Extension of the screed opening. Can this screed break?

Once again for understanding:
- You have a recess in the screed and insulation for the chimney.
- The recess was filled with thinner insulation and thicker screed.
- The recess was too small, was enlarged, and the enlargement then presumably filled with cement screed.

That means you also have a floating screed under the chimney, just executed more heavily than the screed in the rest of the room?

If that is the case, you now need three things:
- Proof of decoupling of both areas by a joint (usually by edge insulation strips)
- Proof of the tensile bending strength of the entire chimney screed slab (up to the edge areas!)
- Load distribution of the chimney and total weight

Since, in my opinion, the strength of the screed cannot be proven with this patchwork, I would do the following:

- Take the structural drawing of the chimney at hand, showing the points of the superimposed load. Presumably once around the outside and in the middle no load at all?
- Refuse the acceptance of the screed construction by the screed installer/general contractor. Presumably the highest load is in the edge areas, and there the screed is weakest anyway...
- Remove everything and build the base as described by Klara.

Regards,
Markus

We were informed that omitting the insulation is not possible because it would create a thermal bridge.

Such nonsense (excuse me)! A fireplace in a living area is something special, but not unusual. Thermal insulation is needed under screeds that either rest on the ground or over an unheated room. Under a fireplace that sits solidly on the raw ceiling, no insulation layer is needed. Why should it be? I don’t understand this whole additional discussion! My notes were clear, they couldn’t be clearer. And they correspond to what is called the "state of the art" in construction. I don’t understand this whole additional discussion. What will it be like when it really gets complicated???? "So whoever can explicitly argue why the variant I presented cannot and should not work exactly like that, let them now speak up – or forever hold their peace!" (Amen) ------------------------------------ Regards: KlaRa

KlaRa

I believe the OP's point is that it was NOT carried out exactly as you described exemplary.

The question is therefore whether it will still work without any problems.

The question is not how it should actually be done, but whether what has already been done is okay.

If I understand you correctly, it is primarily important that the screed under the fireplace a) is directly on the floor slab to distribute the weight force there and b) a working joint was made with the rest of the screed.

It sounds like it is somewhat different with the OP.

@ "chand1986": If I misinterpreted the situation, currently an 85mm thick screed has been installed there, possibly on a layer of insulation, then in that case nothing will happen later if the base area on which the chimney will stand is separated from the rest of the (floating) screed by joints. However, if under this area, which supports the chimney, there is a ductile insulation layer (as impact sound insulation), then the intrinsic strength of the screed mortar may not be sufficient to absorb bending stresses caused by the heavy load without damage. With a firmer thermal insulation, however, there would very likely be no negative effects. All in all, if my assumption is correct, it would be incomprehensible that craftsmen seem overwhelmed by this additional requirement of the builder. Nevertheless: Thanks for the hint!

KlaRa: Good.

Then we derive the following task for the TE:

Determine if joints are present and determine if something runs continuously from beneath the normal screed into the load area. If yes: What exactly?

With this information, it should then be possible to determine whether the work was carried out sufficiently well or not.

Summarized minimum requirements: Joint is always required. If insulation material lies continuously in the load area, this only causes no problems with certain materials for sure. Is that correct?

Is this correct?

Yes!