Flooring advice - DPM, SLC, Insulate?

Discussion in 'Getting Started FAQ' started by Matt Rooney, Nov 23, 2019.

  1. Matt Rooney

    Matt Rooney New Member

    Hi everyone, I’m after advice on the correct steps for the following desired outcome.

    We are converting half of our garage into an internal room. The current concrete floor is uneven, slopes into the middle of itself probably by about 100mm from the sides. Not sure on dampness but the garage is single skin currently on two sides. The garage floor is also roughly 250mm lower than the house we want to level it with.

    Question I need help with is: Are these the correct steps to take to complete the floor before carpeting?
    1. DPM
    2. SLC (to even the existing floor)
    3. Insulation boards (150mm Celotex or similar brand)
    4. SLC or Screed poured directly over the boards
    5. Carpet underlay
    I am in no way experienced with this but the above is what I’ve cobbled together from other posts online, so looking for confirmation on those steps and any advice if I’m way out.

    Thanks everyone!
  2. rogerk101

    rogerk101 Screwfix Select

    Waterproof SLC to level out the concrete floor
    150mm Celotex
    18mm or 22mm T&G chipboard boards glued together but leaving a 5mm gap between its edge and the walls
    Finished flooring
  3. Matt Rooney

    Matt Rooney New Member

    Thanks Rogerk101 - how would the board be secured to the insulation and would the floor be almost spongy to walk on without the solid top layer?
  4. rogerk101

    rogerk101 Screwfix Select

    If the solid base is perfectly flat (which it would be thanks to the SLC), the insulation itself is rigid enough not to wobble or feel spongy. However, with the 22mm chipboard (even with 18mm chipboard) on top, there is no way you'd feel any sponginess. In fact the floor would be more solid feeling than any upstairs floor based off 200mm timber floor joists spaced at 400mm centres.
  5. Matt Rooney

    Matt Rooney New Member

    Makes Sense - how would the chipboard be secured to the insulation?
  6. rogerk101

    rogerk101 Screwfix Select

    It isn't. It doesn't need to be. It needs to be able to expand and contract with changes in humidity and temperature and it will do that at a rate that is different from the insulation, so micro movement needs to be enabled. If you did decide to glue it to the insulation with some sort of adhesive, the inevitable movement of the chipboard would simply rip the insulation, which doesn't provide much resistance to shear forces.
    You do need the 5mm gap around the edges though, otherwise the chipboard risks buckling.

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