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Discuss Wet underfloor heating in the Central Heating Forum area at UKPlumbersForums.co.uk.

  1. CrazyFool

    CrazyFool New Member

    Hey guys

    Ive carried out some rough calculations as to how much of my concrete floor I'll have to break out to fit a UFH system and I'm concerned about the amount of work involved and the number of skips I'll need and was wondering if I can reduce the amount of digging I need to do.

    I was originally thinking of digging 270mm:
    100mm concrete base
    100mm PIR insulation board
    70mm screed.

    Is there any way I can skimp out on any of the above figures?

    My kitchen is 2.4 x 3.1 m
    if all goes well I want to Start digging my conservatory next.

    Thanks again
  2. Craig Watson

    Craig Watson Plumber GSR

    You may need to dig out further, depending on whats under your floor now you may need to add 100mm of hard-core too ;)
  3. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    If you’re cutting corners now then don’t be surprised if your ufh is ineffective
  4. CrazyFool

    CrazyFool New Member

    Thanks for the replys fellas
    Its a solid concrete block under the kitchen.
    What do you recommend I line the floor with after I dig deep enough and how thick should I make this layer?
    I've heard of sand being used, sand/cement, concrete, etc. But not sure what the right answer is.

    Thanks again

    Last edited: Feb 12, 2019
  5. Craig Watson

    Craig Watson Plumber GSR

    How it should be is
    100mm hard-core
    Damp proof membrane
    100mm concrete
    100mm minimum insulation
    60-70mm screed.

    It's alot of work and expense to retrofit underfloor heating. Unless you want to waste your money, do it properly and don't scrimp on materials.
    • Agree Agree x 1
  6. Pickwickpick

    Pickwickpick Plumber

    As above, prep is critical with this sort of thing, the layer of compacted hardcore is critical to everything that goes above it. You will kick yourself if in 5yrs time you've got split UFH pipes, or split DPM due to movement/concrete cracking! It will cost you considerably more time & money than doing it right now.
  7. CrazyFool

    CrazyFool New Member

    Thanks for the advice guys
    Couple of questions

    1) what recipe do you recommend for the 100mm concrete layer? Are there any quick drying concrete options?

    2) what material do I need to use for the hardcore layer? Is there any scope to throw some of the chopped up concrete that I've dug back down again?

    3) do I need to dig out the whole kitchen (even underneath where the units will go) or can I just dig in the actual floor space?

    Thanks again
  8. Pickwickpick

    Pickwickpick Plumber

    For slabs I've always done 1 cement:2 sand:4 ballast, there are fast setting cements out there but I have never used them on slabs so wouldn't like to say either way.

    You want MOT1 for the hardcore layer, in theory you could if you broke it down to the right size but personally wouldn't.

    You need to do the whole kitchen.
  9. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    It’s very easy to tell from your thread that you are trying to do this quickly and by cutting corners. underfloor heating is not a quick job particularly when you are doing it retrospectively. Take your time do it properly to achieve the best results I wouldn’t use something quick drying as you want a nice even setting for when you start to gradually increase the temperature into the underfloor piping
    • Agree Agree x 2
  10. CrazyFool

    CrazyFool New Member

    Thanks guys.
    I'll be sure to take your advice on board.

    Just to make sure I've understood correctly. Do I still need the hardcore/concrete layer if I've already got a concrete slab on the floor? It's not like I'm building on soil. I just want to level the concrete slab that I've broken into to ensure I don't puncture the DPM and so the kingspan can sit on the floor straight.

    Thanks again
  11. Craig Watson

    Craig Watson Plumber GSR

    Is the concrete slab 180-190mm below your finished floor level?
  12. Murdoch

    Murdoch Active Member

    Op are you really thinking of doing this in a kitchen, and keeping the existing kitchen?
  13. CrazyFool

    CrazyFool New Member

    Absolutely not.

    So far I've taken 100mm concrete layer off. And there's another 100mm layer of compacted sand and broken brick.
    Who knows what the next 100mm layer will hold although I wonder if the current bed of compacted sand and broken brick will be able to support fresh concrete, insulation, screed etc.

    Cheers again
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