Croatia - Structural engineer requires 23cm casting against existing interior wall

[mvs]

We have an old house in Croatia for which we are currently going through the building permit process. Attached you will find a simple sketch of the house located among narrow alleys in the old town. Unfortunately, a construction crane is not an option.

We are quite advanced in the process, but now the responsible structural engineer has come up with new requirements just before the building permit application is to be submitted. He claims that since the northern and southern long sides share walls with other houses, we need to cast new interior walls that are 15cm thick (shown in red on the sketch). An additional 8cm of insulation will be added after the casting is done. So a total of 23cm.

Our house was originally two houses. The western part was probably built around 300 years ago and the eastern part about 150 years ago. The northern long side is shared with two other houses (built 100-350 years ago). The southern long side is shared with a house that is about 150 years old.

We have quite different levels on the floors and ceilings (largely because it was previously two houses) and we had planned to address this during the renovation and cast new floors. We will also remove the wall that runs straight through the house.

The architect and the cultural heritage board thought it would be enough to integrate the new floors into the existing walls, but the responsible structural engineer now states that he is doubtful since we are talking about construction techniques from many different eras and with completely different materials and strengths.

I have two reasons to be hesitant about casting new interior walls inside the existing ones:

(1) Since the existing walls have such different building materials, it feels risky to cast a concrete wall against these. I fear that the walls will no longer be able to breathe sufficiently, with all the troubles that may bring.

(2) We lose nearly 10% of floor space in an already small house.

I was wondering if any of you have any thoughts or suggestions on this?

Thank you.

 

[cpalm]

Thought I'd see if others have any thoughts or suggestions on this?

Reading between the lines, is it about converting from wooden joists to concrete joists?

 

[mvs]

Reading between the lines that it's about converting from wooden joists to concrete joists?

Exactly!

 

[Matti_75]

I understand the person as you will be adding a lot more weight and it is impossible to say what the old walls can handle in terms of load.

 

[mvs]

I understand the person since you will be adding a lot more weight, and it's impossible to say what the old walls can handle in terms of load

Yes, I can also see the reasonableness in that, but of course, I hope to find a better alternative. Do you see any other risks with moisture in this case?

 

[cpalm]

Do others also see risks with moisture in this case?

Depends on which material combinations we are talking about. Concrete and trä, for example, are a terrible combination.
Then there might be other challenges as well, such as the materials moving differently.

 

[mvs]

Depends on the material combinations we're talking about. Concrete and wood, for example, is a terrible combination.
There could also be other challenges, like the materials moving differently.

Yes, so we can assume that the walls are full of stone, wood, and sand. Possibly also straw and some paper-like material that looks to be about 200 years old. A concrete wall directly against this does not feel optimal.

 

[Matti_75]

If you are concerned about moisture, use an isodrän board between the walls.

 

[Thomas_Blekinge]

What is the 8 cm insulation that you were planning to put on the new walls?

 

[Thomas_Blekinge]

Instead of casting an entirely new wall, it might work to cast an "arch" against the existing walls that support the new floor joists but do not take up much floor space. The majority of the old wall would then still be unobstructed.

 

[mvs]

If you are worried about moisture, use an isodrän board between the walls

Ok, thanks. What thickness is usually used for it?

 

[mvs]

What is the 8 cm insulation that you plan to put on the new walls?

I actually don't know. It was the architect who wrote that it will be needed. I hope to get more clarity on it when they are back from the holiday.

 

[Matti_75]

Ok, thanks. What thickness is usually used for it?

50mm is enough

 

[mvs]

Instead of casting a whole new wall, maybe it would work to cast an "arch" against the existing walls that supports the new joists but doesn't take up much floor space. Most of the old wall would then still be free.

Yes, that sounds similar to the option presented by the building engineer. He mentioned as an alternative that one can cast pillars in each corner (25x25cm), as well as an additional pillar in the middle of each long side. Then load-bearing beams (25x40cm) are placed on these. Like this:



However, he believes that option 1 (cast inner wall) is a better alternative since, in any case, you will need to apply about 10cm of plaster on the walls to prevent ancient plaster, etc., from starting to fall. And then he thought you might as well do 23cm directly (15+8), and then you only need one load-bearing beam - the one that goes across the house from long side to long side. Like this:

 

[mvs]

50mm is enough

Thank you. Any international brand you can recommend? Most of their material comes from Germany.