Foundation for extension: Is that correct, any experiences?

Hello,

In a concrete foundation, the load spreads out at an angle of about 60 degrees. Below the foundation, depending on the soil conditions, the load then spreads out at an angle of about 45 degrees. If you want to prevent the load from the extension from pressing against your basement wall, the foundations must be correspondingly deep.

If you do not have existing documents for your house, it is definitely advisable to dig a test pit at the place where the extension is to be built. For this, a trench about 40cm wide (a small excavator bucket) is dug. Then you can see how deep your foundations are, for example, if there is a foundation projection, etc. Alternatively, the planner may assume the standard case for your building age based on their experience, with the risk for you that once excavation begins, there may be redesigns. These can, if you are unlucky, become quite expensive and possibly exceed the existing budget. However, by then you are already at a "point of no return."
What is a stepped foundation?

So does my foundation for the extension also have to be 1.5m deep?

Yes, but not over the entire length; usually, you step up in increments of about 30cm, but so that the load distribution angle does not leave the concrete foundation.

Regards BratacDD

Ahh, so quickly light comes into the dark
Thank you!

Can you also tell me something about the moisture barrier? Is the foundation of the extension simply poured flush with that of the house or does something need to be in between? I only know from masonry that a roofing felt is laid between the foundation and the bottom layer of stones.

Thanks again for the quick and uncomplicated assistance!

I think you are mixing things up a bit. The separation of the foundations does not directly relate to the sealing. From a structural point of view, I would place about 2cm of insulation between the two foundations and then pour concrete against it. The issue of settlement plays a role here. It prevents an unwanted interlocking of the two foundations. But most likely, you will find an overhang on the existing foundation, and then I personally would not set the new foundation on the old one. Because then you will almost 100% get cracks in the existing structure.

Regards Ronny

How deep, how wide is already stated in the opening post. How reinforced? Exactly this is the purpose of this thread, to answer my questions and to point out if there is something I did not write that needs to be considered. It confuses me when an architect asks me as a layman how the reinforcement should be executed in such a case...

You did not understand me. I did not expect you to really be able to answer these questions.
I wanted to tell you that many questions have to be answered for building a foundation and a slab.

Good question, what do you recommend?

If I now tell you "Foundation 1.0m deep, 50cm wide. Reinforcement with concrete bar ds 12, slab d = 20cm with reinforcing mesh Q257A top and bottom," will you then take this and start digging your foundation?
I want to make clear that someone has to calculate this. No architect will tell you "Oh, just put in enough concrete and steel, it will hold," although he would probably be right with that.

Do you mean by ring beam the component that sits under the roof frame?

Yep!

A ring beam must be created in the following cases:

[*]Buildings with more than 2 full floors (my house entrance is only single-story)
[*]Buildings longer than 18 m (dimensions are in the opening post, 1.5m x 3.5m)
[*]For walls with many large openings (one door and one window 1.0m x 1.0m, should be okay)
[*]If the soil conditions require it (No)

Aside from the fact that according to the same source I CANNOT decide this, but the necessity of a ring beam is determined by an architect and the size and reinforcement by a structural engineer.

And it’s the same with foundation/slab.

You mean besides the fact that part of my questions would already be answered?

I mean: foundation, slab and all other load-bearing elements must be calculated. And when the structural engineer calculates, he already knows how it must look in detail.
This is not for a layman. That’s the only tip you can get from this forum.

@BratacDD: And once again, thanks for the quick and helpful answers and explanations.

@E.Curb:

I wanted to tell you that many questions have to be answered for building a foundation and a floor slab

You wanted to say something without saying anything, that’s clear to me by now. Otherwise, I would have been able to read by now, probably much earlier than over a page ago, WHICH questions need to be clarified (apart from the roof truss and the thermal insulation that influence my foundation, but even that could be excluded).

I mean: foundations, floor slabs, and all other load-bearing components have to be calculated. And if the structural engineer calculates, he already knows how it has to look in detail.
For a layperson, this is nothing. That is the only tip you can get from the forum

Obviously not the only tip, BratacDD has already helped me a lot. Where there’s a will...

But exactly this "waffling" is the reason why I first gather all the information myself before I call the architect. If I then have him at the door and want to know from him how my moisture barrier on the foundation should look and then THAT kind of thing starts, no thanks.

Please excuse the well-meaning advice I really intended, to also hire professional planning even for such a small construction project. Because for someone who has absolutely no idea, this is not something to do.

In any case, there is (at least from me) no do-it-yourself construction guide for an extension to the existing building, especially not when it concerns structural engineering. It simply doesn’t work, as already mentioned, if you don’t know the exact conditions on site.

But get the information you need from the internet, then the extension will also work. And my colleagues will be happy soon when it’s time again: Why are there cracks in the masonry? Why is water coming in here? Why is there a draft here? Why do I have mold in the corners?

Best regards – I’m going to watch football now