Hello "FelixPrüfer". Unfortunately, I am not always able to respond promptly. I had already pointed out that expertise is necessary to get the water ingress beneath the staircase under control. Without wanting to specifically address the previous answers or proposed solutions: If you put something (waterproof) on the staircase, you inevitably have to ask yourself how a tight connection to adjacent components should be made. Judging by your photos, on one hand it is the rising wall surface and on the other hand the sealing between the stair treads and the receding wall surface. Answer: almost impossible! Bringing concrete onto the old stair treads is certainly the most nonsensical of possible (nonsensical) solution approaches! Because it certainly will not be concrete that you can apply (maximum aggregate size 16mm to 32mm, minimum installation thickness 3 x maximum aggregate size = 48 mm to 96 mm) and mineral building materials are not impermeable to penetrating water. Due to the resulting (relatively poor) compaction of the mortar matrix when applied manually, you might as well leave it and donate the money for the material purchases to a charitable organization. A proper construction would look like this: the load-bearing substructure is exposed (unfortunately that is the concrete of the static staircase structure) and the lateral connection to the building is inspected and sealed. Incidentally, this is a classic area of responsibility for roofers (although this may seem grotesque at first glance)! They would also explain to you that the horizontal sealing in the rising wall surface must be protected beneath a covering against lateral water ingress. A very extensive subject, which certainly cannot be resolved with the use of "window putty" (as one answer suggested). ------------------- Regards: KlaRa