Revisions log for Answer #888
This is the answer to the question Does it make sense to do a reinforced concrete screed on the wood floor in the attic bathroom?
Revision 1
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).