Mouldy floor, tanking membrane?

[showl]

Need to know if the attached photo is likely evidence of a failed tanking membrane?

Background... Recently bought the property and I knew the basement floor needed re-tiling as almost all were cracked. The tiles were laid over an electric underfloor heating system which, the previous owner had indicated was likely no longer working. Started to pull up the tiles and discovered that the floor was very poorly laid; tiles were barely supported (likely the why they all cracked) and the screed over the underfloor heating was non-existent in places. Concluded that the UFH was not salvageable as it has proven to be impossible to separate from the tile adhesive. Decided to strip it back to a chipboard base layer that I found under the UFH insulation boards.

All was well until I reached the section of the floor shown. The chipboard surface is damp to the touch and covered in black "soot". Can also see the tile adhesive has gone a brownish colour. The area of the floor is nowhere near any obvious water source and the ceiling looks undamaged, The water must've come from above or below somehow and the fact it's still damp maybe suggests its still being exposed to water (rather than a recent kitchen flood). But why it's "sooty" is a bit of a mystery.

Anyone got any ideas? Or at the very least who should I call to figure out what the problem is and how to fix it? a surveyor?

I don't want to have to start trying to remove the chipboard (as the interior walls appear to be supported on it) so could use some advice before I take that drastic step.

Would really appreciate any help/suggestions

Cheers

Steven

 

[Madrab]

What wet services are in the room, washing machine/sinks etc?

That's definitely water ingress, there may have been a floor flood and the water has seeped down into it it and just hasn't dried out. Not seeing any tanking there though, as your title suggests.

As far as lifting the chipboard is concerned then that wouldn't normally be an issue, Find some nail/screw points in from the walls, that would usually dictate where the joists are, draw a mean line through the nails with a level/straight line and cut that with a circular saw and up to joins that can be seen. Try 19mm first then up it to 23mm if the floor doesn't go on the first cut. Lift a small section to start with.

The main problem that has been there is prep, there should never be anything laid directly onto chipboard IMO, it just isn't designed to take anything like that.