ᐅ New Build Semi-Detached House – Shared Wall Basement Dampness

Hello dear forum,

we would like to ask for your advice:

We are currently building a semi-detached house, with the basement constructed from waterproof concrete (WU concrete). Yesterday, the construction staircase was installed, so we were able to enter the basement for the first time since the basement ceiling was finished. During this, we noticed that...

• the joint between the basement floor and the basement walls at the corner where the party wall meets the exterior wall is damp
• and this is only on the wall facing the other half of the semi-detached house (see picture: the left side shows the dry exterior wall, the right side the damp wall facing the other half).

• the further away from the exterior wall, the drier it becomes. After about 2 m (6.5 ft), it is completely dry.

We know that a sealing tape was installed externally and cast into the concrete, but NOT on the interior walls between the two halves of the semi-detached house. Therefore, we strongly suspect that water is somehow getting between the two halves. The roof has been installed for about 1 month, so no more rain can reach the insulation between the walls from above (both interior walls are separated by approximately 4 cm (1.5 inches) thick mineral wool insulation). The exterior walls, however, are not unprotected.

Before contacting the site manager, we wanted to get your opinion:
- Do you have any ideas about the cause?
- Could the observed moisture be normal at this stage of construction (see above)?
- Or is this a defect?

Thank you in advance.

Worst-case scenario. Clearly a serious defect. Construction must definitely be halted until this is resolved.

Are both halves of the basement built on a common slab, or were two separate slabs poured next to each other?

How is the basement waterproofing planned: on the sides using mineral-based or bitumen-based coatings or slurry? But what was done underneath? Waterproof concrete (WU concrete) alone is not 100% watertight, only impermeable. This means it only prevents water passage if the substrate below is capillary-breaking dry and the concrete is at least 20cm (8 inches) thick. However, if water is standing under the basement on the concrete, according to DIN standards, it does not have to be fully watertight, only less permeable than normal concrete.