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Drainage Around Detached Garage

thegiz | Posted in General Questions on

Hi hope everyone in northeast was safe with storm. I have a 2 car detached garage with a new roof but during very heavy rain I get ground water in garage. I believe some of it is just traveling down hill under old garage door. However the back left corner also collects water and that is not near the left side door. I have a swole on left side of garage and grass on right side. Besides adding a long drain in front of garage doors wondering if I need to create an exterior drain around the entire right and back side of the garage. The grass was flooding with this storm so it was a lot of water. Pictures attached.

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  1. Expert Member

    If I were Pharaoh, I would have put a perimeter drain around the pyramids. I'm a firm believer that water shouldn't stand at any place on a property if it can be helped.

  2. Expert Member
    BILL WICHERS | | #2

    You may find simple rain gutters will eliminate this problem. Remember that all the rainfall that falls on the roof of a structure is concentrated at the eaves when it runs off. If you let this run off to the ground along the eaves, those areas are getting the equivalent of torrential rains even in a light storm. Rain gutters help to carry this water array and direct it away from the structure so that you don't have problems.

    I'd try installing gutters here. Seamless gutters are best, but PVC gutters are a relatively cheap and quick DIY project. Make sure when you plan your system that you are are draining the gutters far enough away from the structure, and also towards the downhill side of the structure as much as possible.


  3. thegiz | | #3

    Thanks for advice, I do have rain gutters running along side of roof. It’s pitched to rear of property but I suppose there could be an error in the system. Along the back and side could I put something in place to get rid of ground water and drain it? A grate and drain in front of garage on asphalt makes sense but can I cover it with grass on side and back? It’s part of back yard on side and I don’t want to loose any more space in a tight yard.

    1. Expert Member
      BILL WICHERS | | #4

      You might try a french drain. Get some perforated drain pipe with a sock on it, bury it in clean gravel. Drain the pipe to wherever you want the water to go downhill. You can put such a french drain under grass too, but it will be less effective since the water will need more time to go through the grassy turf to get to the drain compared with going through a gravel bed.

      Note that if you do all this work trenching in drains, I'd recommend building the drain system so that it can handle the runoff from your gutters too. That way you can direct ALL the water to somewhere away from the structure.


      1. Expert Member

        I'll add to that -

        Don't run the gutters into a perforated pipe that runs along the base of your building if you can help it. A non-perforated pipe carrying the water away would be ideal, unless you need to drain along the way of the whole path.

        1. Expert Member
          NICK KEENAN | | #7

          Right, ideally you have two pipes running away from the building, one for the gutters and one for the perimeter drains. Once they get one pipe diameter below the perimeter drains they can join and exit together.

  4. Jon_R | | #6

    Hopefully drains + slope/gravity can remove the water from the garage area.

    If not, you can have drain pipes bringing the water to a grass covered dry well. Be sure to make it large enough (perhaps 6'x'6'x6').

    If that won't work for you, you can pump drain water out of a much smaller sump.

  5. thegiz | | #8

    So I drew a picture of what I’m dealing with. Original plan was shoot out water from right side of the garage but there is a retaining wall to the rear of the property. If I dig down and then drill a hole in retaining wall I fear it will undermine the wall. To the rear of property is commercial businesses. Starting at the left side of garage door I was thinking shooting a pipe to the right elbowing it past the side of garage and then elbowing the end to a drywell in yard away from the garage. Not sure if that is too many turns. Also I don’t want to dig a deep hole for piping. What is my starting depth and how deep do I pitch it so I can grow grass on top of side. Also what if I just pitch the front of garage to a drywell in yard. Let the gutters pitch out and over the retaining wall in rear and just dig a trench on side with gravel and cover with grass so surface water will sink down below.

    1. Expert Member
      BILL WICHERS | | #10

      You don't really need any particular depth, but if the line is very near the surface it will be more liekly to get damaged. If you can only get the line in a few inches underground, I would use schedule 40 (or even scheduel 80) pipe, and NOT the cheaper schedule 30 stuff with the bell ends that is thinner walled.

      Typical pitch is 1/8" to 1/4" per foot, which is 1-1/4" to 2-1/2" slope per 10 foot stick of pipe. Steeper slopes create faster flow, and allow for more water to be carried by a given size pipe. There are charts to help you size the pipe based on the expected rainfall rates in inches per hour for your area.

      Note that smooth-wall pipe is much less prone to clogging than the corrugated stuff.


  6. GBA Editor
    Kiley Jacques | | #9

    I realize you are dealing with a slab, not a full basement, but this detail and related resources could be helpful: Underground Water Barrier Retrofit .

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