Sub-Slab Depressurization: How Radon Mitigation Actually Works
A well-built radon system drops the gas in your home by 80 to 99 percent. Most of them use one method to do it. It’s called sub-slab depressurization.
The name sounds technical. The idea is simple. You pull the radon out from under your basement floor before it can get inside.
This guide breaks down how it works, part by part. What the pipe does, and what the fan does.
You’ll also learn why it beats every other fix. And when a handy homeowner can help versus when you want a pro.
The short version:
- Sub-slab depressurization (SSD) is the most common radon fix in Canada and the US.
- It pulls soil gas out from under your slab and vents it above the roof.
- A pipe, a fan, and a sealed pit are the whole system. No filters. No chemicals.
- A good install cuts indoor radon by 80 to 99 percent.
- The fan runs 24/7 and uses about as much power as a light bulb.
- Health Canada and the US EPA have backed this method for over 30 years.
What sub-slab depressurization means in plain English
Radon is a gas. It forms in the soil and rock under your home. Then it seeps up through cracks, the sump pit, and gaps around pipes.
Your basement floor sits on a layer of gravel or soil. That layer holds the gas. When the pressure inside your home is lower than the pressure under the slab, your house acts like a straw. It pulls the gas up and in.
Sub-slab depressurization flips that pressure. “Sub-slab” means under the floor. “Depressurization” means lowering the pressure there.
Instead of letting your house pull the gas in, the system pulls it out. It creates a low-pressure zone under the slab. The soil gas takes the easy path, up the pipe, not into your rooms.
That’s the whole trick. You beat the gas at its own game.
The parts of a radon system
A full system has only a few parts. Each one has a job.
- A suction pit. A small hole cut through the slab, then dug out below it. This opens up the gravel layer so the fan can pull from a wide area.
- A PVC pipe. Sealed into the pit. It runs up through the house and exits above the roofline, or up the outside wall.
- An inline fan. Mounted on the pipe, in the attic or outside. It runs non-stop and creates the suction.
- Sealed cracks and joints. The installer seals big gaps in the slab and around the sump. This helps the fan hold its pull.
- A manometer. A small U-shaped gauge on the pipe. The red liquid shows the fan is working.
That’s it. The pipe and pit can last the life of the house. The fan is the one part that wears out, every 10 to 15 years. We cover that in our radon fan cost guide.
How the system pulls radon out step by step
Here’s what happens once the system is on.
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The fan spins and creates suction in the pipe.
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That suction reaches down into the pit and the gravel under your slab.
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Soil gas, radon and all, gets pulled toward the pit.
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The gas travels up the pipe.
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It vents out above your roof, where it scatters into the open air and does no harm.
The gas never gets a chance to enter your living space. It’s caught under the floor and sent up and out.
A radon system doesn’t clean your air. It stops the gas at the source. There’s no filter to change. The fan just keeps pulling, day and night, for years.
Why vent above the roof? So the gas doesn’t drift back in through a window or a fresh-air intake. Health Canada’s measurement guide sets out where the exit point should go for residential systems.
Why this method beats the alternatives
You’ll see other “radon fixes” online. Most don’t work as well, or at all.
| Method | Does it work? | Why |
|---|---|---|
| Sub-slab depressurization | Yes, 80 to 99% drop | Stops gas at the source, before entry |
| Sealing cracks alone | No, small effect | Gas finds new paths, pressure stays wrong |
| Opening windows | No, not a real fix | Helps for hours, fails in winter, wastes heat |
| Air purifiers / HEPA | No | They catch dust, not a gas from the soil |
| An HRV or air exchanger | Small effect | Dilutes the gas, rarely gets you under the line |
Sealing helps the fan work better, but on its own it barely moves the number. Gas is sneaky. Block one crack and it finds another.
That’s why the pros pair sealing with active suction. The fan does the heavy lifting. The sealing just makes the fan more efficient.
This isn’t new science. The US EPA has recommended active sub-slab systems as the standard fix for decades.
How well does it work?
A good SSD system cuts indoor radon by 80 to 99 percent.
Here’s what that looks like in real numbers. Say your home tested at 600 Bq/m³. That’s three times the Canadian action level of 200 Bq/m³.
After a solid install, that home should land somewhere between 6 and 120 Bq/m³. Most fall well under 100. Health Canada’s goal is “as low as reasonably achievable,” and a working system gets you there.
The only way to prove it worked is a follow-up test. A good installer runs one 30 days after the job. If the number didn’t drop enough, they adjust the fan or add a second suction point.
