How to fix uneven ceiling?

[Småfixren]

After dismantling the room partition, a level difference appeared. To the left of the wall cabinet in the ceiling is a 13 mm thick board, to the right only painted masonite. 2.70 in ceiling height. Would preferably meet the level difference and "smooth" it out to zero. Like a wedge?

There is probably a better solution?

 

[Stefan N]

Plane down and drywall, it's easiest and you'll get a good result that way. Possibly remove what's on the left first to have less to even out.

 

[Småfixren]

Regla ner och gipsa, det är enklast och få det bra då. Ev ta bort det till vänster först så det blir mindre att jämna ut.

Thanks for the input. You have a point in that. To the left, it's some kind of tretex board. It is said to have some insulating property. I would prefer to keep it. Maybe just drywall the surface to the right, and cover the seam with molding?

 

[Stefan N]

I wouldn't have done that, completely different structure and properties between plaster and tretex.

 

[Småfixren]

I wouldn't have done it that way; completely different structure and properties between gypsum and tretex.

Sure, there are differences. The entire surface is 8x3 meters. Spry 68-year-old, but 2.70 and ceiling are deterrents. Long way to rent a lift and material.

Do you have knowledge of moldings with different thicknesses?