Hello. I’m having a Wetroom shower tiled. One side is a new wall made of Wedi boards which have been tanked. The other wall is an existing wall (mix of plasterboard and old lathe and plaster wall). Tilers have applied tile adhesive directly to that wall without tanking. On querying this with them, they have said that the wall was not true so they have used a very thick layer of adhesive in order to bring it level hence tanking was not necessary as you’d never get water ingress through such a thick layer of tile adhesive. Is this acceptable or should they have tanked it?