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Under-Slab Drainage + Radon Mitigation in Same Pipes

user-6445625 | Posted in General Questions on

I am in zone 6 and planning for a crawl space with crushed stone base + 2″ rigid insulation + 6 mil poly barrier + 3″ rat slab. I will run radon piping beneath my slab in the gravel and penetrate with a riser out the roof. Question: Should the sub-slab perforated pipe simply run along the interior perimeter or does it need to criss-cross? I can’t find a good resource for piping layout. Also, I would assume this perforated radon pipe can do double-duty as under-slab drainage? That is what I would ideally like.

thank you!

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  1. Expert Member
  2. user-5946022 | | #2

    Typically they do not share the same pipe.

    For the under slab drainage, you need to provide someplace for the moisture to drain to. That is usually going under the foundation, tying into the exterior perimeter drain and daylighting away from the house.

    For the radon mitigation, you want something that will pull the radon from the soil and exhaust it. If this were tied into a drain that daylights, the radon mitigation system would just pull up air from the daylight location. You want to force it to pull air from the sealed area under the slab.

  3. user-6445625 | | #3

    Thank you for these replies!
    Based on further reading, here is what we plan to do. Please let me know if you think this is sound.
    Under our rat slab/membrane/foam we plan a 4" perforated perimeter drain that will do double duty for radon and under slab drainage. On one end, it will tie to a solid riser through the house and out above the roof - on the other end, it will go out through the footing to daylight. The run which penetrates the footing and goes out to daylight will be solid pipe with a check valve at the end where it goes to light. This system will be independent of our exterior perimeter French drains.

    Would be grateful for your input!

  4. walta100 | | #4

    Interior drain pipes would be a very unusual choice in new construction.
    As exterior drains running to daylight are better, cheaper and more reliable.

    I think the money budgeted for interior drain pipes would be better spent on increasing the thickness of the gravel under the slab that would allow the free flow of both radon and water everywhere.


    1. Expert Member
      Michael Maines | | #12

      It probably varies by region, but interior footing drains (in addition to exterior footing drains) are standard here in Maine, even on budget-oriented projects. We have a lot of groundwater and clay so maybe locals take water more seriously than in other places. They don't cost much and provide a degree of redundancy.

  5. Expert Member


    I agree with Walta. If you have exterior perimeter drains (and that's the preferred location), interior drains are only necessary if you have a spring or very high water-table. If you want some reassurance that any water that accumulates there has somewhere to go, a short perforated pipe running through the stem-walls at any point, will remove that water. It doesn't need to run all around the perimeter.

  6. user-6445625 | | #6

    Thank you both for those replies. Malcolm, if I add the short perforated pipe isn't that introducing outside air under the slab anyway? This house is sitting on a slope with a decent amount of surface water. We have proper swales and perimeter French drains outside the house but were looking for an added layer of security with an interior drain and sump just to be super safe (belt and suspenders). We were hoping that the required interior perforated drain for radon we already plan to install can also be used for that extra layer of drainage. Thus our strategy outlined above. Assuming no worries about cost for that pipe, do you think the strategy is sound? thanks again!

  7. Expert Member


    Thinking about it again I see no downside to your plan. It sounds like a good strategy.

  8. user-5946022 | | #8

    I would be concerned that the check valve would somehow get caught open.
    Agree that funds would be better spent on gravel under the crawl slab, spend more on waterproofing the crawl space walls..

  9. walta100 | | #9

    I say if you install a 6 inch thick layer of clean gravel and your exterior drain should happen to fail you can make a sump anywhere you want and drain the water more effectively than interior pipes could.


  10. maine_tyler | | #10

    Is using a check valve for this common? It seems the air pulling the valve shut and the water exiting are fighting one another. I'm sure water can still escape, but I'm trying to picture exactly how they'd interact.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      I'm going back and forth on this. Relying on value is a worry. In theory Brad's system works, but I agree with User ...022 that it is vulnerable to the valve malfunctioning over time.

      I'm not sure there would be any real concern about air pressure being sufficient to work against the weight of the water, but if I was doing this, I'd like that valve to be accessible for inspection.

  11. Expert Member
    Michael Maines | | #13

    Here's my article on this subject: Summary: I combine the interior footing drain with the radon pipe. I live in a high-radon area and using this system I have never had to add a radon fan. I have never used a check valve.

  12. user-6445625 | | #14

    Thanks again everyone for this helpful feedback. I've been torn between Michael Maines' approach (combined with no check-valve) and Martin Holladay's approach (with check-valve). I'm leaning toward installing the valve and just making sure it is accessible for inspection as Malcolm suggests.

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