Using self levelling compound on a rough floor with a slope.

[Frazer G]

I want to lay vinyl plank flooring in a hallway which has a surface which is is quite rough, bumps and hollows up to 10mm locally. At one end of the hall it also has a slope across the width (about 1.2m) of about 10mm over a length of about 2 meters. I have purchased Everbuild universal levelling compound as I was advised this would be suitable but now I have doubts as it says on the bag "depths of up to 6mm".

My questions are~: would this compound be acceptable to fill local 10mm deep hollows and if not what should I use to reduce the excessive depth of the deeper hollows so that I can finally use the compound I purchased as a normal self levelling coat.

In relation to the "lateral slope", could I use a pre layer along the last 2 meters to raise the "downhill" side of the hallway where the slope is greatest to reduce the total slope to less than 6mm and then apply a second general coat of the self levelling compound to give an even level surface.

The base material is concrete with some odd tiles partly buried from previous activities. Wholesale removal of the floor in the "start again" principle is not a viable option due to buried services and maybe the odd skeleton of a previous occupant (the property is very old!). 

[Mr Punter]

Maybe grind off any high points first.  Clean the floor and prime with SBR.  If you can't return the compound you may as well use it.  You may need 2 coats for the thick areas.  Work out how much you need before you begin.  I have found levelling compound a struggle to use so I only do small areas, like mat wells.  Probably just needs practice.

[Andehh]

Yeah, it isn't foolproof and has caught me out as well before! Have a look at a few YouTube videos before you use it.

[Iceverge]

You could fill the holes with some tile compound first. 
 

Self levelling is mis named I think. More like localised self smoothing compound. 10mm over 2m won’t make much difference and don’t expect it to come out level unless you use 20mm+. It’s a 1:200 slope. 
 

Get it as flat and clean as you can first. Then get all your compound mixed up thoroughly. Mop the floor with water so it’s all soaking wet and get the SLC down in one go. Spiked roller or even a trowel may be useful to get it spread out evenly. 
 

 

[Nickfromwales]

On 04/05/2025 at 18:58, Frazer G said:

the property is very old!

I’d be checking for a DPM, as you may find damp is something of an issue here.

 

Maybe you’ll need to apply a liquid membrane first and then level it.

 

I always use a 2 part self leveller (Ultra Level It) which comes with a 5L tub of solution which mixes exactly 1 bag of compound, so you get consistent mixes. You can lay this up to your max thickness in one sitting.

 

For areas where it’ll be 5-6mm or less, you need to add 1/2 pint of water into each mix to allow it time to flow and to improve the viscosity so it’ll run out to the thin areas before biting and refusing to move any further. For those areas you need a spiked roller (google that and you’ll see examples) to help disburse the compound uniformly.

 

Its not a job for the front hearted btw, and defo you’ll be better off with 2 people and to mix 2 bags / tubs up at a time for the thicker areas so you get a good spread and fill, plus it’ll go nice an level then too.

 

Just be sure to mix the compound with a machine and only put the water into the mixed compound immediately before pouring out, mixing it again thoroughly after adding the water, so the heavy stuff doesn’t have time to settle to the bottom of your flexi-tub.

 

Use decorators caulk or other mastic sealant to make sure the compound cannot disappear into any gas or voids, as this stuff will just vanish before your eyes if there’s any small hole or crack it can go into. For the floor area, spot fill with rapid set tile adhesive for divots, cracks, voids etc.

 

You need to tick the ‘DPM requirement check box’ first to see if you need one. If not, then priming the surface will be the next bit of advice.

 

If you have to use say black-jack as a liquid DPM then this needs sand to be thrown over it as you’re putting it down, to provide a key for the leveller. 
 

Simples, eh? 

[Ajm]

We've recently been using ultraplan renovation screed (we found screwfix cheapest!) to level rough and un-level floors throughout. Priming first with 1:1 diluted primer. Some areas well in excess of 10mm, some only a few mm.

 

I would also vote for using a spiked roller for shallower areas

 

We also found it useful to put screws into the floor on a regular grid so that the tops of the screws were level and at the finished level required. This gave us something to gauge the the levelling compound as we pushed it around gently with a float. As @Iceverge mentioned, it doesn't level over a large area but smooths itself in small area - so a little persuasion is needed to push it to the required level.

 

 

[Nickfromwales]

On 06/05/2025 at 15:55, Ajm said:

it doesn't level over a large area but smooths itself in small area - so a little persuasion is needed to push it to the required level.

That's where the extra 1/2 pint of water works wonders