Uneven floors - self level or ply subfloor?

13 Mar 2016
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United Kingdom
Hello all, Im after a bit of advice.

We are doing an extension which will create a combined kitchen diner at the back of our house. All the downstairs floors are solid screed (house was built early 90s so should also have DPM under the screed and insulation under the slab I believe). We are going to have a continuous floor covering throughout the downstairs of the house (engineered wood parquet, which can be glued or secret nailed - but cant be installed floating).

The problem I have is that for some reason I cannot fathom the screed in the kitchen is around 8-9mm higher than the other downstairs rooms.

I had originally thought we could grind the kitchen floor back and then use self levelling to make good, but I have been told by a couple of people that removing this much screed in this way would be very difficult/ time consuming. I had originally considered smashing out the screed in the kitchen and re-doing but that would be equally if not more time consuming/ costly I think.

Other options I had considered were either to use self levelling compound on the rest of the downstairs rooms to raise them to the kitchen level, or to glue 9mm ply to the downstairs floors to act as a sub-floor for the parquet.

In terms of cost I think the ply option and using something like stopgap 300 would be pretty similar.

To add to my quandry we are (probably) going to be having wet UFH retrofitted to the existing downstairs screeds, and I am unsure whether a ply subfloor has any downsides compared to compound in this scenario.

Then lastly, I am going to be doing this myself. I have done some smaller areas of self levelling compound using F ball products in the past - although I made the mistake of not using a spiked roller/ squeegee and although the result was OK, it wasnt perfect by any means, so I suppose in terms of getting as flat a finish as I can, ply might be the better option...

Any thoughts/ comments would be most gratefully received!
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Don't glue the ply - screw it down at 150mm centres
Yes. It's also a recommendation not to glue the ply down, but to screw it down
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Thanks...just a bit wary of screwing down when there's ufh pipes below.....guess that can be overcome with care.
How thick is your existing.flooring? With 8mm plywood, 19mm chipboard and you are limited to 4 x 25mm screws (19 + 8 = 27) or 22mm modern "1in" redwood - 4 x 30mm screws (22 + 8 = 30) and you won't hit anything.
Currently carpeted. The flooring going down is 10mm engineered parquet. The ply sub only needs to be about 9mm thick, I will glue the parquet to it (preferred method of the manufacturer).

The UFH pipes are being placed in channels that are milled into the existing screed to depth of 16mm, then covered by a small amount of self levelling compound, so theres only about 4mm between top of the pipe and the floor surface....
Sorry, didn"t realise your UFH was so close to the surface - all the stuff I work on it's a lot further down. In that case I think you are going to have to screed it to get the level up. If you glue ply onto screed, then attempt to glue engineered flooring onto the ply I strongly suspect that you risk the wood to wood bond (engineered flooring to ply) being a lot stronger than the (ply)wood to screed bond and the whole lot eventually simply cupping and lifting away from screed. What does the flooring manufacturer have to say on this subject?
Admittedly I havent asked them her but I will! Out of interest. Is the reason it might cup because of slow moisture ingress from the screed into the underside of the ply?
Wood moves with changes in the moisture content in the environment it is in. Screed moves a lot less than wood, so it is a lot more likely that over time the plywood to screed bond will fail. That is why I asked what are the flooring manufacturer's recommendations on this?
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