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Old cottage, 'french' drain

  • 22-05-2021 2:22pm
    Moderators, Social & Fun Moderators, Society & Culture Moderators Posts: 25,503 Mod ✭✭✭✭ looksee

    I have an old cottage as part of my house. The cottage has no foundations other than large rocks, under the rocks is soil. The building is fine and has no issues other than some low down damp on 1 or possibly 2 walls inside.

    When the house extension was built onto the cottage around 20 years ago, a french drain (of sorts) was put across the front of the cottage, which gets a lot of weather.

    The cottage foundation rocks are just at ground level, and the interior floor is about 15" above this, was a suspended floor but has been lined and infilled with concrete.

    It appears that the ground was dug away from the cottage absolutely in line with the wall, plastic (not membrane, actual builders plastic) was inserted - I am not sure if it is like a french drain, or if it is just vertical lining against the sub-foundations. I haven't dug that far yet. The area was filled with stones, a slotted pipe was laid on top (ie, at surface level) of the stones and the area levelled with stone and then sand. Loose laid slabs were put on top.

    Question - are there any circumstances in which a slotted pipe would do anything useful on top of the stone infill? I am tempted to just remove it (to get at the plastic sheeting).

    I am going to try and take out the plastic and just allow the infill to drain the area. The ground is extremely free draining and really I doubt the drain was needed anyway, but I think it is the plastic that is causing issues. I am nervous of disturbing the infill too much though as the whole thing is far too close to the wall.

    Does anyone with knowledge of cottage buildings have any thoughts on this?


  • Moderators, Social & Fun Moderators, Society & Culture Moderators Posts: 25,503 Mod ✭✭✭✭ looksee

    So, I think problem is solved. A bit more muscle turned up and digging was done, and it turned out the plastic was just a ragged 12" deep strip between the soil under the foundation rocks and the drainage pipe. Which makes no sense at all, but it is pulled out now without too much disturbance. The pipe has no fall on it, rather the reverse, it isn't even level, so I am considering whether to remove it completely. The 'french drain' turned out to be just a layer of stone the same depth as the pipe.

  • Registered Users Posts: 1,073 ✭✭✭ Roger Mellie Man on the Telly

    You should wrap the perforated pipe with a geotextile to prevent fines entering. The pipe should transport excess surface water to a soakaway via a silt trap. If the ground is free draining the perforated pipe may carry very little, but no harm in it being there. If rainwater goods or eaves runoff enters the perimeter drainage area, the pipe will see more use.

  • Moderators, Social & Fun Moderators, Society & Culture Moderators Posts: 25,503 Mod ✭✭✭✭ looksee

    Thank you for that. It has been 'wrapped' in sand for about 20 years so I imagine it has a good bit in it, though there is no weight in it. As you say, no harm to leave it.