Subfloor Prep Advice- Half Wooden/Half Concrete

Discussion in 'Subfloor Preparation' started by sprogy23, Jan 8, 2025.

  1. sprogy23

    sprogy23 New Member

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    Hi folks.

    Apologies if this has been asked and answered but did go through a lot of previous posts but couldn’t find one that answered my query.

    I am currently renovating a 1912 property and have a kitchen/dining room where we have removed a wall that seperated the two. One side is a suspended timber floor that has the original t&g 28mm floorboards covered with 18mm plywood (maybe overkill but added this for extra rigidity for screeding). The other side is a concrete slab. This is now 18mm lower than other side due to plywood i have installed. I have no history of the concrete slab material etc. it doesn’t appear to have bitumen on it and appears to be free from damp. There is a 500-800mm cavity under my subfloor.


    The wooden subfloor side is approx 15mm out of level over 4metres.


    We plan to install 15mm engineered wood herringbone throughout both rooms. My preference is to glue this to the finished subfloor as had floating floors in previous properties and never been fully satisfied with the end result.


    My plan was to fix down 18mm cement board (or similar) to the concrete slab side to take up to the plywood height. Then use self levelling compound over the whole space to get rid of the 15mm discrepancy. I’m unsure what prep work (primer/DPM-If needed) needs to be done due to having two different subfloor materials.


    Any advice/guidance would be greatly appreciated as the final flooring isn’t going to be cheap so want no issues down the line. If anyone has a better method I’d be greatful to hear it. Fairly confident DIY’er but all these screed specs/primers/DPM’s fries my brain a bit.

    Cheers!
     
  2. dazlight

    dazlight Super Moderator

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    Would have been better taking the floorboards up and using the 18mm plywood to the joists then overlaying with 6 or 9mm SP101 plywood. Then the concrete side will need an epoxy liquid Dpm. With smoothing compounds. To get the levels correct.
     
  3. sprogy23

    sprogy23 New Member

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    Firstly thanks for your response.

    Ah okay! I did actually have the floorboards up as insulated between the joists. But as they were in decent condition I just re-used them. My logic (maybe flawed) with over sheeting with 18mm was just to reinforce the floor to reduce any risk of “bounce” when screeding level.

    So as it stands now, would you suggest I apply expoxy DPM to the concrete side, then fully screed the room with smoothing compound? I could also take up the 18mm the the concrete side with Ardex NA levelling compound with aggregate. This says no DPM is required. Then I could apply a smoothing compound throughout whole room to get the level correct? This is where it’s a bit of a minefield as unsure what screeds I can use directly onto 18mm ply as Iv not used SP101. As my level is about 15mm out on the ply side I’d rather have something I can apply once as opposed to multiple layers.
     
  4. Ryanf4888

    Ryanf4888 Well-Known Member

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    You won’t get anything to go that deep in one hit, it will also need an epoxy dpm having been built in 1912
     
  5. sprogy23

    sprogy23 New Member

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    Yeah the house was built in 1912 but have no way of knowing with my limited knowledge if the concrete slab is original or was a later addition. As mentioned, the slab is a minimum of 500-800mm suspended above the ground, and no signs of damp so wasn’t sure if DPM was required. But happy to ebb on the side of caution and use one.

    So you wouldn’t recommend products like Ultraplan 3240?
     
  6. dazlight

    dazlight Super Moderator

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    Do the ardex na build up to get the concrete side smooth.
    then ardex DPM1c over it.
    then a fibre compound over the lot at 5mm.

    But massive risk screeding over 18mm plywood as it really needs 6mm sp101 or any other flooring grade plywood.

    are you joists moving ? It would be been best to sort them out 1st as floorboards shouldn’t need 18mm plywood on top of them.

    It’s all about the bond strength for gluing wood down.
     
  7. dazlight

    dazlight Super Moderator

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    Can you see a visqueen sheet under the concrete ? Any concrete before 1990 really needs a Dpm I’d say.
    It’s good if it’s high above the ground though as that’s a good water table.
     
  8. sprogy23

    sprogy23 New Member

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    No the joists are secure. I replaced the cill plates on the dwarf wall and some of the joists due to ends of some of them crumbling. So the floor is now very secure. I didn’t know there was a flooring grade ply prior to installing 18mm as was under impression you could screed on any ply/wooden subfloor going on what I read on the specs for some self levelling products. I clearly should have done more research beforehand as screeding/self levelling was an afterthought as it’s a huge rennovation project.

    I can’t see any visqueen sheets but tbh I haven’t ripped any of it up to have a good look. I can’t imagine it would have been done post 1990’s given the decore of vinyl underneath the top layer of vinyl. Screams 1970’s to me so concrete must pre-date that.
     
  9. merit

    merit Well-Known Member

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    I would go with a ardex dpm1c then cover it with Na. You can then bond your cement board over that. Build up as much of the timber side with ply first as most fibre reinforced compounds cant go that deep over suspended floors. Also most important…you really want a layer of that ply bridging the join between the concrete and timber subfloor so
     

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