Levelling Concrete Slab up to 40mm thick

Discussion in 'Builders' Talk' started by Flewkey, Mar 4, 2020.

  1. Flewkey

    Flewkey New Member

    Hi all,
    I'm levelling a concrete slab in a garage conversion in preparation for a floating floor.

    There is about a 40mm variance between the highest and lowest point.

    What type / brand of self levelling screed would you recommend?

    Can I fill it in one pour?

    is there anything else I need to consider before starting?

    I will of course prime the area and seal any gaps.

    Many thanks in advance.
  2. DIYDave.

    DIYDave. Screwfix Select

    40mm (at its lowest point) of self leveller is gonna work out expensive - a sack doesn’t go that far

    Mapei Ultraplan can pour at 40mm max in 1 hit. It’s a fibre reinforced flexible SL, which you don’t actually need over a concrete slab but, you can pour in 1 go so convenience but your paying for that privilege

    Mapei Ultimate is way cheaper, not flexible but fine over a slab but, max 10mm pour in 1 hit. Further layers can be added after drying time

    Both these SL on special at SF at moment on 3 sack multi buy

    Could look at taking worse areas out with screed/sand cement mix as much cheaper and doesn’t need to be perfectly flat as SL over top to finish

    But minimum depth for screeds/concrete mix at around 25mm otherwise it may crumble and lack strength. Certainly ok though for deeper areas

    Whatever you go for, seal slab with diluted SBR mix before you pour. This will bind a dusty surface and give a great bond between slab and concrete and/or self leveller

    I’m diy only but this is a job I’ve done a couple times over concrete slab and all went well

    Others may have other ideas :)
  3. Flewkey

    Flewkey New Member

    Thanks Dave.

    I'm new to this, but I'm doing the project for someone else so time is money and all that.

    Using the Mapei Ultimate in 4 x 10mm layers sounds like it will involve 4 separate visits as it needs 24hrs drying time?

    Also when it comes to the sand cement screed or any compound that has a minimum depth such as the 25mm you mentioned.

    How does it work with the thinner depths on the edges of the depressions that will be less than 25mm?

    see diagram:

    The areas marked X will be less than minimum depth but are necessary to achieve a level.

    Am I over thinking this?

    Or will these areas flake , crumble.

    25mm depth diagram.png

    Thanks for your response.
  4. DIYDave.

    DIYDave. Screwfix Select

    What I suggested was using a sand and cement mix (can talk ‘recipes’ later) to take up the bulk of the dips

    This has no max depth limitations but the opposite, say a minimum depth of 25mm to give the base strength. Could add fibres or modify with additives to go a bit thinner but you don’t really need to go thinner here

    Self levellers don’t really have a minimum depth, can’t actually feather out the edges but I would think a couple of mm pour would be fine

    What I would suggest is use sand cement mix for the deeper sections, not spreading thinner than 25mm but leave 10mm below finished height then pour self leveller over the lot

    Sand and cement is cheap, easy to work with, don’t need to be perfectly smooth, will greatly reduce the amount of SL you need

    Allow each layer to cure before pouring the next and SBR coat between everything to aid the bond

    Sounds complicated but isn’t, setting levels is the hardest part
  5. Mr Rusty

    Mr Rusty Screwfix Select

    Apol for slightly OT

    Q for DIY dave. Currently building a garage/workshop which will have a concrete slab. Raw concrete finish will be fine for the use, but I need to seal against dust. Is this a good solution? and can you give any info from experience how this would work. If I an going to paint the floor would dilute SBR help? How dilute?

    Not a project I have done before so any tips welcome.
  6. stevie22

    stevie22 Screwfix Select

    Most concrete is dusty because its laid far too wet so it's easy to spread (but then tricky to barrow!!).

    Result is excess water bleeding to the surface pulling cement particles with it resulting in a weaker job with a dusty surface

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