Self levelling compound over floorboards

Discussion in 'Builders' Talk' started by Jacopo, Nov 22, 2019.

  1. Jacopo

    Jacopo Member

    Hello, I’m planning to use self levelling compound over a timber floor

    I will use this

    The instructions say it can be used directly over floorboards, however the general public opinion is that you should board using 6mm ply first

    I’m struggling to find 6mm ply at a decent price (there was a B&Q clearance sale at 7£/sheet but it’s out of stock)

    Do I really need the ply?

    Whatever substrate I end up screeding on, how should it be primed?
  2. koolpc

    koolpc Screwfix Select

    I would never use it over floorboards. Only over a concrete floor myself.
    pvcu_king likes this.
  3. Hans_25

    Hans_25 Screwfix Select

    I'm with kools on this, however it says this screed is for use over underfloor heating. Hence I'd say its not suitable to lay directly over floorboards, but over UFH which would typically have some kind of insulating layer and board first to house the UFH pipes/cables.

    Why are you wanting to lay self levelling compound over a timber floor, what's the problem you're trying to solve?
  4. Jacopo

    Jacopo Member

    My subfloor is not flat, so when I lay my engineered wood it squeaks and feels bouncy
  5. Dave Marques

    Dave Marques Member

    That 3240 is recommended by Uponor for their low profile Minitec ufh system. It only needs 3mm above the pipes and NO insulation underneath.
  6. Hans_25

    Hans_25 Screwfix Select

    koolpc likes this.
  7. Jacopo

    Jacopo Member

  8. Hans_25

    Hans_25 Screwfix Select

    With that kind of difference, most self levelling compound isn't going to work either. I'd think about using plywood over it, may be different thicknesses to make it more or less even as needed, then 18mm ply on top.
  9. Jacopo

    Jacopo Member

    it’s not like there is a step that I can easily bring to level with the right thickness of plywood

    imagine a long spirit level across a room: in the middle there is 1cm gap, so that gap is going to stay there even after I board it with plywood; then there are slightly changes of slope where the joist meet over a downstairs wall
  10. koolpc

    koolpc Screwfix Select

    Imagine the self levelling compound cracking when weight applied to it because the boards below it move and bend etc!

    You need to come up with a better solution.

    Hans_25 has posted a couple of good ideas.
    Hans_25 likes this.
  11. DIYDave.

    DIYDave. Screwfix Select

    I had the whole downstairs of my house prepped as the OP suggests before Amtico was laid

    Suspended timber floor

    Damaged/loose boards replaced and/or fixed down securely

    No major bounce in floor and all solid, but old boards, been up and down several times over history of house for plumbing/electrics so not pretty !

    6mm flooring ply over the lot (usually more like 5.5mm but ....,,) held down with a staple gun, even coverage across the ply, especially edges but ...... loads and loads of fixings across total board area

    The ply will deform to the dips and hollows of floorboards but no problem, ply helps to tie the floor together and improve stiffness whilst closing any gaps between boards. The self leveller goes down pretty runny so will disappear down gaps between boards !

    Mapei Ultraplan used - this is made for timber floors and is flexible and has fibre reinforcement - it’s made for this job

    Mapei Ultimate is for solid/concrete floors, different beast entirely, different spec, around half the price of Ultraplan - great product but DON’T use it on this application

    Try local woodyard for ‘flooring ply’ 8x4 sheets. It’s not a finish timber so a lesser quality but perfect for flooring. Timber from a timber yard is always cheaper than from a ‘shed’ I’ve found

    Go for it :)

    PS. Major defects in floor, split boards, bounce, loose joists/boards, etc all needs sorting before any further work

    Ply and leveller ain’t a miracle cure for the above but for giving a flat surface prior to flooring, then yes
    Last edited: Nov 23, 2019
    Rob Wilson likes this.
  12. Jacopo

    Jacopo Member

    Thank you!

    how did you prime/seal the ply?
  13. DIYDave.

    DIYDave. Screwfix Select

    Hi there and sorry for the slow reply

    Take a look on the Mapei website as there’s loads of information there on all their products, prep required and how to use, etc

    Several Mapei primers available but for timber, they recommend Mapei Eco Prime Grip (catchy name eh) !!

    Again, full data sheets available on website

    I didn’t lay floor myself but used a local flooring company that was well recommended and established and did a superb job, covering 5 rooms

    They main guy actually recommended using SBR as a primer (not Mapei), it’s a latex based primer used for several different substrates, including timber floors prior to tiling, screeds, levelling, etc

    This saved us some cash which was welcomed and we felt confident in his recommendation

    8 years later, we’ll, it’s still stuck down !!

    Have a read up on the above Mapei data sheets and take a look at SBR

    DON’T use PVA :eek:
    It looks like SBR but different beast and not for this job
  14. Jacopo

    Jacopo Member

    I have had a quote and 2/3 of the price is mainly the cost of plywood and the labour involved in screwing it down

    So about 600£ if I want plywood + self levelling compound, just over 200£ if I have the slc screed directly on floorboards (including sealing all the gaps beforehand)

    The person I found to do the job says he would just screed over the floorboards, but it’s my call eventually

    So yes, I’m just trying to understand if spending 600£ instead of 200£ is going to give me any benefit

    Then floorboards are 18 mm tongue and groove ones ( replaced with 18 mm ply in a couple of zones where they had been damaged or taken up too many times)

    I also got this information from Mapei
    We acknowledge receipt of your email and can comment as follows.

