ᐅ Concrete slab in an older building – only gravel with a 3 cm cement layer underneath

Hello,

we removed a partition wall (1.2 m (4 feet) high) in the basement next to the washing machine. Unfortunately, it was not made of simple bricks but, as usual with the previous owner, was cast in concrete. Unfortunately, something unpleasant has now appeared underneath – see photo.

The concrete beneath the tiles is only 3 cm (1¼ inches) thick, and directly below that is gravel.
Question: what is the best way to seal or waterproof this area?

Thanks in advance for any tips and advice.
Best regards, Markus

The house was built in 1952.

Hello questioner.
What you describe as a problem is not easy to solve.
Therefore, the most important question first: Why does this area need to be waterproofed?
It is true that moisture can rise from the rear side here. However, if a moisture-resistant covering (such as the tiles installed next to it) is planned for this strip, I would omit waterproofing.
A sealed connection to the black vapor barrier, or whatever that may be, is in my opinion not achievable.
This transition remains a weak point in the structure—if, and only if, a moisture-sensitive covering is to be applied here. It will be a special construction for which nobody can guarantee a 100% moisture barrier!
I would excavate the strip about 40cm (16 inches) deep, fill it with glass gravel (which is somewhat more expensive but prevents capillary transport). Make sure to compact it well.
At the level of the bottom edge of the tiles, place a water-resistant, load-bearing drywall panel or another (water-resistant) construction board as the installation base. This strip will always behave independently under load pressure. Therefore, plan for an elastic (not rigid) transition on both sides along the length of the excavation, so the eventual covering does not crack or break.
Again: this cannot be a "standard-compliant" construction!
It remains an emergency solution that you will need to test to see whether it works or not.
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Good luck: KlaRa

Hi, first of all, thank you for the response.
I think I should add a few more details.

• The room in question is the laundry basement.
• There is a dehumidifier in the basement.
• Without the dehumidifier, humidity levels rise above 70% in some areas during summer.
• The basement has the usual single-glazed basement windows.
• In the end, a single tile will be installed on that spot.
• Otherwise, the basement is used only for storage, not as living space.
• The black coating on the screed is probably just paint from the 1950s.

I hope this helps for a better assessment.
Best regards, Markus-44240

Don't make it so complicated.

Remove everything loose, maybe dig out about 2cm (1 inch) of gravel. Then fill with around 1cm (0.5 inch) of sand and place a slightly more durable building foil (vapor barrier). Attach this to the bitumen layer using mounting adhesive, then pour the concrete.

Done.