Our tile setter started work today. He built the rough framing for the shower seat and affixed a membrane to the existing concrete bed (poured by our previous tile installer) with a layer of thinset. The membrane is called Schluter-DITRA and, according to the product website, it performs four functions: it uncouples the tile from the substrate to prevent movement in the subfloor from affecting the tile above; it waterproofs the floor; it allows any vapour rising from the subfloor to escape by evaporation; and it distributes the tile load evenly across the floor. I suppose we'll only know if it lives up to its claims if it never fails us. But we admit that it looks substantial and capable and bright orange. Something about that strident colour declares "I am strong. I mean business. I am not mere decoration".
Tomorrow we pick up our tile.