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Suspended timber floor or concrete slab?

Discussion in 'Building Regulations and Planning Permission' started by zedox, 27 Aug 2020.

  1. zedox

    zedox

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    Our semi was built in the 1900s with a traditional edwardian layout and modern extension at the rear. We moved our kitchen to the extension and now renovating the old kitchen. The "old kitchen" is suffering from rising damp on 3 walls (excluding the external wall) caused combination of things... failed/bridged DPC and a poorly constructed concrete slab - only 1-1.5" thick and no DPM. There is only 1 tier of brick (60mm) between the DPC and the ground (inside and out). The finish floor height throughout the house is just above the DPC line. Dimensions are 3m x 3m. 1 wall is external facing, the other 3 internal.

    We had a damp survey done and have been advised to replace the concrete slab and to "Vandex" the walls and here is where I have several questions.

    Should we pour a new slab or construct for a timber suspended floor?

    Option 1 - Suspended timber
    • Concrete oversite - Based on the damp issue, should we lay a concrete oversite or shall just lay 50mm of inert fine aggregate?
    • Internal ground level - Regs state the ground covering should be "above the highest level of adjoining ground". This isn't possible once you total the joist depth (97-170mm) and a 150mm void. Does this matter, is there a solution to overcome this?
    • Ventilation - There might be poor cross ventilation as we can only install air bricks one side. Is leaving a small gap between the floor/wall opposite side sufficient?
    • Dwarf wall - Go for 38x97mm timbers with a central dwarf wall OR 38x170mm and no dwarf? Can the dwarf wall sit on the oversite or does it need a deeper foundation?
    Option 2 - Concrete slab
    • Should we pour the slab to below the DPC and bring the floor level by using timber OR can we lay the DPM up the wall, overlap the DPC and screed to the desired floor level?
    Thanks in advance!
     
  2. Notch7

    Notch7

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    You need to be sure you have sorted out the damp issue before you do either method.

    I would start by digging down by the external wall and see where the foundation is, as it might cause an issue with a solid floor.

    You need a fair bit of thickness for a new solid floor

    150mm type 1, bit of sand blinding, DPM, 100 concrete slab, 70mm celetex (nb to building regs requirement so could be more), 70mm screed = 290mm reduced dig measured from underside of floor finish.

    Mind you suspended is probably 50mm slab, 150mm void, 150mm joist

    I'd be nervous of suspended if you can't get cross ventilation.

    BTW it has to be building regs compliant.

    You might want to run your services etc when you floor
     
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  4. mrrusty

    mrrusty

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    Rising damp? maybe not. I'd put my money on condensation. I have recently finished (1 year ago) a 2 year refurbishment of an edwardian semi.

    When we inherited it, back room was flags on sand, old fella had had a coal fire every day for ever, and the bottom of all the walls, internal and external was wringing wet - like soaking.

    We lifted the floor, took all the plaster off the walls and without any other intervention the walls became absolutely dry. If this was the proverbial "rising damp" they would have remained damp - perhaps even damper with no heating. I believe it was condensation cause partly from living in the room and partly from water vapour evaporating through the unsealed floor and condensing on the cold lower walls

    We installed a concrete floor with insulation etc, internally insulated (our walls are cavity - and this method was discussed with our BC) and everything is AOK. I'm not a believer in "rising damp"; I think most cases are in fact condensation on the lower part of the wall, which is, of course, likely to be the coldest and therefore most likely to attract condensation.
     
  5. zedox

    zedox

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    Thanks. That's the thing, either way we'll have to dig down a fair amount with either method! We do have BC involved so I'm just waiting for their advice too. Also planning to run services within the floor.

    I do agree, I don't really believe in "rising damp" but more in building defects that cause damp to travel up the wall... such as failed or bridged DPC which we have! There was one corner of the room that had been treated with a damp proof slury and this plaster was absolutely bone dry. Removed the plaster and the brick behind it was saturated. Either way though, whether it's damp rising through the floor and condensing on the walls, or rising up through the walls because of the bridged/failed DPC the slab has to come up and be done properly ;)
     
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  7. zedox

    zedox

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    We've dug down 400mm from desired floor height to the bottom of the wall footings. Not going any further. We're actually about 200mm below external ground level. The soil is very damp, you can see moisture on the surface.
    I'm not sure what ground covering is best here? 100mm concrete oversite or 50mm of fine aggregate on a 1200dpm. I understand the floor covering should be above ground level but we don't have the height to do this.
     
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