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Community and Q&A

drainage for shop slab: Wisdom of french drain?

mikeysp | Posted in General Questions on

Hi. I am building a pretty well built insulated shop on a slab in zone 4a.

Do I need a french drain or will surface slope be very adequate for me.

Shop details:
built on high ground.
2″ of foam and radiant heat.
4″ of gravel
Graded slope 6″ in 10 ft
Rain gutters diverting rain on roof to bottom of steep side (3ft drop below slab level).

Thank you.


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  1. walta100 | | #1

    An exterior drain around the footing is cheap insurance if you can drain to daylight.

    If you can not drain to daylight on your site I say you are building on the wrong site. Other may disagree but I will never be flooded those that disagree may not be able to say the same when the 100/500 year storm hits.


  2. Expert Member


    Yours sounds like one of the rare instances when a perimeter drain wouldn't be of much use.

  3. gusfhb | | #3

    I would slope the floor slightly to the door so you can sweep out water from vehicles etc

    I would not want any kind of floor drain if I could avoid it. If something petroleum based leaked[you don't say what kind of shop] and got into a floor drain that is more problematic than it being on the floor.

  4. Expert Member
    Michael Maines | | #4

    If your location uses the IRC for a building code, a sloped floor is required:

  5. mikeysp | | #5

    Michael, I do not have a code requirement; however, maybe I should ad some slope in the entry way area; so, on the occasions I pull a vehicle into the shop to load/unload or do a vehicle repair, any moisture will find its way toward the vehicle entryway.

    My current plan is to make the concrete dead level so I can have an easier time with machinery being level, weather fixed to floor or on casters.

    I am building a 28x80 post frame detached workshop. Can call it a garage if you like, but it will be for machinery and a small home based manufacturing facility 90% of the time.

    I will not do a floor drain. Had one, hated the floor. Got little to no use out of it. If I do anything, it will be a slope.

  6. Expert Member


    If it's a shop I would keep the floor level. Sloping it throws everything off - cabinets, moveable equipment, saws.

    How are you planning to build the foundation? If it's a slab on grade with no stem-walls, a sloped floor also means cutting each stud individually, and tapering the sheathing, siding and drywall.

  7. maine_tyler | | #7

    do you happen to have a sketch?

    1. mikeysp | | #8

      Tyler, attached are two sketches:

  8. gusfhb | | #9

    The amount of slope needed for convenience draining is not relevant to machines or cabinets.
    You have to level machines anyway
    I think I did less than an inch in 10 feet and it did not bother me.
    I did find a wallow in my current workspace[not built by me] while moving a 3000 pound lathe on a wet floor.
    Moved all by itself

    1. Expert Member
      MALCOLM TAYLOR | | #10


      I don't know - For the slope t0 be useful it needs to be 1/4" per foot. That would be an inch or more over the length of a work table, or almost two feet over the length of Mike's shop.

      Our code requires garage slabs to be sloped, but every building with an overhead door isn't necessarily a garage.

      1. gusfhb | | #11

        Nah, 1/4 inch per foot is flat roof
        If it is raining in the garage there are other problems

        All you need is enough to overcome unevenness in the floor and have the water want to run out rather than in. A truly flat floor will keep water in the back corner forever and even a tiny slope will overcome that.

        1/4 per foot would be truly self draining and I do not see the point in that.

        I did not build my current garage and did not notice the floor was sloped until recently.
        I will double check but I think it is maybe 3 inches in 24 feet

        Funny story while reno-ing the house, lotta stuff piled around

        Driveway runs down to the garage, walkway wrapping around the side downhill
        Nice field across the street
        porta potty dropped at top of walkway in front of garage

        Early march starts raining
        And raining
        and raining

        I run to the lumberyard and come back
        The field is flooded
        There is water streaming across the street across the yard digging a trench beside the house
        streaming down the driveway
        It has floated the porta potty, which combines with a stray bale of peat moss to block the entrance to the walkway

        I walk in the man door of the garage and flip on the lights


        hit the garage door openers

        It was like a cartoon, or the elevator scene in 'The Shining'

        whooosh, 3 -4 inches of water rushes out the doors.

        Turns out the field is not a field, it is the dry part of the pond[which is hidden by trees]
        The walkway is not a walkway, it is a culvert

        Point of the story is the garage self drained
        I only skimmed the linked code but it did not seem to specify pitch, maybe I missed it

        1. Expert Member
          MALCOLM TAYLOR | | #13

          Ha - great story!

          I don't think any codes say much more than it needs to slope. The distinction is probably between wanting water not to pool, and having all of it actually drain out the door. My own preference living where snow melt isnt a problem is a completely flat floor, which also has the advantage of being easily convertible to living space d0wn the road.

          1. Expert Member
            Michael Maines | | #14

            Standard around here is a 2" slope for an average-sized garage, 24' to 28' deep. It's not enough to overcome the surface tension of water so I'm not sure why we do it, other than it helps avoid puddles at low spots. It is enough to notice if you are trying to set up a cabinet shop, but not if you're just doing occasional projects with portable tools.

    2. Jon_R | | #12

      I have a 2" step that keeps equipment separated from water from cars. Other than that, it's flat - which means puddles remain in place longer, elevating humidity and causing corrosion. An overhead fan + temporary ventilation has helped to evaporate the puddles, but more drainage would be nice. So I drilled some drain holes in the low spots.

  9. Jon_R | | #16

    Agreed, the #1 factor is how high the slab is above the high side ground level. High enough and you don't need any french drains, exterior water will harmlessly flow around the building on the surface.

    Hmm, quite odd how my comments are being placed above what I'm replying to (#15).

  10. Expert Member
    NICK KEENAN | | #15

    We've gotten off topic here, the original poster was asking if he needs french drains, not floor drains. I'd say if you can guarantee that any water that is above the level of the bottom of the footing will drain away you don't need french drains.

    1. gusfhb | | #17

      I think my image of french drains makes them into defacto floor drains.

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