Laminate on a wavy underlayment board?

Hello dear forum community

I ask you for your advice.

We are currently having our attic converted (apartment building built in 2000, maisonette apartment with expansion reserve in the attic. The ceiling between the two maisonette floors is a beam construction). At that time, the developer already laid a 22 mm chipboard subfloor (tongue and groove) on the beam construction for the previous owner, on top of which there is a similarly thick insulation mat (looks like glass wool), and then floating on top another 22 mm chipboard subfloor (tongue and groove, each with about 1 cm expansion joint to the wall). I don’t know whether the lower subfloor is screwed to the beams.

Now we want to install a floor and have decided on laminate (Tarkett Woodstock 832). Underneath as impact sound insulation ewifoam Silenza 5 mm (an XPS insulation material). 5 mm is chosen so that the floor height later matches the top stair step well. The material is already at our house, bought from a store that also arranged the floor layer for us. When this floor layer came to us to start work, he found that the upper subfloor is quite uneven: the individual panels have warped upwards along the longitudinal edges by about 5 mm. When walking on the edges, you can clearly feel that the floor gives/wobbles at these spots.

Now the floor layer tells us that he cannot lay laminate here or if he does, he does not want to provide any warranty. In his opinion, the tongue and groove construction of the laminate will eventually break because the floor is so uneven. Sanding is pointless because the problem is too extensive. In his opinion, the cause of this unevenness is that the subfloor panels have absorbed water, perhaps from the paint that was brushed onto the wallpaper a few weeks earlier.

We have sent him away for now and are quite confused and uncertain. Is it really as serious as he describes? Of course, we do not want a floor that wobbles with every step and possibly even cracks. But are there better solutions than just saying that no warranty will be given?

Would it be conceivable to screw all the upper subfloor panels to the lower ones so that the floor is basically “pressed flat”? But then the floating floor would be over. Would one then possibly cause other problems?

Or should one try to even out the unevenness of the floor when laying? What happens if the floor warps again?

Thank you for your opinions and tips.

Yes, unfortunately there is no other choice than to try to level the floor somehow. If you want to do it properly, you unfortunately have to spend a lot of money in this case or alternatively invest a lot of your own time in the renovation.

Good morning, I am also currently laying new laminate. The subfloor is also uneven and here I have laid chipboard. I read this here beforehand and then acted accordingly. It was really worth it because the laminate can be laid wonderfully on it. Everything else is rubbish and then there is only double work, but also double the cost. Best regards

I know that too, and our room in the attic has a wavy floor as well. I also laid chipboard and filled the joints. Then I laid the laminate on top. I hope it lasts forever. :)

What surprises me here is that the floor layer did not see this beforehand. If I hire someone for the job, they must inspect the floor beforehand. That way, they would have noticed that the floor is uneven. Action could have been taken in advance. Now to say, I do not assume any warranty. That is weak.

Sometimes the defects only come to light during work. That is not uncommon. But he recognized it in time. Others would have laid the laminate and taken the money.

After a short time, the edges of the laminate would have torn.