ᐅ Proper Waterproofing Below the Screed: What Is the Correct Layering?

Hello, I’m new here and have a question about the floor construction.

First, the background: This is a building that has been gutted, and the floor was completely removed (tiles and underneath, frost-damaged concrete from the 1970s). The walls have been fully protected against rising damp by cutting into them and subsequently inserting wedges plus pressure injection. The plaster was completely removed and reapplied. There is no more moisture in the walls. Now to the problem:

The building has been vacant and ventilated for a year. Still, the residual moisture in the ground does not go away because the building is located on a slope and the neighbor’s land becomes slightly damp when it rains, which then causes moisture to seep into my building. The neighbor does not want to take any measures such as waterproofing.

The cut edge is about 10 cm (4 inches) high, so it’s already below the future bottom of the floor.

Now to the actual question: How should I build the floor so that I can definitely live in a dry space later on?

The idea was (from bottom to top):

- Sand (already filled in, about 10 cm (4 inches))
- Bitumen membrane (welded?)
- Foil
- Thermal insulation / rigid foam boards
- Impact sound insulation
- Flooring system plate for underfloor heating
- Screed
- Tiles

I have a construction height of about 18 cm (7 inches) available, possibly a bit more if necessary.

Thanks for your help. If you have any questions, I can gladly provide more information.
Regards,
ralf_anne

Hello Ralf,
I’m not sure from where you measured the possible 10cm (4 inches) of construction height. If 10 cm (4 inches) of sand is already used for the base layer, then that’s not feasible.
Starting from the waterproofing layer, we need 10 cm (4 inches) of thermal insulation for a slab-on-ground foundation.
The waterproofing layer must be applied above the horizontal damp-proof course in the masonry and secured there (for example, clamped with a plaster rail).
With 6.5 cm (2.5 inches) of screed as heated screed and the system panel for underfloor heating, we are likely already at or even above the specified 18 cm (7 inches) total build-up height.
However, in principle, this should work because installing impact sound insulation at this point in the structure makes no sense. It can safely be omitted since it serves no purpose with a slab-on-ground foundation!
Best regards and good luck: KlaRa

That was phrased unclearly. The 10 cm (4 inches) of sand is already underneath and not included in the build-up height.

So I will do the following:

- Bitumen membrane (heat-welded?)
- Foil (pulled up to the horizontal barrier and fixed in place)
- Thermal insulation / rigid foam boards
- Floor heating system panel
- Screed
- Tiles

Is that correct?
Do I need to heat-weld the bitumen membrane, or is overlapping sufficient?
Thank you!

Hello Ralf,
I am happy to provide information about the necessary structural setup. From your message, I gather that you probably intend to carry out the work yourself.
I strongly advise against this! As an expert who regularly sees execution errors—even from so-called “specialist companies”—I can tell you that the cost of repairs later often far exceeds what would have been needed for proper installation.
A waterproofing layer, in particular, must be executed flawlessly. Even the smallest weak spot can become inaccessible and impossible to fix once the floor construction is completed.
Even screed installers have the waterproofing work on floor slabs carried out by the trade typically responsible and trained for this type of work: roofers!
Using waterproofing membranes simply overlapped is completely insufficient and therefore not functional.
Best regards, KlaRa