OBC Radon Requirements Ontario: What New Home Builders Need to Know

What Is Radon, and Why Is It in the Building Code?

Radon is a naturally occurring radioactive gas that comes from the breakdown of uranium in soil, rock, and groundwater. That sounds dramatic, but the important part for homeowners is simple: radon can move through the ground and enter a building through cracks, gaps, sump pits, slab openings, pipe penetrations, crawl spaces, and poorly sealed foundation details.

You cannot smell it, see it, taste it, or wave your hand around and say, “Yes, definitely a bit radon-y in here today.” The only reliable way to know the radon level in a home is to test for it.

Health Canada’s indoor radon guideline is 200 becquerels per cubic metre, usually written as 200 Bq/m³. If a long-term test shows the average annual radon concentration is above that level in a normal occupancy area, Health Canada recommends taking corrective action.

What Do the OBC Radon Requirements Actually Require?

For Ontario homeowners, builders, designers, and owner-builders, the most important thing to understand is that the radon requirement has two sides: soil-gas control and radon rough-in preparation.

Soil-gas control is the basic building envelope work that helps stop gases from leaking into the home from below. This includes the air barrier or soil-gas barrier under slabs, sealed laps, sealed penetrations, sealed sump pits where applicable, and proper continuity where the slab meets the wall.

The radon rough-in is the prepared pathway that allows a future active subfloor depressurization system to be connected if the home later tests high. A typical rough-in includes a pipe connection point below the slab or within the granular layer, an airtight cap, and labeling so nobody confuses it with a mystery pipe and cuts it off later because it was “in the way.”

Many Ontario municipalities now expect permit drawings to show radon rough-in details. That is not a bad thing. It is much easier for the designer, builder, inspector, and homeowner when the plan clearly shows what is happening before excavation, underslab plumbing, stone, insulation, poly, and concrete all start happening at the same time.

Where Do the OBC Radon Rules Apply?

This is where people get mixed up. Ontario’s Building Code has long had specific “designated areas” under Article 9.1.1.7 related to radon concentration limits. Those areas are generally understood as Elliot Lake, the Township of Faraday, and the geographic Township of Hyman.

However, the newer OBC soil-gas and radon rough-in requirements are not something you should ignore just because your project is not in one of those three named areas. The 2024 OBC approach has expanded the practical expectations for new Part 9 residential construction, especially around rough-ins for subfloor depressurization and protection from soil gas ingress.

The safest working assumption for a new house, garden suite, addition with a new foundation, or other Part 9 residential project is this: your designer should address radon and soil-gas control on the permit drawings, and your builder should know how it will be installed before the slab is poured.

Local building departments can have their own drawing checklist and inspection preferences. That is why it is always smart to review the requirements with the municipality early, especially if the project has a basement, crawl space, slab-on-grade living space, garden suite, or unusual foundation condition.

The Difference Between a Radon Rough-In and a Full Radon Mitigation System

A radon rough-in in Ontario is preparation. A full mitigation system is the finished active system that actually depressurizes the soil below the slab and vents radon safely away from the building.

The rough-in usually includes the under-slab collection point and capped pipe location. If post-occupancy testing shows high radon, a certified radon professional can convert that rough-in into an active system by adding the fan, exhaust routing, electrical connection, and proper termination.

This is similar to roughing in plumbing for a future basement bathroom. You still do not have a finished bathroom. You have made the future work less destructive. Nobody enjoys cutting finished concrete, opening ceilings, chasing mystery pipes, or explaining to the homeowner why the “small fix” now involves dust, noise, and a bill that feels like it ate lunch at an airport.

What Should Be Shown on Permit Drawings?

If you are submitting drawings for a new home or addition in Ontario, the radon detail should not be treated as an afterthought. Building departments increasingly expect to see how the rough-in and soil-gas control will be constructed.

A good permit set should show enough information that the builder, inspector, and homeowner understand the basic system. The drawings do not need to turn into a 400-page engineering textbook, but they should clearly show what is under the slab, where the pipe is located, how the barrier is sealed, and how the rough-in is labelled.

If you are already preparing a full permit package, this should be coordinated with the architectural drawings, foundation details, mechanical layout, and any required energy or HVAC documents. For homeowners trying to understand the broader permit process, our guide on how to obtain a building permit in Ontario is a good companion article.

What Inspectors Are Usually Looking For

Inspectors are not trying to make your life miserable. Well, not usually. They are trying to verify that the system shown on the drawings is actually being built before it disappears under concrete.

The most important inspection moment is before the slab is poured. Once concrete is placed, it is very difficult to confirm whether the barrier was continuous, whether penetrations were sealed, whether the granular layer was correct, or whether the rough-in was properly located.

