Reducing concrete floor for underfloor heating

Discussion in 'Builders' Talk' started by Jayme2189, Apr 10, 2020.

  1. Jayme2189

    Jayme2189 Member

    Hi, after years of saving just bought our end of terrace 3 bed semi :) Now renovating it. Doing work with my sparky brother and getting professionals when needed.

    I want to put underfloor heating in, and take the radiators out for space(and energy efficiency). But ceiling is already low at 2350mm, I'd rather lower the concrete than retrofit make it even lower and have to worry about doors and staircase.

    I plan to take the concrete floor down 35mm, put a fresh chemical damp proof membrane. Then use low profile boards and lay either a UF heating screed or direct apply flooring.

    Just had some builders in to take down a wall and put a padstone and RSJ in. When they excavated the stone got to see the concrete.

    The intention is to use a floor grinder with diamond cutting over a long weekend to take it down and level it.

    From the pictures can anyone see a problem with doing it to the concrete floor? Or general problems from this approach?

    I'm redoing all the pipework as it was complete mess mixed and all over. I will get a Corgi Engineer to connect the boiler into it's new place and UF after all pipework is done.

    Attached Files:

  2. nigel willson

    nigel willson Screwfix Select

    You would have to remove all of the floor. You need insulation underneath your underfloor heating. Then suffice to screed depth for strength and . The screed becomes ur radiator
    WillyEckerslike and David Hatim like this.
  3. David Hatim

    David Hatim Active Member

    Hmm! The big problem you have is the efficiency of the UFH, if it's not insulated well you will be heating up the world beneath your house, and your energy bills will go through the roof. We have retrofitted UFH quite a few times, your not going to like this, but I'm going to advise you on the best way to get good efficiency anyway. Dig down 390mm, compact 100mm of hardcore, cast 100mm of concrete, lay a damp proof membrane, 125mm of ridged insulation, fit UFH pipes and 12.5cm perimeter insulation, 65mm sand/screed. Said you wouldn't like it!
    Jayme2189 likes this.
  4. Jayme2189

    Jayme2189 Member

    Well I have access to a Jack hammer and cement mixer so wouldn't have to hire one Win! What's not to like? :p

    Not disagreeing but trying to learn more. I thought the screen panels that retrofit systems use were insulated?
    As normally they'd sit onto of the concrete? Why if I shave a few centimetres of it changes?

    Question that follows on from this. If I take a jackhammer to the floor and go down 40cm or so. Am I damaging the structural integrity and risk causing damage to the house?

    Attached Files:

    David Hatim likes this.
  5. David Hatim

    David Hatim Active Member

    Good couple of questions. Firstly the overlay panels are only say 40mm think, they may be assuming that there is more insulation beneath the existing concrete, but you don't, 40mm is not enough on its own, 125mm is over 3 times thicker, and is the minimum amount in a new building for it to work efficiently. As long as you don't dig down below the existing footing it will be fine. I'm just trying to give you the best advice. Good luck!
  6. Abrickie

    Abrickie Screwfix Select

    It does look like you have a 50mm screed on top of the floor slab
  7. Jayme2189

    Jayme2189 Member

    Sorry, is that a help or a hinderance?
  8. Hfs

    Hfs Screwfix Select

    Won’t be worth a **** without at least 100mm of celotex. That’s a lot of work and expense to dig out that much.
  9. Jayme2189

    Jayme2189 Member

    Just to say to everyone thanks for the advice. I have headed your warnings and decided against this.

    Looked up the options, after what was said discovered the retrofit systems seem more for suspended timber floors which doesn't get stated on the sales page "suitable substrate". Had to be told here and find an eco efficiency site essentially detailing that point. The extra cost, added delay, inability to get a dust respirator, and that the misses and brother both thought it was not worth it to begin with means its firmly dead and buried downstairs :(

    I might still install it upstairs between the joists where I can add insulation but probably not worth the hassle having the 2 systems and cost just for upstairs. When it was downstairs being open plan I was worried about.

    Ill upgrade the radiators and put a electric fireplace in. But thanks for saving me a costly mistake!
    Hfs likes this.
  10. Abrickie

    Abrickie Screwfix Select

    If you intend to proceed with your original method you don’t need a concrete planer or breaker just a shovel and a little effort, so it’s a help. But personally I’d be digging the floor up, 18mm or even their 25mm panel it’s just not enough for an uninsulated concrete floor
    Jayme2189 likes this.
  11. CGN

    CGN Screwfix Select

    Stick with radiators. Better things to spend the money on.
  12. Jayme2189

    Jayme2189 Member

    Ah okay. Thanks for the clarification. Decided against that plan as just cost x benefit really not worth it. But useful to know should make it easier to chase out fresh radiator pipes then.

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