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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorsteveray
    • CommentTimeApr 6th 2021
    I am renovating a late 1970's bungalow which had joisted floors which were supported by small sleeper walls built off an oversite concrete slab which was typical then.
    We want to remove all the joisted floor construction and build up the void from the oversite slab to finished floor level screed incorporating underfloor heating pipes.

    Normally if starting from scratch we lay a 100mm concrete base over compacted hard core, DPM, 100 Celotex PIR insulation, UFH pipes followed by 75mm Screed.

    I have core drilled the oversite slab in several positions and it is 100mm > thick overall and it is 300mm below finished floor level.

    Is there any reason why I need to pour another 125mm slab of concrete on top of the existing oversite slab?
    ie. + 125mm new Concrete Slab + DPM+100mm PIR Insulation +75mm Screed = 300mm
    or is there any reason the following would not produce a stable solid finished floor?
    +125mm Well compacted blinding topped off with sand or quarry dust(instead of concrete) + DPM +100mm PIR +75mm Screed = 300mm

    This would save quite a lot of time and money, but I don't want problems in years to come.

    Would be glad of any comments.
    • CommentAuthortony
    • CommentTimeApr 6th 2021
    Blinding is dodgy as it can percolate into the hardcore leaving voids, I never use it.

    Should be ok if sheets sit flat
    • CommentTimeApr 6th 2021
    Why not 225 mm insulation? EPS say instead of PIR. Or maybe 200 mm or so plus some T&G sheets to guarantee a flat surface for the screed.
    Posted By: steverayI have core drilled the oversite slab in several positions and it is 100mm > thick overall and it is 300mm below finished floor level.

    Chectkthe quality of the oversite slab, often the oversite was short on cement as it was not meant to be a load bearing slab just a covering for the subsoil.

    I agree with djh - EPS instead of PIR, - usually better bang for buck if you have the space.
    • CommentTimeApr 6th 2021
    Posted By: tonyif sheets sit flat

    Thinking more, this is going to be the killer whatever solution. The sheets need to be flat with no voids so the screed gets good support everywhere, otherwise it will crack.
    • CommentAuthorrevor
    • CommentTimeApr 7th 2021
    Any mileage in considering bison beams and blocks as a base to work from. It would spread the load on your oversite layer and be level. You do not state how big the area is so handling beams may be awkward. It will take up about 150 mm of your depth so would leave 150 mm for your insulation and screed. You can get back some 25mm depth if you use liquid screed instead of traditional "dry" screed. What is your floor finish going to be, if tiles would thoroughly suggest you include a flexible membrane.
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