Want the full breakdown of Ontario pricing and what changes it? See our radon mitigation in Guelph guide. For the Canada-wide view of system cost, install day, and who pays, read our radon mitigation system cost guide.
DIY sub-slab depressurization vs hiring a pro
Can you build a radon system yourself? In Ontario, there’s no law that stops you. But it’s one of the harder home jobs to get right, and the cost of getting it wrong is invisible.
Here’s the honest split.
| DIY install | Hire a pro | |
|---|---|---|
| Cost | $500 to $1,500 in parts | $2,800 to $3,800 |
| Skill needed | Concrete cutting, pipe fitting, fan wiring | None |
| Risk if wrong | Radon stays high, you don’t know | Low, with a verify test |
| Fan sizing | You guess | Measured for your home |
| Tarion warranty claim | Not accepted | Accepted |
What you can safely do yourself
Some parts of radon work are fair game for a handy owner.
- Seal small cracks in the slab with the right caulk.
- Put a sealed lid on an open sump pit.
- Swap a dead fan on a system that already works.
These are low-risk and can save you money. We walk through the fan swap in our fan cost guide.
When to hire a pro
The full system is a different story. Cutting the slab, sizing the fan, and routing the pipe all take skill.
Get a pro when:
- You’re building the system from scratch.
- Your home has a crawl space or a tricky foundation.
- Your post-test number is still high after a DIY try.
- You’re filing a Tarion warranty claim.
A fan that’s too weak leaves radon high. A fan that’s too strong can pull furnace gases back into the house. That balance is why sizing matters. A certified installer measures it instead of guessing.
There’s no shame in either choice. Seal a crack yourself. Leave the full system to someone who does it every week.
When your home needs a second suction point
One pit works for most homes. Bigger or oddly shaped homes may need two.
The gravel under your slab isn’t always connected. A footing or a buried wall can split it into zones. One pit can’t reach across that divide.
Signs your home may need more than one suction point:
- The house is large, over 3,500 square feet.
- There’s an addition with its own slab.
- A first test after install didn’t drop the number enough.
An installer can test the suction reach before cutting. That tells them if one pit will do or if you need two. This is one more reason the full build is a pro job.
Where Breathe Radon Free fits
We’re an Ontario-based testing service, owner-led, based in Guelph and serving the Greater Toronto Area plus surrounding Southern Ontario. We publish our prices and we tell you when you don’t need us.
The first step is never the system. It’s a test.
- If you haven’t tested, a $99 test tells you if you even need a system.
- If your number is high, we walk you through your options with no pressure.
- If you bought a newer home, you may not pay a cent. Tarion may cover it.
Our published pricing:
- Residential test: $99 flat. 3 to 7 days, written report.
- Real estate test: $399. Faster turnaround for closing timelines.
- Long-term test: $299. 91-day test for ongoing peace of mind.
- Mitigation install: $2,800 to $3,800. Full system, 5-year workmanship warranty.
- Tarion claim help: free for test customers.
The Canadian Cancer Society calls radon the leading cause of lung cancer in people who’ve never smoked.
Book a $99 radon test to start. Or see full service details.
Frequently asked questions
What is sub-slab depressurization in simple terms?
It’s the standard way to fix a radon problem. A fan pulls soil gas out from under your basement floor and vents it above the roof. The gas never gets a chance to enter your home. A good system cuts indoor radon by 80 to 99 percent.
How much does a sub-slab depressurization system cost in Ontario?
A full professional install runs $2,800 to $3,800 in Ontario. That covers the pit, pipe, fan, sealing, and a follow-up test. If you bought a newer home, Tarion may cover the whole cost. We help test customers file that claim for free.
Can I install a radon system myself?
There’s no law against it in Ontario, but it’s hard to get right. Cutting the slab, sizing the fan, and routing the pipe all take skill. A weak fan leaves radon high, and a strong one can pull furnace gases inside. Most people are better off hiring a pro for the full build and DIY-ing only small tasks like sealing cracks.
Does the fan run all the time?
Yes. The fan runs 24/7 and uses about as much power as a 60-watt light bulb, roughly $5 to $10 a month. It has to run non-stop to hold the low pressure under your slab. If it stops, radon climbs back up within days.
How do I know the system is working?
Two ways. First, check the manometer, the small gauge on the pipe, where the two sides of the red liquid should sit apart when the fan pulls strong. Second, run a follow-up test 30 days after the install. The number should drop well below 200 Bq/m³.
Book a $99 radon test for your home →
Authored by the Breathe Radon Free Team. Every claim is backed by a primary source linked above. Full bibliography at /sources.
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