Uneven concrete slab

dav71 · Thursday at 2:39 PM

Hi

My builder has poured a very uneven concrete slab for our kitchen extension. 60mm differences between highest and lowest spots. I need to lay rigid pir insulation over the top of this and then a 50mm screed, followed by tiles.

What are my options to level this slab out? If I put PIR across this uneven slab it will rock and there will be voids. This will cause the finished screed/tiles to crack over time. How best to even the slab?

HayMaker · Thursday at 3:05 PM

My first port of call would be the builder, and tell them to fix it. You should not be doing remedial work if you've paid for a level slab

dav71 · Thursday at 3:08 PM

Thanks. I will. I guess I just wanted to see what the actual solution should be because I know he is just going to say "just put the pir down and by time you've screeded it will be fine". Not sure if it it will be

endecotp · Thursday at 4:09 PM

Is the concrete thick enough for the required strength?

If so, consider adding more insulation - that's actually beneficial. Polystyrene is cheaper than PIR. Sand to fill remaining gaps.

(Just some ideas. Not my speciality!)

stevie888 · Thursday at 6:35 PM

At 60mm across the slab it needs breaking out and redoing. If the high bit is too high then the insulation is compromised, if the low bit is low then the strength is compromised.

How far along is the build? This doesn't bode well.

Deluks · Thursday at 11:08 PM

Sloppy! I bet it was ****wet too, and has a loose, flaky spalled surface.

You can level with sand, but at that depth, I'd be levelling off with a weak cement/sharp mix. Once the insulation is down you'll never see it again.

noseall · 2026-06-19T05:07:38+0100

dav71 said:
Hi

My builder has poured a very uneven concrete slab for our kitchen extension. 60mm differences between highest and lowest spots. I need to lay rigid pir insulation over the top of this and then a 50mm screed, followed by tiles.

What are my options to level this slab out? If I put PIR across this uneven slab it will rock and there will be voids. This will cause the finished screed/tiles to crack over time. How best to even the slab?

As above, fill any pockets with sand. We’ve worked on much worse especially some (deliberately) sloping garage floors.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Uneven concrete slab

Similar threads