Levelling a concrete floor

Do what I did. Buy a spirit level, some string and an awful lot of quick setting tile cement.

And roughly fill in the worst bits with scrap cement and tiles left over from the process as its cheaper to use broken tile and cement to fill that 18mm gap, than straight cement.

Sounds like a hard way to do it. I would use a sand and cement screed to half fill the deepest "holes", say bringing them up to only 5mm below level, and then slap half a dozen bags of self levelling latex cement on the top.

I'd say as a first thought, bloody hell, that kind of removal is going to create the mother of all dust problems for years, no matter how careful you are. How did it get to be like this? Brian

There would be lots of dust if I went this route, that's for sure.

I think the house was probably built like that. There are no signs of cracks in the floor, although there was subsidence 20+ years ago (that was remedied).

Not a bad idea. 3:1 sand cement mix?

I would use an SBR regime too - SBR added to the mixing water for the screed, and paint the existing floor with SBR where the extra screed will go.

You can take it down to 10mm without fear of the new screed crumbling and it will bond like iron to the old stuff. Also tile cement loves SBR treated surfaces.

No - like this:

Thanks. That'd work. Or a sand cement screed as John suggested.

I had the same problem - I took a tungsten flail floor grinder (looks like a cylinder lawn mower) to the floor first. The house wasa building site and empty.

I taped the room's doors shut.

Dust got everywhere and we were sweeping it up by the bucket load.

Ta. When you say "you can take it down to 10mm" do you mean that's the thinnest the screed can be?

Yup, (possibly with plasters or silver sand - dpending on how thin you want to feather it), and use a good admixture (SBR being idea as Tim suggests)

Yup, sand and cement screeds can normally only go so thin (else the edges crack away when walked on). However you may be able to go a bit under that for something that will be re-screeded in latex since that will protect the edges.

So basically fill all the big holes with sand and cement (+SBR), then cover everything with the levelling compound.

Yes.

And you won't do that with regular screed - not without it crumbling.

That's what I did :)

So, using SBR and Tim's linked doc, the ratio is 3.5:1 plus some SBR. How much sand and cement (in bags) would 1m3 be (not that I need that amount)?

sand + cement alone will crack and lift sub around 20mm

You need sonmethi8ng designed to stay together in thin layers, like levelling compound or - gasp - tile cement.

One of these will probably do you:

While you are at it, you may find:

handy...

That doc says mix using a forced action mixer. How important is this, compared to using a £10/day standard mixer?

It's not - a regular cement mixer works just fine. I worked to a different document (Cementone IIRC) which did not mention forced mixing.

You could even mix by hand, but mix well and keep the water down as teh SBR acts as a plasticiser so you need less water than expected.

The important thing is to soak the area in 1:1 diluted SBR first and allow to dry, then paint on an SBR/water/pure cement slurry *just* before you apply the screed. This is the primer they refer to (the Cementone document went into more depth).

The end result IME is that the screed is bonded to the top 10-20mm of the substrate.

Don't have high spots as I did in one area, about 10sq inches. It took

Also - wear gloves and take heed of the cleaning your tools bit in the doc :) This stuff will bond to stainless steel forever - I still have a mixing bowl that has slurry on it.