How can laying plywood level a floor? Don’t understand how it still won’t be a bit wonky…!

[Londonandon]

As in the title really. We are having flooring laid and he is going to put down 5/6mm ply underneath to help level it. But won’t the ply just follow the contours of the floor underneath?

We put green fiberboard under ours - thick woven matting which evens out the lumps n bumps. Also insulates too. Got ours from Wicked

Sounds good - we have had four floorers in to quote though and they all use plyboard?

Is your floor on joists with an air gap? If so then I think this is the way they do it. My understanding is that levelling screed is not a good idea on a joist floor as it expands which could crack the screed.

When they did this in my kitchen they packed the gaps. They used slivers of wood to get the ply level and the ply was strong enough to bridge between the packing.