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Community and Q&A

Internal vs. Exterior Garage Water Floor Drain

zetetic | Posted in General Questions on

I’m trying to achieve a reasonably air tight garage in CZ7b in case we someday have an EV and need to keep the garage warmer for the EV batteries.

The garage and the family room below it  is a new build, so I have some flexibility.

This is heavy snow country (400″ to 700″ annually), so a snow-covered car will often be parked in the garage, and the garage floor will have floor drains below each of the two bays.

The garage drains via a gravity feed pipe in the concrete garage floor, then down 25 feet inside an insulated exterior wall (living area) where it reaches a below-grade french drain.  I’m assuming this will keep the garage relatively air tight.

I don’t expect anything other than dirty water to go down the floor drain.

My question is whether to run the drain inside the exterior wall where it will create a thermal bridge and be difficult to service, or on the outside of the wall where water will likely freeze inside the drain.

My gut feeling is to run the drain inside the wall.  What is the best way to insulate the drain pipe?  Simply a few inches of CC spray foam, or are there better ways to detail the drain?

Is it wise to use a smaller drain pipe in the wall to make the thermal bridge smaller, although this might increase the risk of clogging?

Maybe an exterior clean-out in case the wall pipe gets inadvertently clogged?  Is there such a thing as an insulated clean-out?  Or am I over-thinking this?

There is a third option that I just realized as I’m writing this – there is an insulated interior elevator shaft from the garage to the two levels below the garage.  Might this be a better candidate for draining the garage?  Perhaps more difficult to service the pipe, though, should there be a clog (has anyone ever had a garage drain clog?).

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  1. Expert Member
    Michael Maines | | #1

    I recommend sloping the garage floor to drain toward the overhead doors.

    1. Expert Member
      DCcontrarian | | #2

      Out the door is what I was thinking too.

      In particular, this won't be clean water -- it will have road grime in it. You don't want to be dumping it into french drains, you'll clog them. If the door is at grade that's a good place to be shedding the water.

  2. Expert Member
    Akos | | #3

    The problem with out the door is that it will freeze. This is the usual failure in lot of heated driveway installs around me, they create a nice ice pile right by the road where the heating stops so instead of having to shovel a bit of snow you are now chipping inches of solid ice.

    I think the best would be an internal drain, make sure there is a large, effective and easily cleanable sediment trap, you don't want any soil or sand to get into your French drain as it will clog it over time.

    Most residential drains would be plastic, so not a big thermal bridge. If running it inside a wall, I would make sure there the drain is towards the inside of the wall and there is at least a couple of inches of insulation behind it to keep it from freezing.

    Extra cleanouts are always a good idea for any drain.

    1. zetetic | | #4

      Thanks for keeping me from building regretful designs - great advice and suggestions!

      Instead of a French drain, I’m thinking maybe a landscape style valve box with 6-8 inches of gravel on the bottom and the garage drain entering from the side (below grade). Some screen on the pipe to keep pests out. Fairly easy to inspect and service (before the snow falls in earnest).

      1. Expert Member
        DCcontrarian | | #6

        Where do your rain gutters go? In cold places it can be tricky to have drains go to daylight without a risk of them freezing up.

        If you want to have floor drains, and you're worried about them creating an air leakage path, the customary way to block air in a drain is to have a trap. It would have to be somewhere protected from freezing because it would stay full of water.

        1. zetetic | | #7

          We have no gutters. The eaves are 48” and the grade slopes away from the foundation.

          Are gutters helpful with 4-8 ft of snow on the roof, and 10-13 ft on the ground? We get more snow than rain, and I do not know how well gutters will hold up?

          If we do floor drains, the pipes will be protected inside insulated exterior walls down to below grade.

          Bill Wichers suggested a dry well to collect water and filter sediment. Seems like a good idea.

          I like your trap idea, too. It will also collect sediment and needs to be easy to inspect and service. A trap does require water to prevent drafts. If we are away for a few weeks in the winter, will the trap go dry and the garage be less tight?

          Maybe drain to the sanitary sewer with a serviceable sediment trap?

          1. Expert Member
            DCcontrarian | | #9

            I think trying to drain underground is going to cause you more grief in the long run than just draining onto the ground. Especially with a steep slope, it's easy to get the water away from the house.

  3. Expert Member
    BILL WICHERS | | #5

    I would slope the floor slightly towards the overhead doors as others have mentioned, then put a channel drain (sometimes called a "trench drain") across the driveway just outside the overhead doors.

    You can probably just drain to a drywell, unless your area requires garage runoff to go to a sanitary sewer (which is now usually the requirement for commercial truckwells, since the water those collect tend to have oil and other "vehicle runoff" in it). I don't think a valve box is going to be big enough to handle this though.

    BTW, you don't have to heat your garage to be able to have an EV. The EVs will typically condition the battery using utility power from the charger. My Volt, for example, will also pre-heat the battery if you use the remote start function to get things ready before you leave.


    1. zetetic | | #8

      Very good point that the EV can maintain the battery temp in a cold garage. Just need to remember to plug in the EV, otherwise the EV battery may be too drained when I go to use it next.

      I also like your drywell and sanitary sewer suggestions..

      Like a drywell, the drain to a sanitary sewer will need an easy to inspect and service sediment trap. But would the garage smell of sewage? We’re connected to the neighborhood sewer system.

      We will have a good seal between the garage floor and the garage door. Will the seal defeat or impede drainage if we slope the floor to the driveway? Is it possible for ice to form at the seal, adhering the overhead door to the garage floor?

      Would a trench drain eventually fill with ice?

      It’s a somewhat harsh environment where we are. We’re not quite CZ7b with 8500 HDDs, but we’re definitely on the upper end of CZ6b. I think we’ll see 400 inches of snow this season at our home (ground snow is 13 feet now, and code requires us to design the structure for 420 lb/sqf snow load). The garage will have plenty of snow melting off the cars.

  4. nuthin2fancy | | #10

    I'm in Zone 5b and our unheated garage is sloped towards the overhead door.
    This results in ice dams and the occasional frozen shut overhead door. I am contemplating a "trench drain" as Bill suggests, but wouldn't you want this inside the door to catch the meltoff before it reaches the door?

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