I want to make a bathroom on the attic floor. Now the floor of the attic – boards. Is it possible in the bathroom to throw a thick film for waterproofing and top pour reinforced concrete screed 5-6 cm? Or better to do otherwise? The final coating will be tile.
Don't do that.
There will be a lot of weight of the screed, plus the weight of tiles and glue, plus the weight of the bathtub with water and other plumbing equipment, your own weight, a lot of it will be.
You have to calculate carefully. Film, even thick film, when laying a screed usually gets breakthroughs. This is all unreliable and unrepairable.
It is better to put on the boards plywood 18mm, aquapanel under the tiles on self-tapping screws in the plywood, then sprayed waterproofing, tile on adhesive (technology worked out at KNAUF, look at their website album of technical solutions for the floor with aquapanel for wet rooms).
I did it that way for myself, but I reinforced it with additional mesh. And instead of the film, I used a roofing felt with a rise on the walls.