Waterproofingmembranetested before tile.

Tile and grout are both porous, and a tiled shower is designed on the assumption that water gets behind them. What stops that water reaching the framing is a continuous membrane — either a sheet product bonded to the substrate or a liquid applied in coats to a specified thickness — carried across the whole wet zone, lapped at internal and external corners with the manufacturer’s corner pieces rather than improvised, and sealed at every penetration for a valve, a spout or a fixing.

Then it is flood-tested: the drain is plugged, the pan filled, and the water left standing long enough to prove the assembly holds. It either does or it does not, and finding out at that point costs an afternoon. Finding out afterwards costs the bathroom and usually the ceiling below it. This is the step that quietly disappears from a cheap quote, because it is invisible in the finished room and it is the single most common reason we are called to rebuild a bathroom that is only a few years old.