I have a flat area under a 6 by 30 ft deck in front of my split level house. I am currently replacing the deck boards on this (Pictures are below). The soil underneath this deck is relatively flat and does not drain well, which causes moisture penetration into my basement (below the grade under the deck frame).

I am considering install a drainage system (similar to a french drain) in this area while I have the deck up, to fix this. It will run water to the right of house where it will flow down the hill and away from the house. My intention is to overbuild this feature, since it will be next to impossible to modify once I put the new deck in on top of it.

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My plan:

  1. Re-level the grade so water runs to the right of the house.
  2. Dig a trench running downhill with the new grade as well (parallel to the front of the house in pictures).
  3. Line the entire area under the deck with 6 mil plastic.
  4. Lay a couple inches of gravel in the bottom of the trench
  5. Run perforated drainage pipe (with silt net) down the length of the trench until it drains on the side of the house. This pipe will be lower than the grade, due to being laid in the trench.
  6. Cover rest of the exposed pipe and 6 mil plastic with gravel
  7. Replace my deck boards and not have to mess with it for 20 to 30 years when the deck boards give out next

I have looked around online and have not seen anyone doing anything quite like this. How long should I expect the 6 mil plastic to hold up in this almost buried state?

It seems to me that this would work better than a french drain because I wont have to put soil on top of it, which will eventually fill the thing up with silt and the plastic will ensure that none of the water gets into the soil below.

Should I expect any other issues with this approach?

Thanks for any comments or suggestions!

  • are you sure that the grade is what causes moisture penetration? I've seen small springs(or what look like springs) that continue to cause moisture penetration
    – depperm
    Aug 9, 2019 at 12:50
  • Yeah the area in front of the house was graded improperly without great drainage. Also I live in the mountains. When water comes out of the ground as a spring around me it goes for the lowest point's possible and comes out in gully. My house was was not built in gully.
    – space97
    Aug 9, 2019 at 15:32
  • Have you considered a perimeter foundation drain that runs beside the foundation footer?
    – Danger14
    Feb 8, 2021 at 8:58
  • I had finished this project up a while back as I had outlined. But I did consider that and I think that would have been the right way to fix the issue. Would have been a lot more work to dig down that far a and remove the other deck on the side of the house to do the same over there, but I would have had a much better drainage system. Thanks!
    – space97
    Feb 8, 2021 at 12:41


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