Damp proofing a concrete slab

Discussion in 'DIY' started by Spacehopper383, Apr 25, 2010.

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  1. I'm in the process of laying a concrete slab for the base of a outdoor store with two compartments. One for storing my daughters play equipment and the other for storing chopped wood for the wood burner.
    I've got a roll of DPC to lay under the blocks, but has anyone got any advice on damp proofing the rest of the slab? I have two ideas.
    1. Painting the concrete with bitumen before screeding with a sand cement mixture.
    2. Mixing the screed with a waterproofing admix before laying.

    Any thoughts out there on either method.
  2. I'm a builder. For something like this, you just need to put a membrane underneath the slab really. The proper stuff is expensive but any bit of polythene that fits in the space underneath the slab will do or you might find a cheaper membrane from B&Q or somewhere.

    If you were building something habitable, it would be a different ball game but for a storage area, why go for anything other than what I have suggested.
    • Bullshit Bullshit x 1
  3. Thanks rgj, would cutting up the All-in ballast bags overlapping and duck taping together work?
  4. For what you are talking about yes. If it's just a slab in the garden for a shed or some simple structure, do you really need any damp membrane under it, but what you are talking about would be good enough for that and, it's almost nil cost.
  5. If where you are going to lay the slab is pretty wet I would probably bring the DPM up the sides of the slab.
  6. This is just another thread about sheds..
  7. Don't let the Duke hear you say that!!!

    A shed is built of wood and I'm building this of block so it can't be a shed and therefore this can't be a thread about sheds :D

    Edited once because I can't believe that the banner at the bottom of the page is selling sheds, they are all out to get me!!!!
  8. I'm a builder. The product I use is called Bakor 770-06. It costs about $70.00 Cdn per 5 gallons. It is a polymeric bitumous membrane that cures through solvent evaporation. Only thig is, you have to cover it with cheap foam insulation befor backfilling to avoid punching holes in the membrane.
  9. ...bituminous....

    Also, I would never even bother with waterproofing a slab for this application. Waste of time and money. The Bakor I spoke of is more for leaking foundations. Just throw in super 6mm under the plates of the framing and you shoulsd be fine. Hope this helps.

    Also, you will have a VERY bad time of it if you try to adhere sandmix concrete to tar. WILL. NOT. WORK. Parge the slab with portland mix (2 portland to 1.5 units of sand) will provide a hydraulic seal. I think you might be overthinking this
  10. the stuff you need is called VISQUEEN

    Visqueen DPM - Visqueen Building Products..Its a durable thick polythene membrane and is used throughout the building industry in the UK...comes in rolls...quite pricey but does what it says on the tin..unless you rip it...
  11. Provided that you lay the slab all in one go and have a reasonable thickness, having no joints, the slab should be pretty well watertight without any further messing about.

    Any water entry is likely to come from the blockwork. I don't know what your plans are there, but a masonry paint should stop water entry through the blocks and mortar. That just leaves the very bottom of the blockwork, where it sits on the slab... You can avoid water entry at this joint by rebating the perimeter of the slab. You wouldn't need a high rebate, so the easiest way would be to support scaffold boards horizontally such that the tops of the boards are at the level that you want the top of the slab. Hold them in place with some battens fixed to pegs while you're concreting to prevent them lifting. If you're smart, you'll put a very slight outward fall on them so that any water that gets on the rebate (or on top of the slab) will run outwards. Make sure that the concrete is vibrated well to avoid honeycombing beneath the boards.

    When the concrete of the slab has cured, remove the boards to leave the rebate. When you build the blockwork wall on the rebate, leave a bit of the mortar out at the bottom of every third vertical joint to assist drainage. (You could install brick weeps, but I doubt that you'd gain much over and above just leaving gaps. As an alternative to missing out the mortar, you may find it easier to just drill out the mortar later).

    edited to add:

    Before casting the concrete, make sure that you've oiled the boards so that you can remove them later - alternatively, wrap them in clingfilm, then you can recycle them, either for your next project or for use in your woodburner.
  12. Before you put the slab down, I've got this small child sized object that needs ..........
  13. any old bit of polythene? Feckin cowboy - I'd run you off my sites!