Outside the shower, there is no waterproof membrane. There is no layer. The protection is junctions — the lines where one material meets another. A basin is a hole in the vanity. Water splashes at the basin, runs down the wall behind it, and the protection is the caulk in the joint where the wall meets the vanity top. A bathtub is a container sitting on the floor. Water runs down the side of the tub, and the protection is the caulk in the joint where the tub edge meets the floor. A toilet is mounted on the floor. Water sometimes escapes from around the base, and the protection is the caulk in the joint around the toilet.
These junctions are protected by silicone caulk, which is waterproof and flexible. The caulk seals the joint, stopping water from running down into the wall below. This is different from a waterproof membrane, which is a continuous layer that stops water outright. Caulk at junctions is how moisture is controlled outside the wet zone — the zone where tile and waterproofing are doing the heavy work.
But not all junctions are sealed. Some are deliberately left open so water can drain and dry. A junction that is sealed but collects water underneath can trap moisture and promote mould growth on the back. A junction that is left open allows water to drain and air to circulate, drying the area. The choice of which junctions to seal and which to leave open is a design decision based on how water actually moves in that part of the bathroom.
The floor perimeter is the line where the floor meets the wall. Water stands at this line if there is any low spot in the floor. The seal here is complicated because water sitting against the wall will find its way inside if the seal fails. The detail at the floor perimeter is usually a coved base — a small radius where the floor curves up to meet the wall — so water does not collect in a sharp corner. The junction is sealed but with an understanding that this is the lowest point where water collects.
A sealed joint is not permanent. Caulk has a life. After five to ten years, depending on the caulk type and the exposure, the caulk loses flexibility, cracks, or separates from the edge. When that happens, water begins to get past. This is maintenance work. The caulk has to be removed, the joint cleaned, and new caulk applied. A bathroom owner who understands this schedule — and plans to re-caulk every five to seven years — can keep junctions sealed and dry.
The weight of water is against the bottom of the room. Water runs down from above — from the basin, the tub, the ceiling — and collects at the floor level. This is where protection is most critical. The junctions at the floor level, especially at the perimeter, are where water concentration is highest and where failure is most consequential. These are the junctions that need the most attention and the most durable sealing.