ᐅ Waterproofing slurry applied above the foundation slab

Hello forum members,

I am currently reviewing our scope of work description. It includes, among other things, waterproofing against ground moisture for 10 cm (4 inches).

Our house walls have been erected, and underneath there was already a type of aluminum adhesive foil (probably bitumen-based adhesive) installed, which was then glued onto the concrete slab. The entire concrete slab inside the building was covered with this. Finally, the mentioned 10 cm (4 inches) on the outside at the baseboard was also covered. The item in the materials list is listed with over 1000 square meters.

Now I have read that the scope of work specifies that the concrete slab in the 10 cm (4 inches) area should be pretreated with a slurry waterproofing layer. Unfortunately, this was not done. What could be the consequences of this? It states that the slurry is only intended as a bonding primer for the foil. If I’m not mistaken, the aluminum material is actually bitumen-based adhesive.

Is this state-of-the-art construction today? Has anyone ever installed it this way?

Best regards

meister keks schrieb:
It includes, among other things, waterproofing against ground moisture at 10 cm (5 inches).

The "aluminum material" is most likely a bituminous membrane or a cold-applied self-adhesive bitumen membrane with an aluminum coating for radon protection (usually installed with an aluminum coating, but versions without it also exist). This serves as the main barrier against water penetrating from the outside or below.

The mineral waterproof slurry is either used alone as waterproofing against splash water above the building foundation or as a bonding bridge and protection against moisture from behind for bitumen coatings/membranes in areas in contact with the ground. Bitumen adheres only to dry surfaces and provides protection only against moisture coming from outside onto the waterproofing layer. It is ineffective against moisture coming from inside or behind and can even detach because of it. Therefore, on construction elements with residual moisture, applying a mineral waterproof slurry before the bitumen layer is advisable, as it also blocks moisture from behind or inside (though it is less effective from the front than bitumen and is often used in combination).

Additionally, in foundation areas, a mineral waterproof slurry should ALWAYS be applied behind the bitumen membrane, and the membrane must be securely bonded. Otherwise, it can more easily detach due to moisture behind it caused by rain or splash water. As far as I know, this is also required by general technical building regulations (a.r.d.t.).

On the floor slab and beneath the walls, I see this as less critical since the continuous load prevents detachment.

Thank you for the detailed explanation.
The base area has already been fully filled and plastered.
Does it matter if the concrete slab is slightly above ground level?
The XPS formwork was above ground level, with the slab positioned a bit higher.
Around the entire perimeter, there is a roughly 80 cm (31 inches) wide layer, compacted down to frost depth, filled with gravel.
On top of that, we laid a nice layer of loose gravel.
Does the concrete slab still come into contact with the soil?
If so, I would eventually have to remove the plaster and insulation in the base area and apply bitumen or waterproofing slurry...

meister keks schrieb:
So, does the foundation slab still come into contact with the soil?

No moisture will penetrate through the foundation slab as described.

meister keks schrieb:
Then at some point I would have to remove the plaster and insulation in the base area and apply bitumen/sealing slurry…

It is not YOU who should do this, but either the general contractor (if available) or the company carrying out the work should do it immediately, not at some later date.

Applying sealing slurry at the base area is, as mentioned, state of the art and also required by the relevant DIN standard (don’t ask me for the number) for building waterproofing.
That does not mean it can’t work otherwise! However, if there are deviations from the DIN standard, the client must be informed and their approval obtained. If this has not happened here, you should point out this defect as soon as possible – at the very latest, refuse acceptance during the final inspection. Defects in building waterproofing are significant and justify refusal of acceptance – you can use this as leverage if needed.

Retrofitting sealing slurry onto insulation is, by the way, NOT professional practice and can only be done additionally to protect the base plaster.

I cannot assess if and how damage might occur here due to non-standard-compliant building waterproofing. However, you should not accept any “compromise” often offered by construction companies, such as extending the warranty while keeping the faulty workmanship. By doing so, you accept the current condition and weaken your position.