Building a wet room on a porous cinder block wall

[Emil Stenqvist]

I am working on a complete renovation of a 1940s apartment and am fascinated by the choice of materials during the wartime period.

The wall that separates the bathroom from the dining room is 10 cm thick and consists of, what I believe is, a mixture of slag stone and something else porous, which is then plastered. The material is quite porous and lacks real strength: when pulling out old concrete nails, often ~20-40 mm slabs of both plaster and slag come with them, and drilling leads to large fractures. This makes me a bit skeptical about building a wet room wall with waterproofing and tiles on it, or attaching a vanity unit.

So my question is: can I trust this wall, or should I demolish it and build a proper wall with 70mm steel studs and plywood instead? One advantage of demolishing is that it would make electrical work easier, and I could run water through the wall.

The wall is about 3 m long, so it's definitely a bit of work. Additionally, it impacts my project planning quite a bit – if I demolish the wall now, I will have ruined the bathroom that I planned to use until the dining room and kitchen are completed. And I will be lowering the ceiling in the dining room, kitchen, and hall, so it would be a bit tricky to do that first and then demolish the wall. Or maybe I'm overcomplicating it?

Attaching some photos. In the blueprint image, it's the blue-marked wall I'm talking about.

 

[tommib]

If the wall is so porous, I wouldn't trust it. Especially not for mounting a vanity unit. The risk is that someone leans a little on the sink and the whole thing comes down.

Tear it down and redo it, I would say, even if it's a hassle. Why wouldn't the toilet work without the wall? You can shower at work

 

[ZipLock]

I have the same interior walls in my apartment from 1936. Even though it's worked since then, I would definitely tear down the walls and build stud walls if I were to renovate my bathroom.

If you need to attach something to an existing slag stone wall, lightweight concrete screws work really well. I have used that where I've attached a new stud wall to a slag stone wall.

 

[Emil Stenqvist]

I agree with you! It feels silly to compromise on such a thing, especially since I'm redoing everything anyway. So I'll gather a demolition crew for next weekend then!

It's the same with the wall between the kitchen and the walk-in closet, where the kitchen fan is installed. Maybe I should tear that down too, or possibly build out on it and sacrifice some space.

Yes, lightweight concrete screws are handy as well, just drive them in without pre-drilling!

Regarding the toilet: Then I would have to poop in front of an audience! Actually, the toilet is on the other side now, so there's really no problem at all.

 

[ZipLock]

I can also warn you that it will dust an almost unbelievably large amount. After a few different attempts, I concluded that the best way to take down the slag stone wall was with a wide chisel in the rotary hammer.

 

[tommib]

If slag stone is like aerated concrete, I would try a coarse-toothed handsaw of the cheapest model. Saw the wall into suitably large pieces and carry it out. Produces less dust than chiseling, but I don't know if it works with slag stone the same way as with aerated concrete.

 

[PNO]

That doesn't look like slag stone. Slag stone is a product primarily found in places that have had blast furnaces. It's simply blocks of blast furnace slag mixed with cement. If you chip away at the blocks a bit, you'll usually find some pieces that look like green bottle glass, approximately, and some rusty iron clumps. It tends to be hopeless to drill into slag stone since there are so many iron remnants in the blocks.

I think it's more likely to be some variant of lightweight concrete.

 

[Emil Stenqvist]

I have actually taken down a wall like this in the apartment already (between the kitchen and dining room) and can confirm that a coarse-toothed handsaw for aerated concrete works excellently! I started with a reciprocating saw, but as you say, it created an insane amount of dust. After some experimenting, I made cuts with the handsaw and then knocked down large pieces with a sledgehammer.

But I'll try using a chisel! I also thought about spraying water with a spray pump in the meantime in hopes of reducing the dust.

 

[Emil Stenqvist]

That doesn't look like slag stone. Slag stone is a product primarily found in areas that have had blast furnaces. It's simply blocks of blast furnace slag mixed with cement. If you chip away at the blocks, you usually find pieces that look like green bottle glass, roughly, and also rusty iron chunks. It tends to be impossible to drill in slag stone because there are so many iron remnants in the blocks.

I think it's more likely some variant of lightweight concrete.

Interesting! Here are some pictures of the first wall I removed. It had two or maybe three different filling materials in it.

 

[tommib]

If you can use a handsaw (+sledgehammer) then it's probably the least dusty way to do it. A jackhammer on a wall that can be sawed will just go straight through and make small holes.

Interesting filling, I've never seen anything like it before. It almost looks like lecakulor that have melted together, but that type of product feels quite far-fetched for a 1940s house.

 

[Emil Stenqvist]

Yes, that's right. I was thinking mainly to simplify the removal – live 1.5 floors up without an elevator, so it's best to crush and pack the debris into bags to get it out.

No, me neither. I assume it's wartime – they filled with whatever was available. It was with dismay that I discovered this when I was drilling 8 mm holes through a wall and ended up with a crater on the other side.

By the way, I'm sitting here reading your basement thread. Damn!

 

[tommib]

Sitting by the way and reading your basement thread. Damn!

Hehe.... yes. However, it is a bit slow there right now due to other projects. Hopefully, I can resume it in a few months.

 

[Emil Stenqvist]

I used a handsaw, and sawed vertically and hammered down large blocks – it was quick and created less dust than chopping down pieces. A generous amount of water spray before shoveling up the debris worked wonders for the dust!