Help figure out what is going on under my floor

[Tedds23]

Hello, I have noticed some funny things going on under the floor of our recently bought house, which was originally built in 1905.

Eventually I will rip out the old floor and replace it but I would like to understand (as much as possible) what is going on under it without taking it off.

I attach some pictures of an unusual hole in the fireplace. Here you can noticefrom bottom up):

1) A black DPM membrane
2) 2cm concrete slab
3) 2 cm void
4) 2-3 cm timber joist
5) Chipboard panel
6) Thin underlay
7) Engineered floor

My questions are:

1) Is this type of floor construction common (dpm, thin concrete slab, void, timber joist...)
2) Could the DPM (or the bad installation of it) be causing our damp wall issues?
3) Shouldn't a floor concrete slab be thicker than 2cm?
4) Are there any options to remove timber joists and maybe screed concrete slab to gain some ceiling height in the room?

Thank you

 

[foxhole]

The hearth stone was probably removed and timber installed to support new floor.
Where is damp ? In chimney breast or nearby?, check roof .

 

[Tedds23]

Foxhole, thanks for the reply. Dampness was only measured around chimney breast and surrounding walls. Readings were high and we were told that this was probably due to failed DPC on walls. However, looking at how the DPM under the floor was not connected to the wall's DPC when it was installed makes me think that this is the true issue. No leaks from roof and the fireplace is on the party wall. Any advice would be welcome