In real construction, the problem is usually not one giant mistake. It is twenty little gaps that nobody owns. The plumber cuts the poly. The electrician pokes through it. The labourer folds it back. Somebody steps on it. The sump lid gets treated like an optional suggestion. By the time the slab is poured, everyone assumes someone else sealed it. That is how buildings get into trouble.

Radon and Basements: Why Ontario Homes Need Special Attention

Ontario homes often have basements, and basements are naturally where radon concerns show up. They are below grade, surrounded by soil, and affected by pressure differences between the home and the ground. When the house is heated, exhausted, and mechanically ventilated, it can create pressure conditions that draw soil gas inward.

This does not mean every basement has high radon. It means every basement deserves proper construction and testing. A finished basement with bedrooms, an office, gym, rec room, or basement apartment is a normal occupancy area where people may spend many hours. That is exactly the kind of space where radon should not be ignored.

This is also one reason we like building high-performance foundations properly from the start. A well-built foundation is not just concrete and insulation. It is drainage, waterproofing, air sealing, mechanical planning, slab detailing, and future serviceability. For related foundation planning, see our article on ICF foundation cost in Ontario.

Does ICF Construction Help With Radon?

ICF construction can help with overall foundation performance because the walls are solid reinforced concrete with continuous insulation. Properly built ICF foundations can be very airtight, durable, and thermally stable. That said, radon does not care what brand of block you used or how nice the basement feels on your feet.

Radon usually enters through openings, cracks, penetrations, sumps, and poorly sealed slab details. So even with an ICF foundation, the slab and under-slab detailing still matter. The best wall system in the world cannot make up for a sloppy hole around a pipe.

If you are planning an ICF home, the radon rough-in should be coordinated with the foundation, underslab plumbing, radiant heating if used, sump system, and mechanical layout. On full custom homes, this is why we prefer to think through the whole assembly instead of treating each trade like a separate island. You can learn more about high-performance ICF construction at ICFhome.ca’s custom ICF home construction page and about ICF foundation work at ICFPro.ca.

Radon Testing After Occupancy: The Part People Forget

The OBC rough-in is only part of the story. The real answer comes after the home is occupied and tested. Health Canada and C-NRPP recommend long-term testing for at least three months, preferably during the heating season, because radon levels can change from day to day and week to week.

A short-term test can be useful as a quick snapshot, but it should not be treated as the final word. A home can have a low reading one week and a higher long-term average over the season. That is why the long-term test matters.

Testing should be done in the lowest lived-in level of the home, such as a finished basement family room, office, bedroom, or apartment. An unfinished storage room that nobody uses is not the same thing as a basement bedroom where someone sleeps every night.

What Happens If the Home Tests High?

If a long-term radon test comes back above the Health Canada guideline of 200 Bq/m³, the next step is mitigation. In most homes, the most common approach is active sub-slab depressurization. That means a fan is used to draw soil gas from below the slab and exhaust it safely outdoors before it can enter the house.

If the home already has a properly installed radon rough-in, the job is usually easier. The radon professional has a place to connect. The system can often be completed with less cutting, less guessing, and less disruption.

If the home has no rough-in, mitigation can still be done, but it may involve coring the slab, routing piping through finished spaces, finding a suitable exhaust path, and dealing with existing finishes. That is where the “we saved a few dollars during construction” decision can become a “why is there plastic over my furniture?” renovation moment.

Common Mistakes With OBC Radon Rough-Ins

The radon requirement is not difficult, but it does require coordination. The most common mistakes are simple ones, and simple mistakes are the ones that happen when everyone is rushing to pour concrete before lunch.

How This Fits Into the Bigger Permit Package

Radon is just one part of a complete Ontario permit package. A good set of drawings may also need site planning, foundation details, structural notes, energy compliance, HVAC design, ventilation, septic information, grading, and municipal forms.

If the project is a new custom home, this is where coordination matters. Radon rough-ins can conflict with underslab drains, plumbing, insulation, radiant floor heating, sump pits, footings, thickened slabs, and mechanical room layouts. It is much easier to solve those conflicts on paper than while the concrete truck is idling in the driveway.

If you are planning a new home, it is also worth reviewing Ontario Building Code changes for 2025, heat loss calculations for a new home, and septic system requirements in Ontario if you are building outside municipal services.

For high-performance custom homes, related building systems such as radiant floor heating, Nudura ICF installation, and whole-home envelope planning should be considered early. Radon is not a separate little checkbox. It is part of the building envelope and mechanical planning conversation.

Practical Radon Rough-In Checklist for Ontario Permit Sets

Here is a practical checklist to discuss with your designer, builder, or building department before the permit set is submitted and definitely before the slab is poured.