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Community and Q&A

Best guidance for EXTERIOR “sump” pump for yard drainage

Terryssr | Posted in GBA Pro Help on

Where can I find the best state-of-art guidance for constructing/installing an EXTERIOR “sump” type pump for exterior drainage pipes to drain ground water away from house? My house/yard is located near bottom of hill in our neigborhood & the “back yard” also is about 15′ above the part of the yard the house is built on. We already have the below-grade foundation retro-fitted with dimple board system draining to a footer drain pipe system that has solved the “wet basement” problem. With all the climate change related extreme weather events & long periods of heavy rain, we now have a new problem: after the high backyard ground becomes saturated from extended heavy rains, there appears to be high ground water pressure being exerted against/under the carport & garage SLAB, that water starts to push thru tiny surface cracks in the carport/garage cement. Enough water seeps & runs thru those existing tiny slab cracks, that our garage floor starts to flood. I believe the solution is to install horizontally across the backyard, a deep [4-6 ft] drain system between the high backyard area & the carport/garage so that heavy groundwater flow is intercepted before it reaches the carport/garage. this horizontal drain system would be connected to a drainpipe to run DOWN along one side of the house to daylight at the street area. Here’s the problem: there is NOT enough gradient from the backyard to the street for gravity drainage to work if the backyard drainage system is installed 4-6 ft deep. So I am wondering if installation of an EXTERIOR [outdoor, in-ground] “sump” pump type system could solve this problem? And if so, where I could find the best state-of-art guidance on how to install such an outdoor sump pump & the most reliable pump & backup pump system to use? I appreciate your time & assistance in directing me to the best info. THANKS!

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  1. Jon_R | | #1

    Why so deep?

  2. FixitAgainSam | | #2

    I have occasionally wondered about this too, as I have a low lying back yard that sometimes floods. I have used a regular pump with 2-inch plastic discharge hose when that happens, but I don't have it permanently installed. I don't know if it's state of the art, but an interesting article on this topic is on the website of a Charlotte contractor: Best of luck to you with this.

  3. GBA Editor
    Martin Holladay | | #3

    The first step is to intercept rainwater at grade by installing a swale. The swale should drain around the house to the street, if possible.

    The second step, if possible, is to install a buried drain pipe -- a French drain -- that slopes to daylight. This French drain does not have to be 4 to 6 feet deep, as you imagine. It can be quite useful even if it is 8 to 10 inches deep. The key is to have it slope to daylight.

    Only if these two steps are impossible should you resort to an exterior sump pump. The link provided by User-6934637 (names, please!) gives you a guide. More sophisticated but similar systems are used for commercial or municipal systems; see, for example, this site: "Dry well pumping stations."

    If you end up installing an exterior sump pump, this sump pump will be located in a dry well. If you don't know what a dry well is, Google it. The problem with this type of sump pump is that sump pumps fail, and electricity is intermittent during storms. Solutions depend on your budget, but you can install two sump pumps if you want, or a battery backup system, or a sump pump alarm. Not a fun system to maintain.

  4. Terryssr | | #4

    thank you all for your responses. "User": i appreciate the ref & info re Zoeller pumps. Martin, as always your responses are very helpful. I apologize I forgot to mention [duhh! brain fog] that there already is an existing, working French drain around that circles that area, but it seems clear the French drain is not intercepting enough of the ground water when the ground gets saturated. I think it's deeper ground water flowing from the elevated back yard that must be creating the water pressure pushing up under the carport & garage slabs. It's a good point that maybe I don't need to go as deep as 4+ ft, but it apparently needs to be deeper than the French drain. I'm going to have my contractor do some exploratory digging so we can better assess where the ground water is coming from & why the French drain is not capturing it all. I also appreciate the cautiion/warning about failure of sump pumps--& if I decide one is needed, I will indeed have a backup system. We already have a large KW whole house backup generator because in our area the electricity ALWAYS goes out during storms which of course are of increasing frequency & more extreme due to climate change. We have family members with med conditions requiring reliable backup power. THANK YOU!

  5. Jon_Lawrence | | #5


    Check out Matt Bowers built his own passive house with an exterior sump and he is very happy with its performance. I did the same at my house (after reading about Matt's experience), and I am using a temporary pump at the moment to remove water. My excavator installed a 3' diameter corrugated plastic drainage pipe that is connected at the bottom to the footing drains. We will use PVC secured to the sidewalls to hold the permanent sump in position. My electrician will build conduit to the pipe to provide power. For now, I am using 1/6 hp pump with a check valve so it kicks in automatically after heavy rains. I had a chance to test it last fall after we finished framing the roof and before the slab was poured. I left the pump off after a monsoon storm and the lower part of the basement flooded with the water about 3" above the top of the footings. I turned on the pump and within a few hours the water was gone.

    I don't think there will every be enough head pressure to cause problems in my basement, but my reasons for doing it this way were to limit radon, avoid damp smells and the thermal bridging of a sump. I have a radon meter that I have been using for the past 9 months to measure the radon and after sealing up the basement for the winter, but before pouring the slab, radon was running in the 4pCi/L. Then my plumber stubbed out the radon vents and it jumped to 10. I capped the vents, and it went back down to 4. After adding 4" of EPS it dropped to around .7. After a 10mil vapor barrier, it dropped to .4 and now that the slab is poured (glass fiber reinforced concrete without mesh so no issues with punctures in the vapor barrier) it is hovering around .15. So for anyone concerned about radon, I would recommend placing the sump outside, and insulating the slab.

  6. Terryssr | | #6

    Jonathan, thank you for that info. Definitely will ck it out. Glad for you that your efforts almost totally eliminated your radon! best wishes.

  7. beeker4747 | | #7

    I put an exterior in and cannot tell how it's harder to maintain, except that if you need to work on it in bad weather. It's not fun going down to the basement either, but I suppose it is relatively easy since the sump will be shallower.

    Details: I stacked two cheap sump pits in a whole deep enough to go below my footing drains. I capped with an ordinary sump pit cap, a slab of rigid foam, a slab of shale, and then gravel around. Power is right there at the exterior wall and the sump pump just plugs in. I could be done a little more cleanly, and I like the idea of a 3' section of pipe on end, but my way is not hard. Just have to reach down and grab the pump and haul it out to inspect/repair. I used a float that flops around free on it's tether, but a more reliable way might be to have one that rides up and down on a straight rod.

    Now it's around zero, so I should go take out the cap and look down there well in advance of the coming spring thaw. Will I?

    1. Expert Member
      BILL WICHERS | | #8

      You can use septic tank level alarms in these sump pits too. That gives you an early warning of pump failure.

      You can also use precast manhole and sewer "fittings" to build up your sump too. This is sometimes the best option if you want a larger sump.


      1. beeker4747 | | #9

        That's a good idea. Those culverts are very long, so a lot of waste or storage when you buy one. I was thinking of a normal pit with a slightly larger ring for the next level up. Then there would be a ledge to work with.

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