We recently had a french drain installed, and after the first couple of rains, we've noticed that there is always standing water at the bottom of the french drain, about 2 inches, even after several days of zero precipitation. I believe that water will flow properly once is rises high enough to overflow into the piping, but I definitely don't want 12x12x2" of perpetually standing water for mosquitos to thrive in.

Is this how a french drain is supposed to work? Shouldn't the standing water at the bottom of the drain slowly seep into the ground?

enter image description here

  • Did that box have a plate cover, or a grate cover?
    – Huesmann
    Commented Aug 5, 2023 at 14:49
  • yes, it has the typical grate cover/lid, I just took it off for the photo
    – swv
    Commented Aug 11, 2023 at 16:29
  • Does the box have an open bottom?
    – Huesmann
    Commented Aug 12, 2023 at 12:07

1 Answer 1


Yes, that's a surface drain box (it may be connected to the pipes of a French drain, but it's not a part of a standard French drain) where the pipes are elevated from the floor so that you can remove debris before it gets into the pipes to clog them. Or you can never clean it out, let the debris pile up and flow out, and get the pipes throughly clogged. Your call.

You can drill weep holes (1/8" or less) in the bottom of the box to allow that little bit of water to seep out if it's failing to evaporate in a timely manner and breeding mosquitoes. In the event that the box has no bottom, that's the level of the ground water right there. I'm guessing it's a closed bottom with no weep holes, though.

  • ok, I think this is the information that I need, despite referring to the wrong part ( surface drain box vs french drain). Thank you for the quick info!
    – swv
    Commented Aug 5, 2023 at 13:46
  • That's step negative one, that's not in the manual; drill forty, 1/4" holes before you put it in. 1/8" holes for potted plants to drain into their pans clog up IME.
    – Mazura
    Commented Aug 5, 2023 at 20:46
  • To be sure, this box SHOULD NOT have perpetually standing water, right? The landscaper keeps saying "that's normal" and ignores the problem of mosquitos. @Mazura
    – swv
    Commented Aug 11, 2023 at 18:21
  • 1
    @swv - that someone wouldn't go the extra mile and drill holes in it first doesn't surprise me. Nothing should ever have standing water in it except a trap. That's not your landscaper's job, it's the plumber's, or your handyman's whose job it is anyway to fix everyone else's mistakes.
    – Mazura
    Commented Aug 11, 2023 at 23:41

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