Cracks in tile and stone grout lines allow water through the finishes, even when those finishes aren’t submerged, which can cause damage such as that shown here.
Shower curbs are typically framed with wood, which shrinks and expands with changes in humidity, putting stress on grout lines and often causing them to crack.
Simply running the liner to the outside edge of the curb isn’t enough to prevent water damage. Water can still seep through cracked grout lines and wick under the liner edge.
Whether a sheet membrane or liquid-applied, the liner needs to cover the top of the curb, extend down the outside face, and lap onto the floor.
This application, like the one in the previous slide, left critical gaps in the waterproofing where cracks in the grout are apt to form.
One end of the tub is inside the shower enclosure. No damage is apparent, but check out the next two slides.