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Green Building Bible, Fourth Edition
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General: French drain

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  1.  
    • CommentAuthorArtiglio
    • CommentTimeApr 5th 2019
     
    I’ve a wall in an 1800 house, semi basement 9” brick, at some point in the past the original wood floor was removed and filled with concrete and the walls “tanked” externally the pointing has been redone in sand and cement. The wall is pretty much saturated and damp penetrates around the windows and points where the tanking has failed / was done poorly.
    Pulling up the floor and redoing it, removing the tanking are neither economical or practical.
    Its been suggested a french drain along the wall will at least prevent the wall getting any wetter and repointing with lime will help the wall dry out a little over time.

    Plan is for a 600mm wide trench along the wall to 100mm below the concrete floor level, a studded dpm against the wall , a geotextile in the trench then backfilled , suggestion is to use cockle shells ( available locally) as this will give plenty of air space and quick drainage.

    The wall will then be repointed in lime.

    The homeowner understands that this will not make any immediate improvement , but that it should prevent further water being sucked into the wall and over time allow the wall to dry out a bit.

    Is this a sensible way forward? Being a diy project its not overly expensive . Any suggestions to improve or new ideas?

    TIA
    •  
      CommentAuthordjh
    • CommentTimeApr 5th 2019
     
    Posted By: ArtiglioI’ve a wall in an 1800 house, semi basement 9” brick,

    Sorry to be ignorant but what is a 'semi-basement'?

    100mm below the concrete floor level

    A diagram, or at least a list, showing the levels of everything in relation to ground level would be helpful.

    My instinct is that 100 mm below the concrete isn't deep enough. 600 mm width is more than enough; I presume that's because of ease of digging?
    • CommentAuthortony
    • CommentTimeApr 5th 2019
     
    For me 150 wise 300 deep bottom 75mm filled with beech pebbles repoint in lime
    • CommentAuthorArtiglio
    • CommentTimeApr 6th 2019
     
    Evening Djh

    The house has differing ground levels front to back, at the rear the kitchen opens onto the garden , to the front the ground level is 1100mm above the finished floor level in the kitchen. I’ve called it a semi basement for lack of knowing the correct term.
    The 600mm as you say was selected purely for ease of digging. Buildings in the area are usually built straight onto the chalk with spread brick and often don’t go much below floor level, i’d not want to go below the bottom of the wall and into the chalk. Though if the brickwork goes deeper i’d be quite happy to go down a bit further.

    Thanks gents
    • CommentAuthorgyrogear
    • CommentTimeApr 6th 2019
     
    Posted By: Artigliothe rear the kitchen opens onto the garden


    sounds like you have a walk-out basement, then.

    (like me)

    gg
    • CommentAuthortony
    • CommentTimeApr 6th 2019
     
    If you are on chalk then can you deal with surface water and your own gutters and ensure NONE is getting into the ground on the presumed front of the house.
    • CommentAuthorcc64
    • CommentTimeApr 16th 2023
     
    https://youtu.be/bEVBOEIktr4
    Usefully better than many other resources I've frittered time away on
    Recommended for French drain detailing
    • CommentAuthorJonti
    • CommentTimeApr 16th 2023
     
    With a French drain depth is more important than width. I would go as deep as you can, pipe the bottom and backfill with a suitable drainage medium. If you have a correctly installed drain and the outside wall as a whole has suitable airing then I would have thought it should dry out over time. Check your guttering including where it drains to as this is the cause of the majority of moisture in walls in my experience.

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