I remember tearing out an old tile shower early in my remodeling career and being surprised to find not greenboard but a layer of cement under the wall tile. Even more surprising was the lead pan liner beneath the mortar bed on the floor. Like most people, I had mistakenly assumed that a grouted tile surface was waterproof; it had never occurred to me you might need backup moisture protection.

In the 20 years since, I’ve installed hundreds of showers, all of them with pan liners — although I’ve adopted more modern liner materials. When it comes to the walls, however, I often still do it the old way, with floated mortar walls. I work a lot in vintage homes, and there’s no better way to create a flat, plumb tile substrate over the...

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