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How to prepare base before screeding?

So, we are renovating our ensuite, taking it back to the bones (and even replacing some!)

We are currently taking up the old flooring - think, 1980's original tile, tile on top of that, and no waterproofing 😬😬😬) Screed on screed on "what is that mixed in" 🤣

My question is, how flat/perfect do we need to get the foundation layer before we get the guy doing our rescreeding in? He is currently on family vacation interstate and I refuse to contact him while he's on holiday.

I think we might have to get back to original foundation, as flat an smooth as possible? We are doing the demo work ourselves, to try and save a few dollabucks, as we're getting the pro's in for everything else.

Any input/advice would be welcome!

Pics of the archaeological dig site......

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MitchellMc

Hi @Kelli-H,

You’re essentially aiming for a solid, well bonded substrate that sits below the final screed height. It doesn’t need to be perfectly flat or pretty at this stage. The whole purpose of the new screed is to level things out and give a smooth, workable surface for waterproofing and tiling.

You don’t need to keep chipping away until you reach a pristine concrete slab unless the material underneath is loose or crumbly. Screed can be laid over a reasonably rough surface without issue, provided it’s sound and well-adhered. If you’re having to use a hammer drill to remove what’s there now, that’s usually a good sign that what remains is solid enough to stay.

The key thing to watch is height. Any high spots in the existing floor need to sit below the finished screed level, otherwise they’ll telegraph through. If you can knock down obvious peaks and ridges and generally even things out, that’s more than sufficient.

A good clean is important. Once the demo is done, give the area a thorough sweep and vacuum so there’s no dust, debris or loose material left behind. From there, a competent screeder will be able to work over minor undulations without any trouble.

That said, screeders can have their own preferences, so it’s still worth checking with your guy once he’s back from holidays. Most will simply want a sound, clean base rather than a perfectly flat one. If you want to post another photo once it’s cleaned up, that can help confirm you’re on the right track.

Please let me know if you have any questions.

Mitchell

Kelli-H

This is all extremely helpful, thank you!

The current state of play is that it is all still very crumbly and loose, it's been water damaged and has all different levels, (kind of like the Himalayas!) which is what we are using the hammer drill for.

Will continue to cut takes the peaks and ridges down and repost a picture!

Thank you again!!

Kelli-H

@MitchellMc here are some updated photos post a good vacuum.

There are some areas that are foundation (already there when we took the tile up) and as you can see, some troughs and valleys.

Do you think we should do a little more?

EricL

Hello @Kelli-H

I suggest trimming it a little bit more so that you don't have little valleys forming on the surface. I would also suggest trying to get it a little bit more level if you can.

Eric

Kelli-H

I wish to thank both @EricL and @MitchellMc for your ideas and suggestions.
We did a final go through and then got our screed guy to come take a look and he's happy with the way it is now and will be able to work with what we have to get our flooring perfect.

MitchellMc

Nice one @Kelli-H! I'm glad to hear that it worked out well.

Mitchell

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