipe to increase stack effect. They’re better than standard passives but still limited by weather conditions.

Active Soil Depressurization (SSD) systems use fans to force radon out. They’re the most effective, especially in areas with high radon levels or challenging site conditions.

Fan selection, location, condensate and freeze concerns

Fans can be indoor, in the attic, or exterior. Indoor fans are quiet but may cause condensation. Attic fans risk freezing and need weather protection. Exterior fans expose the fan to elements but offer better ventilation.

Condensation can form on cold pipes, so insulate if needed. In freeze-prone areas, consider a freeze-resistant or heated fan.

Ensure fans have proper clearances for service and adequate power supply with GFCI protection. Use weatherproof enclosures for outdoor installations.

Monitoring: manometers, alarms and commissioning checks

A manometer measures system pressure. Install it near the fan to monitor performance. Regularly check for proper operation.

Alarms can alert you to system failures. Consider installing one if your system is critical or hard-to-monitor.

During commissioning, check that:

1. The fan runs when the system is activated.
2. The manometer shows adequate negative pressure (usually -1.5 to -4 inches W.C.).
3. The vent terminates as per code (typically 10 feet above the roof or 2 feet above the highest point within 10 feet).

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Outline pre-installation testing to establish a baseline and inform slab design, noting detector placement and duration. Understand which tests fit your situation, and document results for comparison after installation. Coordinate with local codes to ensure your method meets expectations.

Describe post-installation testing to verify there are no bypass paths and to confirm the system performs as intended. Plan follow-up tests after depressurization or sealing, and select detector types suited to your setup. Establish a monitoring cadence that suits your site, and track results to identify transient spikes. Prepare a remediation plan with clear steps if readings stay high, and keep records of maintenance and consultant communications for future reference.

Short-term vs long-term tests and detector placement

Before installing your radon mitigation system, it’s crucial to understand the two types of testing methods – short-term and long-term. Both have their purposes but differ in duration and accuracy.

Short-term tests are quick, taking only a few days to a couple of weeks. They’re great for initial screening but can be affected by temporary conditions like weather or open windows. Use these to get an idea if your home has elevated radon levels.

Long-term tests, on the other hand, take several months to a year. They provide a more accurate representation of your home’s average radon level over time. These are ideal for establishing a baseline before installing your system and should be used to inform your slab design.

When placing detectors, ensure they’re in the lowest lived-in level of your home, at least 20 inches above the floor, and away from exterior walls, doors, and windows. This placement helps capture representative readings of your living spaces.

Post-install commissioning and verification testing

After installing your radon mitigation system, it’s essential to confirm that it’s working correctly. This involves immediate post-build testing and follow-up tests after any soil depressurization or slab penetrations are sealed.

Conduct immediate post-install tests within 24 hours of completion. Use a continuous monitor or a short-term test kit to ensure no radon bypass is occurring. Place the detector near suspected entry points and away from any ventilation systems that could skew results.

After sealing, perform follow-up tests. These should be done after at least 24 hours but no more than 30 days post-sealing. Use a long-term test kit for these follow-ups to ensure sustained reductions in radon levels over time.

If your initial testing shows high levels, don’t panic. It can take time for the system to fully depressurize the soil. Re-test after a few weeks and monitor regularly to ensure your system is working as expected.

Routine maintenance, troubleshooting and fan replacement considerations

Maintaining your radon mitigation system is crucial for ensuring it continues to protect your home. Regular inspections, usually every two years, can help catch any issues early.

Common failure modes include clogged pipes, leaks in the system, or fan malfunction. If you notice radon levels rising again, check for these issues first:

• Ensure all seals around pipe penetrations are intact and well-maintained.
• Check that your fan is running smoothly and isn’t blocked by debris or dirt.
• Inspect the system for any cracks, holes, or gaps where radon could bypass the mitigation efforts.

Fan replacement may be necessary if your current one is old, noisy, or no longer effectively reducing radon levels. When selecting a new fan, consider its airflow capacity, energy efficiency, and noise level. Always follow manufacturer guidelines for installation and maintenance.

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Explain how national and local codes interact with radon retrofit work, including when permits and inspections are required. Note how ongoing maintenance or disclosures may be mandated by code for slab-on-grade homes with radon systems. Use this as a guide to stay compliant from the start.

List core standards and best practices to follow, and align them with recognized guides from regional authorities. Discuss certification programs such as NRPP, NRSB, or CARST, and what each credential covers. Build a contractor-vetting checklist that includes licenses, insurance, references, and a detailed written proposal with scope and milestones. Describe a due-diligence workflow that compares bids, clarifies testing plans, and anticipates potential renovations. Finally, highlight contract clarity and communication pitfalls to avoid, such as vague scope statements or missing permit documentation.

National, provincial/municipal code variations and permits

Radon mitigation work is governed by national and local building codes. It’s crucial to understand these provisions before starting any rough-in work.

National Codes: In the U.S., the International Residential Code (IRC) provides guidelines for radon systems in new construction. For retrofits, follow the International Mechanical Code (IMC).

Local Codes: Check with your local building department to understand any additional requirements or variations from national codes. They can provide specific permit information and inspection schedules.

Address budgeting by outlining typical material costs, labor scenarios, and inspection fees, while noting that actual figures depend on local conditions. Emphasize building in contingencies for unexpected site specifics. Use these points to form a realistic project budget without promising exact numbers.

Cover frequent mistakes to avoid, such as misrouting or pinching the vent line, failing to seal penetrations, or neglecting an airtight envelope under the slab. Discuss frost depth and slope concerns and the importance of cleanouts for future service. Explain how regional climate and soil conditions affect design choices, including durability considerations for seals and routing. Offer a simple approach to rough-in cost modeling that ties region, soil type, and climate to expected material and labor needs. Finish with a checklist to verify clearances, vent path continuity, and documentation for inspectors.

Cost drivers and budgeting guidance

The cost of a radon rough-in under a slab varies based on several factors. Slab access, the number of suction points needed, fan choice, and permitting fees all play a role.

Material costs typically include pipes (around $2-$5 per foot), seals ($0.50-$2 each), couplings ($1-$3 each). Add 10-15% for contingencies like unexpected site conditions.

Labor can range from $40-$80 per hour, depending on whether you DIY or hire a contractor. Permitting and inspection fees vary by location but expect around $200-$500 in total.

To estimate realistically, get multiple quotes, consider all factors, and add a 10-15% contingency for surprises.

Regional soil types, frost, and climate considerations

Soil composition, water table height, freeze-thaw cycles, and local climate all impact radon rough-in design. Here’s what to consider:

Soil type: Clay soils can be impermeable, requiring more suction points. Sandy or gravelly soils allow easier radon movement, needing fewer points.

Frost depth: In cold regions, pipes must be buried deep enough to avoid freezing and potential damage. Slopes should also account for frost heave.

Climate: High moisture areas may need additional seals or drainage considerations. Seismic zones might require extra pipe reinforcement. Always check local building codes for specific requirements.

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