Hello Matteo,
From my point of view it should not be a problem to mount the holder in the roof slope. I had a beam 8 cm wide, and I made a flat iron 150/8 mm as a substructure and screwed it to the beam and threaded it with the axis template that is enclosed.

Is it only for 55" TV or can a 47" TV also be mounted?
Many thanks for the answer

Threaded screws fastened into the UK. and thus fastened the TV holder.
Greetings Moreno