Concrete slab on a concrete slab?

Discussion in 'Builders' Talk' started by Wottsy2000, Jun 30, 2020.

  1. Wottsy2000

    Wottsy2000 New Member

    Hi folks,
    (Sorry for no pics, will add later)
    We are currently looking at ways of getting a bit more space in the house. Loft conversion is preferred but we are trying to suss out whether a rear extension would be easier and cheaper.

    We currently have a lean-to conservatory type thing built by previous owner. It is connected to the outside toilet and has a concrete slab floor. Can’t tell how deep it goes but will investigate this week. The slab is about 180mm lower than finished floor level inside.

    So I’m wondering whether it’s possible to just pour another slab over the top of the existing one, rather than destroying it. We would put dpm and insulation down first. The new extension would be larger so would still need to have hardcore and sand round the edge of current slab. Is this doable? With 50mm of insulation, is 130mm enough clearance to fit a slab and a very thin screed?

    Alternatively, would It be acceptable to simply suspend a timber floor above the existing slab. The regs I found say suspended wooden floor needs 150mm clearance above site ground level, which should be concreted over. Assuming 220mm joists, and 18mm boards, this would put unfinished floor at about 390mm above existing slab and about 210mm above inside floor level. Little step up but no big deal.

    Also, don’t suppose anyone could hazard a guess on the chances that the existing walls of the outside toilet are built on a foundation adequate for a timber frame extension with brick skin? It’s side wall is a party wall with neighbours old outside loo (now converted to extension). Is it likely that it will need underpinning somehow?

    Finally, the large air brick on the rear of our house (currently in the lean-to) is partially above inside finished floor level. We have solid brick walls so can’t really tell how that’s ventilating the subfloor. Is it just oversized?
  2. Northernlad999

    Northernlad999 New Member

    We did exactly this when building our extension, we extended over garage area which had a slab already down but a lower level, so we used Dpm, 100mm cellotex and then dpm, then a 100mm concrete slab over the top of it, buildings inspector said a concrete slab under a slab is the best sub base to have.
    Wottsy2000 likes this.
  3. Dan Parkinson

    Dan Parkinson Active Member

    It will most likely be venting the cavity. The inner skin will have missing bricks lower down to create airflow throughout

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