Bathroom questions...also, Japanese sliding doors?

Where the bathtub meets the surround, there's a ~1cm tiling lip (?). What should fill this gap? A strip of greenboard or something? On the topic of greenboard, should it form the walls from the floor to and including the ceiling of the tub/shower?
Also, the toilet is currently drained by 4" cast iron (original, house was built ~1957, Toronto, ON, Canada). I'm going to swtich to ABS with a low-flush toilet. Is it preferable (for flow-rates, etc.) to use 3" ABS? It's about a 12' straight run to the basement floor.
Finally, does anyone know of a source for Japanese sliding doors in Toronto?
TIA! Darryl.
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No expert here, but similarly engaged myself right now. My Jacuzzi tub has a "tiling flange" which is a vertical edge above the tub rim around the 3 sides that fit against the walls (surrounds?). The Jacuzzi installation diagrams show the tile or whatever extending over this flange. The flange is screwed to the studs. The greenboard goes behind the tille or marble or whatever on the walls around the tub. No need for it to be all the way to the ceiling.
Ed

house
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