    The primer we would recommend for use over plywood prior to applying Mapei U
    ltraplan Renovation screed 3240 would be Mapei Eco Prim T Plus or Mapei Eco Prim Grip.

    Whilst it is preferable to over board, it may be possible apply the Mapei Ultraplan Renovation screed 3240 onto the floorboards providing they are clean, solid and free of all movement and deflection. Any holes/gaps would of course require to be treated using a preparatory type filling agent (builders caulk etc.). The prepared boards would need to be primed using Mapei Eco Prim T Plus.
  15. Jacopo

    Jacopo Member

    Ok, I’ve covered everything with 6mm plywood using 15mm screws at 15cm centers, no nails and no glue; then I had a “pro” self level it using Mapei Ultraplan
    I actually had to use the proper Mapei primer (he said there was no need) and he then “painted” the plywood with the leveller. I said PAINTED because he used 5 25kg bags for 36sqm.
    I had to use another 7 bags myself because he didn’t fill the low spots...

    Anyway, the floor is still empty (finished fllor still to be laid), but when every morning I go there and step on it, it makes a cracking sound, but it doesn’t come from the spot I step on, it always come from a different part of the room; if I walk on the entire area of the room a couple of times then it stops doing that, but if I leave it for one day and come back it starts again

    It seems to be worse when it’s cold

    I think I noticed this before the SLC was laid but I didn’t address it because I thought it would go away, instead it seems to be getting worse rather than better
    I repeat, it’s not the plywood or the floorboards rubbing against a nail or a screw, the noise doesn’t come from where I step...
  16. wiggy

    wiggy Screwfix Select

    Sounds like you have made a real bodge job of it. You could fire some screws in to the joists in suspect areas
    but I think you will have to live with it or rip it up and do a proper job.

    How much did you save?
    Last edited: Apr 4, 2020
  17. Jacopo

    Jacopo Member

    How much did I save on what?

    Fire screws in to the joists? Please read that again
  18. Jord86

    Jord86 Screwfix Select

    So in the order of things, did you prime the ply before the leveller was laid by your man? There was no need for it to be THE Mapei Primer, SBR would have been fine as an alternative, but some sort of primer was required. After the guy had used the 5 bags, what happened then, did you use the seven bags immediately, or did you wait until the next day or two then go over the top with the extra seven bags? If the latter, did you prime between layers of leveller?

    Personally, if a top finish was required I would have taken the existing floorboards up and either laid 18mm ply and potched about using latex over that, or more likely planted on timber alongside the existing joists and flattened the area out by packing and planing, then laid 18mm ply. Sounds like a lot of work, but weighed up against the cost of 12 bags of latex and 13 sheets of ply with maybe further remedial work needed, knowing there’s no comebacks would always be first choice.

    6mm ply shouldn’t really be used for any form of levelling substrate, it’s thinner than Victoria Beckham and 15mm screws aren’t long enough to even tickle the surface of the floorboards in order to provide a strong enough grip, 9mm at the very least should have been used, and glued down too. A lot of of DIYERS also just drive a screw in and they think that’s it, job done, but when overboarding a timber floor you need to be standing on the ply in the area you are screwing down in order to hold the ply down fully against the area you wish to fix it to. Small screws with a thin gauge don’t have the capacity to pull the ply tightly against the floorboards on their own, which can result in gaps between, and cracking, popping and squeaking noises, which is another reason expanding polyurethane glue should be used too, to bond and fill any gaps. Before all that though, the existing floorboards need to be scrutinised and inspected, and screwed down fully to the joists before a sheet of ply even enters the room.

    Note I’m not saying you did or didn’t do any of this, but it’s always a possibility no matter how careful you are, as is boarding an area of floor without realising there’s a small offcut or stray screw getting trapped between the layers and causing hell until it’s cut and lifted out o_O

    Do you have heating pipes under the area where you say it’s cracking? You say 36m as the area, what’s the span of the joists?
    wiggy and TheMorg like this.
  19. Jacopo

    Jacopo Member

    I did not glue the ply as I’ve asked here and there and nobody mentioned it was necessary
    The screws were 3.5x16 (not 15 actually) and I clearly observed the ply being pulled against the floorboards with each screw

    I did use a primer (Mapei Eco prim Grip) the night before the first 5 bags where laid
    I then used the other 7 but without primer in between, there was no need (written on the instructions)

    However I noticed this behaviour even before there was any SLC at all so it must have something to do with the plywood

    Yes, there are heating pipes but it doesn’t correlate with the areas that crack (all of it)

    It’s 3 bedrooms and a hallway and the max span of the joists is about 3m (they rest on top of an internal downstairs wall)
  20. Jord86

    Jord86 Screwfix Select

    Having just read the first page of ultraplan technical spec, it states maximum 10mm thickness in one lay, minimum 3mm thickness where floor is to be overboarded, and to prime between layers if laying when dry the following day, but no primer mentioned if a second layer is added after approx 3 hours, whilst lightly set.

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