Where bathroom damp comes from
There are three causes: moist air does not leave, water gets behind the tiles through joints and junctions, and water sits on the floor because of a wrong fall. All three are dealt with at the design and first-fix stage, before any tiles go down. After that, every fix means ripping out.
Mould on the ceiling and black grout lines are the visible part. What you cannot see is worse: water seeping for years through a crack in the grout into the screed and the wall. In a block of flats, the first person to notice is the neighbour below. So when we renovate a bathroom in an Eixample flat, we look not at the tiles but at what is underneath them.
Extraction: size it, do not just reuse the old one
The extractor fan is chosen for the bathroom's volume and how it is used: four showers in a row every morning is a different job from a guest bathroom. An old duct in the wall or a shared shaft in a sixties building may be partly blocked or sized for something else, so it is checked rather than simply having a new fan connected to it.
Two details matter more than power. The first is control: a fan with a timer or humidity sensor runs as long as needed, not the five minutes the light is on. The second is where the replacement air comes from. If the bathroom door closes tight to the floor, the fan has nothing to pull; there needs to be a gap under the door or a transfer grille. How bathroom ventilation fits into the air of the whole home is covered in our ventilation article.
Waterproofing and the flood test
Tiles and grout are not waterproof. Water is held back by the waterproofing membrane underneath: across the whole bathroom floor, turned up the walls, and in the shower zone to the full height the water reaches. Corners, wall-to-floor junctions, pipe penetrations and the drain are the weak points, and they are reinforced with tapes and collars, not just given an extra coat.
Before tiling, the waterproofing is flood-tested: the drain is plugged, the floor is filled with water and left for a full day. Then the water level is checked and, if there is access, the ceiling of the flat below. It costs one day in the programme. A leak found after tiling costs a stripped-out bathroom and a repaired ceiling downstairs. Ask for photos of the waterproofing before it is covered: you will never see that layer again.
The fall towards the drain
A level-access shower with no tray looks best and forgives least. Water has to reach the drain from any point in the shower area, with no puddles by the glass and no back-fall towards the door. A linear drain along the wall allows the fall in a single plane, so large-format tiles do not have to be cut into an envelope pattern.
In older buildings the drain runs into height problems. To get the fall and the depth for the drain, the bathroom floor sometimes has to be raised or the drain recessed into the slab, which becomes a question for the structure and the ceiling of the flat below. That is decided before the quote, not on site. The finished fall is tested with water before tiling: pour out a bucket and watch where it goes. The installer's word is not evidence here.
Joints, sealant and materials
In the shower area epoxy grout makes sense: it does not absorb water and does not darken like cement grout. Silicone at the junctions, bath to wall, glass to floor, basin to worktop, is a consumable. Even when applied perfectly it eventually lifts, and every few years it has to be redone. Ten minutes of maintenance protects the wall behind the tiles.
The rest is choosing materials with moisture in mind. A ceiling in moisture-resistant plasterboard and paint made for wet rooms. A wall-hung vanity or one on legs, so floor water never reaches the board edges. A door with or without a threshold: a decision that determines where water goes if the shower ever overflows.
What to check before tiling and at handover
Before tiling: the fan is connected and pulling, the duct has been checked; waterproofing covers the whole floor and turns up the walls, with corners and pipes reinforced and photographed; the flood test lost no water; the fall has been tested with water. At handover: shower water drains with no puddles, the silicone is continuous, the fan switches off on its timer or sensor, and there is a gap under the door.
The Urgel flat in the Eixample has two bathrooms for four bedrooms. With that load there is little margin: the more people share a bathroom, the less margin the ventilation and waterproofing have. When a bathroom is part of a full renovation, we run it through the same stages, with contract-based staged payments and one person responsible for the site.