We looked at a job a few weeks ago. Deck over occupied space.
Slate tile on a pitched to a center drain, not so good.
Owner said water puddled in a few area’s and to the right of the drain,
but that wasn’t the main problem……………
Seems like the builder had some not so good primary waterproofing under it.
The tile sub sealed the slate and thought that would keep the water out of the mud, well it didn’t and rain water saturated it. The drain was not a clamping ring drain and no thought was put into this deck or where water would go if and when it got into the mud bed.
Within a year the grout showed efflorescence everywhere
and the drywall ceiling below came crashing down on his lanai.
The homeowner was sold on Schluter Ditra as an uncoupling
membrane and a mud waterproofing to stop the mud from getting
saturated and to also stop efflorescence.
So we gutted slate, mud & cut out up the wall a little.
Fixed plywood decking, sistered some rotted beams.
Re waterproofed with ProtectoWrap as a primary roofing waterproofing.
Did the mud bed next.
Then we did the Ditra, Kerdi-band & Tile.
Grouted and finished.
Stucco & painters will finish wall above tile base.