laying new floor on concrete slab (1910?) no DPC

Discussion in 'Building' started by rexwexford, 15 Mar 2021.

  1. rexwexford

    rexwexford

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    hi all

    i have a concrete slab in the rear kitchen which doesnt appear to have any DPM (probably well before they existed) There was evidence of a bitumin lining to the lower 800mm of the exterior wall, but that was in pretty bad shape too and clearly not doing much.

    The floor is circa 100-150mm below the adjoining room. I'd like to have LVT flooring in both rooms. Im wondering how best to proceed with putting in the new floor on the basis it hasnt got a DPC. The slab seems pretty dry but im assuming i should lay a new floor direcly onto it and would be better off insulating and lining it in some way?

    (Few pics attached before floor was completely out...)

    would the best process be to lay insulation directly onto slab, and then run a DPC over the insulation and around the perimeter of the walls up to a certain height to tank the room? This will obviously raise my internal FFL so would there be any issues in raising it deliberately to tie into the height of the dinning room? im cautious that the slab level is basically the same as the outside level which i know is bad news.

    Is there a standard detail to rectify a slab without a membrane that i could consult?

    Thoughts welcome
     
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  3. rexwexford

    rexwexford

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  4. rexwexford

    rexwexford

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    upload_2021-3-15_19-40-18.png

    heres a quick sketch of what i was proposing. just FYI, the diagonal lines represent the ground under the slab, i presume there is no insulation under there. so

    - lay 50mm insulation on the slab
    - build a floating subfloor and then
    - lay the LVT on the subfloor.

    does this seem feasible? how high would you terminate the DPC if i cant tie it into the wall ?
     
  5. SpecialK

    SpecialK

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    Why wouldn't they be the same level? Seems odd...

    Is the wooden floor suitably vented or does the concrete floor get in the way?

    If it was my project I would DPM going above the DPC. As much insulation as I could fit and then screed.

    If there is no ventilation to the house floor then I would rip it out. Dig down, add vents and build a wooden floor
     
  6. rexwexford

    rexwexford

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    thanks for your reply. yes the concrete gets in the way of venting a large portion of it at the rear - hope the plan clarifies...

    there is one(!) airbrick to the back of the house. It seems very inefficient + looks like its been replaced with a more modern one at some point with tiny holes. I suspect this may have been done when the outside was rendered by someone trying to "improve" the damp situation in that corner

    There are two air bricks at the front which have much larger openings.

    i thought its quite common for the kitchen to be stepped down in old houses? I believe this was a deliberate design move to prevent to possible flooding if thats where the original water came in. not sure where i heard that from though..



    upload_2021-3-17_12-44-34.png
     
  7. rexwexford

    rexwexford

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    it does feel like it needs venilation - particularly in the corner under the stair...
    Theres no DPC to take the DPM above... so do i just terminate it 150 above the external level?
     
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