Levelling compound over plywood cracks and crunches under foot - remedy?

[maxdavie]

Hello, 

We have an issue; the levelling compound we've used has now dried and it cracks/crunches when walked over. In one area, it has popped up and bubbled. In another room, a hairline crack has formed along the joint of the 4mm plywood boarding beneath. We are laying a lino floor over this. Are these issues anything to worry about? Can we just sand down and re-smooth the bubble? Is there anything that can be done about the cracking? 

The floor build up from floor finish down is:

2mm Lino floor on adhesive

4mm plywood

18mm floated tongue and groove board glued at joints and screwed down into concrete slab below

50mm PIR board

Concrete slab

 

Any thoughts greatly appreciated! 

 

 

[saveasteading]

I would expect the screed to continue to delaminate. 

4mm plywood is very thin. Perhaps not enough fixings and it is flexing. Why does the board beneath need this extra ply layer

What thickness is the screed?

[markc]

So you have some thin levelling compound over 4mil ply over a floating floor over PIR? It’s going to flex and destroy the compound.

can you ditch the compound and ply?

[maxdavie]

Ah. Is that what is happening? Does 'delaminating' mean the bond is just breaking between the plywood and the screed? 

The screed is between 3-5mm I think. 

 

[maxdavie]

1 minute ago, markc said:

So you have some thin levelling compound over 4mil ply over a floating floor over PIR? It’s going to flex and destroy the compound.

can you ditch the compound and ply?

Do you mean break through the compound and remove it? Remove the plywood floor too? 

Then I'm left with a glued T&G floor over PIR. Its technically not 'floated' anymore as we screwed it down to the concrete. 

[markc]

11 minutes ago, maxdavie said:

Do you mean break through the compound and remove it? Remove the plywood floor too? 

Then I'm left with a glued T&G floor over PIR. Its technically not 'floated' anymore as we screwed it down to the concrete. 

Yes, the compound is always going to break up on a flexible floor. The T&G is the ideal surface. Might need to go back around and adjust the screws to get it level but levelling compound isn’t the answer.

the plywood could be kept but you still need to level what’s under it

Edited October 31, 2022 by markc

[Hobbiniho]

4mm plywood is really a bit thin, i usually use 6mm, i take it you glued and nailed in a 100mm grid?? and im presuming that you used a latex based screed??

[maxdavie]

6 minutes ago, Hobbiniho said:

4mm plywood is really a bit thin, i usually use 6mm, i take it you glued and nailed in a 100mm grid?? and im presuming that you used a latex based screed??

Def not a latex based screed and tbh I don't think the guy who did it was screwing a those C's.

DAMM! Reckon its going to be necessary to re-do this before the lino goes in? 

 

[PeterW]

16 minutes ago, maxdavie said:

Def not a latex based screed and tbh I don't think the guy who did it was screwing a those C's.

DAMM! Reckon its going to be necessary to re-do this before the lino goes in? 

 

Ok so even latex screed cracks but tbh I’m not sure what 4mm ply then 2-4mm screed will do over just using 6mm ply under Lino.

 

If the floor was uneven in the first place then that should have been fixed - SLC there would have helped and then it’s a case of PIR and T&G flat over it all. Final finish of ply is only really needed for amtico or karndean.