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Community and Q&A

Exterior drainage plan feedback

rossn1 | Posted in General Questions on

As part of my remodel of my lower level (~4′ below grade, zone 5a Colorado), I found evidence of some moisture and bulk water issues, though I wouldn’t say I classically have a ‘wet basement’.

Per the attached drawings, I’m planning on adding drainage as shown, and would welcome any feedback on the plan. I think it is mostly a surface water issue, versus groundwater.

A few open questions:

1. I had intended to tie the radon system into the sump, and thus the perimeter footer drain. Since I’m using the dimple mat, I’m not sure if I will lose suction out the top when it is capped and a few inches beneath the surface, or if there are any ideas how to prevent this.
2. I’m not sure the trench/french drain approach will work for me near the surface, given my soil is 60%+ sand, and drains easily. I added a liner, though am not sure if that will work, and ultimately I’d tie it into the rest of the gutter system.
3. Anyone see any significant issues with the length of the perimeter drain being up to 110′ from the sump, and being almost flat?

Any ideas of other forums where drainage is routinely discussed would be welcomed.


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  1. GBA Editor
    Martin Holladay | | #1

    In general, your plan looks sound.

    1. Your underground roof is a little close to the surface. Either erosion or a curious dog digging a hole might expose the underground roof -- so if site conditions allow, you might lower the underground roof.

    2. In general, you don't want to connect a radon mitigation system to any pipe that leads to daylight or is connected to the open air unless the "daylight" pipe has a check valve. Check valves can fail, so that's sub-optimal. I'm not sure about the connection in your case -- but separate radon piping connected to the sub-slab crushed stone layer is best, without sharing drainage piping.

  2. rossn1 | | #2

    Thanks for the review, Martin.

    The red piping system is at the footer, and does not go to daylight on the S & W sides... only to the sealed sump pit. My concern is that if the dimple mat is mounted just a few inches below the surface, it could be sucking air from the top and lose vacuum.

    Any idea if using a trench drain with or without a liner in well-draining soil (60+% sand) is useful and/or feasible (water won't run the length of it without a liner, but then there is the issue of how to get that water back in the pipe)? Perhaps I should scrap that drain all together or make it a solid pipe with some catch basins and grates above it.

  3. GBA Editor
    Martin Holladay | | #3

    Your plans do not really show the radon piping, do they?

    Radon pipes are usually under the basement slab.

  4. Jon_R | | #4

    Do measurements show that you actually have a radon problem?

  5. Expert Member
    Dana Dorsett | | #5

    Jon R: In Colorado you almost don't even NEED to measure it to know estimate that the test will be above the EPA's recommended remediation level. Pick a county, any county:

    If it's red you really don't need to bother it's a pretty safe bet that it would average above 4pCi/L without a remediation system.

  6. rossn1 | | #6

    Ah yes, not so clear - the radon is shown more indirectly. The red line on the S and W sides are the footer drain and are hooked up to the radon. The radon fan lives in the shed abutting the house, and originally tied to only the one location called out on the print. The intent was to tie it into the sump (and thus the footer drain). Now that I'm using dimple mat on the walls, I'm trying to navigate how I prevent it just sucking air from the top (guess I'll have to seal it).

    1960's slab on dirt, so no radon piping, capillary break, vapor barrier, etc.

    Dana's about right. Radon, was around 7-8 picocuries per liter originally, and around 4 picocuries per liter after remediation, but now I'm tightening up the house.

  7. GBA Editor
    Martin Holladay | | #7

    Your radon system is unusual. For an older house, the usual approach would be to cut a few trenches in the slab, and install the perforated radon pipe under the slab. [Later edit: This is inaccurate. See Comments #8 and #9 for more accurate information.]

    The radon riser would be connected to the sub-slab perforated pipe, but not to the footing drain. That approach is preferable to what you've done.

    For more information, see "All About Radon."

  8. rossn1 | | #8

    Hi Martin, I'm not sure about new construction, but that's not how they do retrofits here in Colorado where radon is quite common. They may do that in other areas of the country.

    Almost all radon system retrofit installations here are merely poking a hole in the slab and putting a pipe through, which has a fan on the opposite end of the pipe. They often look to use a sump as their first point, since the perimeter drain also serves as a distribution network. If those options don't lower the radon enough, they either add a second point where they pull from, look for seams to seal, or add a stronger fan. In addition to being the practice, this is also the EPA recommended approach.

    In my case, I do have about 20' cut in my slab for relocation of a bathroom and am extending some gravel from it to the sump.

    My challenge is that I'm adding the dimple mat, which would come close to the surface, and serve as a potential leak point.

  9. GBA Editor
    Martin Holladay | | #9

    You're right. I mis-remembered the usual retrofit approach -- and when I reread my own article, I realized that cutting a trench in the concrete isn't usually done when retrofitting a radon mitigation system in an existing house. I'm sorry for my unhelpful advice.

    Dimple mat is usually terminated at the top with a horizontal termination strip or flashing (as my recent article explains), so it shouldn't be too hard to perform air sealing work at the top of the dimple mat before the termination strip is installed.

    I think you'll be OK. Of course, you'll need to measure the radon level in the first floor of your house after the work is complete. In the unlikely event that the radon level is still high, you'll need to call in a certified radon abatement contractor for advice.

  10. Jon_R | | #10

    If the EDPM is tightly sealed to the wall, then I don't see an air leakage path (other than through lots of soil).

  11. rossn1 | | #11

    Odd -- the prior response I posted seems to have disappeared... or maybe I forgot to hit post. In any event, no worries Martin.

    Jon, the dimple mat has a raised surface on the back, so some sort of sealing is needed.

    My latest thought is to put the dimple mat a little lower, use the termination strip at the top, then cut a strip of water and ice guard in half lengthwise, and seal that at least 6" onto the foundation and a foot below the termination strip. Anyone think this might work?

  12. Jon_R | | #12

    >the dimple mat has a raised surface on the back, so some sort of sealing is needed.

    With airtight, sealed to the foundation EDPM above it, where would the air come from to flow behind the dimple mat?

  13. rossn1 | | #13

    In talking with my excavation crew, I'm getting the message that on other projects, they've had engineers specify for the foundation drain holes to be beneath the *bottom* of the footing, but outside the cone of compression, with the soil sloped down from the top of the footer going outwards. Suggesting a foot or so away from the edge of the footer to ensure when fall is added, the slope is no greater than 45 degrees. Soil is at least 60% sand. Does anyone have experience with or knowledge on this?

    Jon R - yes, getting it airtight is key. The EPDM rubber won't be all the way around the house, just on the side where it is quite flat and subject to flooding. It will also have penetrations for things like the deck posts, so will be pretty effective for water, but not so much for air sealing.

  14. rossn1 | | #14

    Correction... should have said that the soil would slope down from the bottom of the footer. Perhaps it goes outwards flat a few inches before it begins to drop.

  15. GBA Editor
    Martin Holladay | | #15

    My own opinion is that the drainage holes on the footing drain pipes have to be below the level of the slab, but the drainage holes don't have to be below the bottom of the footing.

    You'll hear lots of opinions on this issue. GBA published an article on the topic a while back; here is the link: "How to Install a Foundation Drain."

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