For a tile shower built on a mortar-bed floor, use a heavy 40-mil plastic pan liner. Wrap it over the curb and seal it in place at the drain with a two-part subdrain assembly, as shown below.

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In a typical tiled shower, the pan liner extends above the curb height on walls and is sealed in place by the two-part subdrain assembly. Any moisture that penetrates the tile floor and accumulates will exit through weep holes.
Tim Healey In a typical tiled shower, the pan liner extends above the curb height on walls and is sealed in place by the two-part subdrain assembly. Any moisture that penetrates the tile floor and accumulates will exit through weep holes.

For more on working with ceramic tile, visit the JLC Field Guide.