Extension of the screed opening. Can this screed break?

Hello everyone,

in our new building, the screed recess for our chimney kit (approx. 600kg) was initially made too small. After a complaint, it was expanded as can be seen in the pictures. For me personally, as a layman, it looks quite "botched," but I could be wrong, which is why I would like to know your opinion on this.

What is noticeable is that the expansion of the recess was obviously done with a different screed material, possibly concrete screed, instead of calcium sulfate screed.

Is this a "normal" and "safe" procedure? Can we confidently place our chimney on the attached expansion of the recess without it "breaking off"? What should we pay attention to, do we need to complain again here?

Thank you very much for your assessment.

Best regards,
Seol

Hello "Seoul". Your notes are contradictory. That is probably why no one has answered so far. A screed recess is achieved by cutting and demolition. And not by the additional use of a mineral filler! An expansion is thus an enlargement of an opening. You can put on it whatever the concrete substrate allows. Regards: KlaRa

Hello KlaRa,

thank you for your feedback and the note that my post might be ambiguous. Therefore, I will try to describe it a bit more precisely:

In our new building, KfW-40, without a basement, the underfloor heating was poured in calcium sulfate screed onto the insulation lying on the floor slab. Since underfloor heating is not allowed to be installed in the area of the fireplace, none was planned there either. According to the screed installer, the normal thickness (6 cm) of the calcium sulfate screed is not sufficient to bear the weight of the fireplace (approx. 600kg), so a thicker screed (approx. 8.5 cm) was planned in the area of the fireplace. I call this spot a "recess" because there is a clear optical separation. The result was as follows:



Unfortunately, this "recess" was not sufficient, as previously agreed, to accommodate the footprint of the fireplace. The width of this area was about 3 cm too small, so the fireplace would have partly stood on the weaker screed. This had to be corrected.

From my point of view, what happened is that a strip about 5 cm wide next to the "recess" was cut out and poured with "hopefully" stronger (hopefully also 8.5 cm) screed. It is noticeable that this is a different material. The result is as follows:



Is this easier to understand? Since I am an absolute layman, I have the following questions:

[*]Is this procedure understandable and comprehensible for you? What is your assessment of this?

[*]Is the use of a different material as an "extension" of the "recess" okay, or do you see obvious disadvantages/defects from this?

Thank you very much for your feedback.

Best regards,
Seol

I can't say anything about the fiddling. But I am surprised that 600 kg on (roughly estimated) one square meter should already be a problem. Calculated to the cm, I also generate such a pressure...

The tinkering is rubbish. However, the question is where exactly and how is the chimney standing?
For example, I have completely omitted both screed and insulation under the chimney boiler as well as the buffer.

Buffer approx. 1 ton on 1x1m. But it stands on a narrow metal ring and has a separately concreted pedestal
Boiler also approx. 600kg, but it stands on 4 steel feet with a masonry base on the floor slab as a height compensation.

If you have a masonry insert and the load is distributed accordingly, that should be okay.

Hello Seol.
It is true that a 600kg heavy chimney on a load distribution layer (screed) equipped with impact sound or thermal insulation will cause problems. Screeds in residential construction are designed for loads of up to approx. 200kg/m². In room corners, as can be seen in your pictures, much higher bending moments occur under loads than on the surfaces. In this respect, the advice from the screed installer was completely correct!
Large loads, whether in residential or industrial construction, are ideally transferred directly to the floor slab or ceiling slab. In your case, I would have installed a plinth of the necessary height in composite (with the raw ceiling) where the chimney would then be placed.
Between the composite and floating construction, a movement joint approx. 5-10mm wide is formed over the entire cross-section of the structure so that the whole system can function without cracks later on.
Which materials are finally used with which colors is actually unimportant. I assume the ocher coloration and suspect it is a calcium sulfate screed. The gray stripes in the current photos indicate a cement screed.
It should not be done that way because the two binders do not get along well, especially when moisture is present, but if the entire system remains dry (which can be assumed), then there will be no problems later.
The joint chamber will later be well cleaned from fine sediments by vacuuming, then after installing the top covering a round cord approx. 5-7mm deep will be inserted and elastically sealed on top with a color-matched elastic sealant applied to the floor covering. The round cord serves as a depth limiter for the sealant.
To cut a long story short: the color deviations would not particularly worry me now.
It is a pity, of course, that the recess for the chimney was initially made too small, but in construction there are indeed worse things!!
Subjective comments like "botch job" etc. really do not help and only show a lack of technical expertise!
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Good luck with the further finishing work: KlaRa