Flooring for a concrete slab over basement

[OneReason]

So here we have a ground floor flat in a 1930’s building. This is my first experience with a concrete slab flooring, let alone with a basement below it.



The existing flooring consists of:

1. New tiles (just roughly dot and dabbed on to the old vinyl tiles, to sell the place I think)
2. Very old small square vinyl tiles
3. Bitumen (broken/compromised)
4. Red brick tiles (broken in places)
5. Concrete slab

Property Notes:

• High humidity inside that roughly matches outside (usually 80%).
• No mould or condensation, assuming due to good air flow/ventilation (vent in living room closed chimney, double glazing vents)
• Upgraded the inline extractor fan, is on 24/7
• Humidity doesn’t change if extraction is off or on
• Powerful dehumidifier can bring it to 50%, but after 1 hour is back to 80% (or roughly outside humidity)
• High heat loss
• When lifting up the old flooring, there is damp along the floor, along the outside wall, just underneath dot and dabbed tiles, additionallyy underneath bitumen, where moisture is probably getting trapped underneath due to compromised dpc
• The revealed red brick floor area dried out nicely once top layers were removed. Now dusty and dry.
• Rest of tiles probably have the same damp/wet underneath along that outside wall.
• Blown plaster removed and brick work is dry
• Leaking plumbing and leaking waste were remedied
• Basement has a humidistat extractor fan setup
• Corner of basement ceiling seems to be missing some of the concrete slab (see photo)

Idea 1

• Repair broken red brick areas by concreting in the damaged areas.
• Apply a ‘blue seal’ dpm to all floor including couple of inches up brick work.
• Lay underlay
• Lay laminate wood flooring for warmth

Idea 2

• Same as 1 but instead of painting a dpm, lay something like Diall Aquastop underlay directly on the red tiles, tape joints, which is mould resistant and has inclusive vapour barrier.

My Concerns

• Is sealing this kind of floor the correct action?
• Applying a dpm may push moisture coming from the basement through the concrete slab floor, horizontally to the wall as it tries to escape, which will cause damp rising up the walls and damaging the plaster again?
• If I don't seal it, will this be a constant source of humidity?
• Using Dial Aquastop underlay may cause damp/wet to build up underneath, but this will be more easy to check and remove.

Thanks all, any and all advice is appreciated to point me in the right direction for preparing and laying the correct flooring on this elevated concrete slab.

 

[Trevorpeterson]

given the high humidity levels and the complexity of the existing floor structure in your ground floor flat , addressing moisture control is crucial before laying new flooring. Idea 1, involving repairing the red brick areas and applying a 'blue seal' DPM (Damp Proof Membrane), could effectively block moisture but might redirect it to the walls, potentially causing damp issues there. Idea 2, using Diall Aquastop underlay, offers a mold-resistant and vapor barrier solution that might be easier to monitor and adjust if moisture problems arise, yet there's a risk of trapping moisture underneath; sealing the floor with a DPM or similar solution maybe good thing to mitigate humidity but it's need to make proper moisture management to prevent it from affecting other parts of the building, id ask smb abt best method to balance moisture control while preventing damage to walls or the basement ceiling is advisable, cause building quite old