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Just bought a new home the back patio is sunken with no drain. I know I need to dig down and put a French drain out side of it. Where is the best place to penetrate the concrete and put a drain ie. Base of the wall, though the floor, ... ?

back patio

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    Where is this and how much does it rain there? Does it collect water in any rain or just blowing rain? How much is the surface of the patio below the grade outside? After a lengthy heavy rain does water from the soil drain into the patio at the base of the retaining wall or elsewhere? – Jim Stewart May 16 '17 at 14:56
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    I see some dampness on the surface - I don't see pools, puddles or ponds. So perhaps you don't need to add a drain at all. – Ecnerwal May 16 '17 at 17:09
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    I appreciate the comments relating to whether or not I need to install it, but when i have to use my shopvac to prevent the accumulating water from flowing into my home i am certain i want to do this. That is why I asked for recommendation on HOW to install it. Thanks – BD72 May 16 '17 at 17:15
  • Where do you want the water to drain to? You need a way to get rid of it (like a sump pump that you pipe to somewhere else). – fixer1234 May 16 '17 at 17:33
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    Water flows downhill. The patio is already a few feet below yard level. If you dig a passive drain out into the yard, it will have to go even deeper. As pdd describes, that is likely to collect yard water and route it back to your patio, making things worse. A dry well in the yard would need to be huge, and most of its water would come from the yard rather than the patio. If your goal is to drain water from the patio, put a sump pit in the lowest spot and pump the collected water through a pipe that discharges onto the yard as far away from your house as practical. – fixer1234 May 16 '17 at 18:52
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It really depends on what type of drain grate you want and what tools you have available to cut the hole. If you can rent a core drill that matches the drain grate hardware, you'll probably want to put it in the floor. It would be difficult to cut a hole in the wall tight against the floor (to allow complete drainage).

I'd find the lowest point in the floor, or as close to it as the drill allows if it's against the wall, and go there. You'll be able to excavate outside the wall and under the slab to that point fairly easily, I'd think.

  • Very doable, connecting the drain under the patio slab via the trench for the french drain will be difficult, but possible. The trench for the french drain will need to be big enough to do what is asked of it and that will allow the drain to be excavated under the retainer wall of the patio and the connection made. The OP may want to do a dry well (closed top with dirt) instead of a french drain (open top with gravel exposed) unless it is preferred. I think I have the terminology correct.... – Jack May 16 '17 at 14:37
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I don’t think that a French drain will benefit your situation unless the issue is surface water flowing over the top of the retaining wall. If that is the case, then it might help, but you would not want to connect a drain inside the sunken area to it as it could actually allow water to enter the lower area.

A French drain is simply a tench filled with gravel (may also include perforated pipe) that directs water away from one area to another area.

If you want to get water out of the sunken area, you want to add a drain and pipe it to a storm drainage system, or depending on ground drainage conditions, pipe it to a dry well.

The main issue when dealing with unwanted water is finding a place to put it. A French drain is a way to move it not a place to put it. I think you should make sure that you know where you are going to redirect the water to before going any further. As for how or where to add the drain itself, see @isherwood's answer.

  • If there's water pressure at patio level now it'll show itself. That's not a swimming pool, after all. :) – isherwood May 16 '17 at 17:40
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    As long as the French drain is deeper than the level of the slab it will work great when the water is removed by the drain there will be no water to push into the slab area. – Ed Beal May 17 '17 at 15:39

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