Exceeding the construction time, contractual penalty

I recommend observing §12 paragraph 5 VOB/B

So the engineer caused the delay, not the construction company. Therefore, the construction company is not liable for damages. Now one would have to investigate the contracts with the engineer to what extent he was bound to deadlines etc. and whether there could be any liability for damages here. What financial damage have you demonstrably(!) suffered due to the delay?

What financial damage have you demonstrably(!) suffered due to the delay?

That doesn’t matter, at least in relation to the construction company. That’s exactly why flat rates are agreed upon, so that not every euro has to be proven and disputed. Nevertheless, the responsible party must be clear. If the engineer had the scaffolding wrongly placed, that is more on his account. Besides the additional costs for the scaffolding, if there is no agreement, proof must then be provided as to whether any damage has occurred.

Weren't you present at the handover of the apartment yourself?

Were you not present at the acceptance of the apartment yourself?

No. The appointment was not disclosed to us despite our request. It only took place with the engineer we commissioned and the companies.

That doesn't matter, at least in relation to the construction company. That's exactly why flat rates are agreed upon, to avoid having to account for every euro and making it a matter of dispute.
Nevertheless, the responsible party must be clear. If the engineer had the scaffold set up incorrectly, that's more on his account.

I agree with that.
It just became clear to me that our engineering firm might be responsible. Then you cannot demand a contractual penalty from the roofing company. The possible additional costs, however, are damages. These would have to be claimed from the engineer. Of course, everything would have to be checked. In any case, now some of the engineer's statements as well as possibly the final invoice make sense to me.
Thank you very much for your support.