moisture damage via concrete stairs

[keijoo]

I am new to this forum, but I hope this is the right place to get help.
We have a house from the 60s with a large, nice slate-clad concrete staircase in front of the house. Since some stones were loose, I unfortunately discovered that about 10 of the fine concrete under the stones had turned to sand and that water had leaked into the house, as the sills are below the level of the stairs.
The damaged fine concrete is now removed, and the rotten sills have been replaced. My question is how I should best pour the stairs back. The reason for the damage to the sill is probably that it is poured up against the wooden beams (with roofing felt in between) and that the sills are below the level of the stairs.
Am I forced to lower the entire staircase, i.e., break away another 15 cm of very hard, reinforced concrete and make an extra step (open) up to the door - 2 in the picture. Or can one make a drainage channel that is below the foundation level - 1 in the picture. Other suggestions are also welcome.
I hope the picture is reasonably clear.

Best regards,
Marcus

 

[justusandersson]

A bit difficult to answer without seeing it on site. Concrete is normally not waterproof, nor are joints between stone. So there must be some form of durable moisture insulation between the stairs and the exterior walls. Additionally, one must take into account that the stairs move relative to the house, thereby causing some wear on the moisture insulation. I could imagine a new asphalt or rubber-based moisture insulation covering the exterior wall and the basement wall. Then some form of insulating material, e.g. styrofoam, serving as a "friction protection" which is covered with a thin layer of plaster on top.