I am renovating a bathroom in an old house built in 1905, so of course the floors slope towards the centre of the house. We decided to use this to our advantage and build a curbless marble tiled shower with a line drain at the entrance to the steam shower that will be enclosed with glass later on. Since the floor in the shower already slopes towards the drain 3/4" over the 3 feet, is it possible to just build up the subfloor in the shower to meet the subfloor outside the shower then apply a waterproofing membrane directly to the subfloor? Or is a mortar or prefaced foam base an absolute necessity? I would prefer not to use another base if possible to keep the slope to a minimum. Also, if plywood isn't the best could a sheet of kerdi board be used to raise the subfloor instead?
Kerdi board would be good, the only problem i see is that the mortar wont stick on a regular membrane, unless you use a liquid based waterproof like aqua defense or the schluter kerdi system.
Also you might have to raise /level the outside subfloor of the shower so that might change the slope differences.
Plywood and rubber is old school ..it leaks and a waste of time ...use the new kerdi base systems at any tile store...they aint cheap but a properly installed membrane will be 100% better or better yet just put in a acrylic pan and then tile surround...even better...I have no idea why people still waste time with tile bases when better and no leak products are out there.
No first playood is not enough stable for wall, not good for base as you need slope on drain direction for the water evacuation. Kerdi board don't have slope in, Schluter Kerdi line base got the proper slope for shower. Yes Kerdi 1 1/2 on wall and drain at the of shower also membrane will stik with unmodified mortar and tile also over membrane.
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