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Basement tanking best practice

Discussion in 'Building' started by antz87, 30 Jul 2019.

  1. antz87

    antz87

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    Hi there, new to this forum and grateful in advance for any advice.


    Have a property that used to be a pub, so has a large cellar/basement, and looking to make the space a bit more usable by tanking the walls and floor to create a space for storage that's a bit dryer.

    The walls had a layer of what looked like cellar paint on (no mould and little condensation) on top of a render in places and straight onto brickwork in others. I've taken this off to get back to the brickwork to reveal some reasonably damp mortar that needs some repointing.

    My plan as a next step is to repoint the brickwork prior to tanking, and then do two layers of tanking, horizontal and vertical using something like KA tanking slurry, and am assuming that the mortar in its current condition won't be able to support the tanking.

    But I'm wondering a few other things about the process.

    1. Is there a massive benefit to rendering the walls first? Some tanking slurry suppliers suggest you can go straight onto brickwork if it isn't loose?

    2. Is there a benefit in using a chemical DPC to protect the joists (above the fireboard in the pictures), or to reduce the water building up in the walls from the ground for the internal walls (half of the walls are internal, the others are external and have more salts on them), or is this irrelevant?

    3. Should the walls be tanked their full height, or should I leave a course or two of bricks untanked at the top?

    The wall construction seems to be 1880s, London stock brick and lime mortar. Walls aren't massively damp and there is no water under pressure, pooling water etc, but not suitable for keeping things dry from a storage point of view as it is. Have got some pictures of the walls as they currently look (after I've removed most of the paint on a couple of them and the old rendering):

    https://ibb.co/Fzrtb91
    https://ibb.co/mJP0Yhd
    https://ibb.co/QXvXVbb

    Cheers in advance for any thoughts.
     
  2. ^woody^

    ^woody^

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    Tanking is high risk and you can't just make it up. You need a system designed specifically for the site in accordance with BS8102, not cobbled together.
     
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  4. antz87

    antz87

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    Fair point, thanks woody.

    I'm familiar with BS8102 and am working towards grade 2 water protection rather than grade 3 in this case. There seem to be so many preferred methods for floating around online for tanking I wondered if anyone had some concrete (ahem) advice.

    Structural engineer has surveyed all walls in the basement and recommended areas for repointing, so thought I'd explore making the older damp proofing more efficient in the process.
     
  5. ^woody^

    ^woody^

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    I would not rely on a structural engineer for anything else other than structural matters. Its not his area of expertise (unless he has had specific training in waterproofing).

    The crucial thing with basements is not just knowing the standards and potential methods, but to correctly apply them. The amount of risk you are prepared to accept should relate to the use of the basement and the potential impact of failure.

    So for instance it may be relatively dry now not because the ground is dry, but because of good evaporation. Coat the walls, and there may be a lot of ground pressure to cope with so would say a drained cavity be better? Or say, perhaps a dash of render just like the existing and an extract fan for humidity might be enough for the purpose?

    A building regs application will be required not just for any conversion and use, but for the waterproofing system.

    Your questions are IMO too specific to individual circumstances and can't be answered other than in general terms. But FWIW, adding render adds another potential point failure of a tanking system. Trying to protect joists from water build up is dealing with results of a problem, and not the actual problem - rot can still happen. Any wall underground is a source of water penetration, so if left untanked, then how is moisture to be dealt with?
     
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  7. antz87

    antz87

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    That all makes a lot of sense.

    I've put some forced ventilation in there already, even though there was no mould buildup before. Water ingress is minimal so perhaps the right breathable surface and decent ventilation is the answer.

    Cheers
     
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