The second half of the house will be mansard, but the rafters there are 6.2 m, and the insulation will be laid about two meters from the wall on the length of the rafters. And all that is above, it will also be a cold attic.
The builders say that in my case, the trellis is not necessary.
If it is still needed, can I make a 25x100 board the necessary size 50x50 and use it as a counter-batten?
Counter battens are definitely needed. Insist on it. And yes, the boards, if available, can be converted to the format.