"His height reference line matches ours exactly. And the elevation levels were communicated in writing. He did not adhere to his own height reference line.
How much deviation, and according to which standard, is permissible for the elevation level?"
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Well, DIN 18560 Part 1 Table 1 "Screed Thicknesses" provides information on this.
In the bill of quantities "Screed Works," a nominal screed thickness was specified. By the way, this is specified by the planner.
If we look at the aforementioned Table 1, we see, for example, a nominal thickness for unheated screeds (commonly used in residential construction) of 45mm.
The smallest individual value there is 40mm, so there is a difference of 5mm.
The minimum thickness primarily has to do with the load-bearing capacity of the load distribution layer, yet we can apply the basic principle to the problem in your house when it comes to minimum thicknesses (and thus the smallest installation thickness).
Added to the nominal thickness specification is the instruction (to the screed installer) regarding which height level must be maintained.
The minimum thickness of a screed is thus specified by DIN 18560-1, but there is no normative requirement concerning the maximum thickness.
This usually results only after leveling (task: screed installer), so that he can later invoice extra thicknesses/extra consumption.
The height specification given to the screed installer compared to the binding height reference line is ultimately decisive!
Suppose the screed was demonstrably installed too low. What happens next?
Well, a written defect notice is issued by the client to the contractor (here: screed installer) with a request for rectification within a period X.
The screed installer will usually first check whether there is indeed an incorrect height.
If it was installed too low, he will probably not want to carry out the necessary extra thickness himself but leave that to the following trade (floor layer) against cost reimbursement. The latter then takes over the leveling, which, depending on the height difference, does not necessarily have to be done over the entire surface. Usually, it is sufficient to adjust at component transitions (in this case from one room to the next).
This is how it is mostly done in practice.
There is one more alternative:
Suppose the screed was installed too low in every room, then, with the exception of the main corridor (in multi-family houses) or the main entrance (in a single-family house), the screed can also be leveled only where we encounter critical points. This would be, for example, the front door of the house.
However, if the house or apartment has a staircase to the upper floors, the situation is quite different because the step height of the stairs has been aligned with the correct height measure of the screed (in that case, the step height of the first step would be larger than the following steps; that would be unacceptable!).
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You may recognize that the topic is very extensive and the solution depends on many aspects.
At this point, I only wanted to provide the relevant notes. Ultimately, you (from my practical experience) will only get further with a construction specialist representing your interests (this can be, for example, a good site manager or architect).
Because a craftsman’s willingness to compromise also depends on the economic pressure they are under....
And the quick invoice payment you mentioned already indicates that.
Good luck (and keep your nerves): KlaRa
