Tiles, grout and seamless coatings are finishes, not waterproofing. A properly built bathroom has four layers in the wet zone: a sound, flat substrate; a continuous waterproof membrane with reinforced corners and sealed drain and pipe penetrations; the adhesive or base coat; and the finish. In an apartment, a failure here usually shows up first on the neighbour’s ceiling, which makes it a shared problem as well as yours.
Why is waterproofing so often missing in older bathrooms?
Many bathrooms in Spanish apartment blocks from the seventies, eighties and nineties were built with tiles set in mortar directly on brick and on the floor screed. Bathtubs with a curtain kept most water inside, and the tiled floor only got splashed. Nobody expected a floor-level shower with water running across the floor every day.
When those bathrooms are updated by simply swapping the bath for a tray or a tiled shower, the missing protection becomes a real risk. Water that passes through grout, around a badly sealed drain or along a pipe soaks into the screed and travels, sometimes a long way, before it appears as a stain below.
What are the layers in a waterproofed wet zone?
- Substrate. Brick, block, render, board or screed that is sound, flat, clean and dry. Falls towards the drain are formed at this level, not in the tile adhesive.
- Primer, where the membrane system requires it, so the membrane bonds to the substrate.
- Waterproof membrane. A liquid-applied or sheet membrane that forms a continuous skin over the floor and walls of the wet zone.
- Reinforcement at internal corners, wall-to-floor junctions and around penetrations, usually a tape or collar bedded into the membrane.
- Adhesive or base coat compatible with the membrane.
- Finish: tiles and grout, or a seamless coating and its sealer.
Each layer has its own job. Merging them, for example relying on a sealer to do what the membrane should, is where problems start.
Where exactly should the membrane go?
Across the entire shower floor and up the shower walls well above the height that water reaches. Around a bath, on the walls above the rim and on the floor beside it. Across the bathroom floor wherever splashing is expected, and turned up slightly at the walls so it forms a shallow tray. Into niches, including their base and sides, because a niche is a hollow in the wettest wall of the room.
Membrane manufacturers publish their own coverage and height recommendations; we follow the instructions of the system used on each job rather than a single number for every bathroom.
Which details fail most often?
- The drain. The membrane must bond to the drain body through a flange or collar. A drain simply set into the screed, with tiles around it, leaks at its edge.
- Pipe penetrations. Every shower arm, valve body and tap passing through the wall needs a sealing sleeve.
- Internal corners. Movement concentrates here; without reinforcing tape the membrane cracks.
- The screen fixings. Holes drilled through the finish and the membrane need to be sealed.
- The threshold. Where a walk-in shower meets the rest of the floor, water must not be able to run out under the finish.
Our bathroom renovation mistakes article covers these and other common faults from the owner’s point of view.
Why do the electrics matter for a wet zone?
Water and electricity are planned together in a bathroom. Spain’s low-voltage electrical regulation, the REBT, has a specific technical instruction, ITC-BT-27, for rooms containing a bath or shower. It defines zones around the bath or shower, numbered from 0 to 3, and limits what can be installed in each. Inside the bath or shower tray itself only equipment designed for that zone is allowed, and switches and sockets are generally excluded from the zones closest to the water; sockets further away must be protected, for example by a residual-current device of no more than 30 mA.
In practice this decides where light fittings, extractor, mirror light, shaver socket and towel rail can go. Those positions are fixed by our authorised partner installers before the membrane is applied, so the cables are in place and the membrane is not punctured afterwards.
How can you tell whether a bathroom has been waterproofed?
Once tiles are on, you cannot see it. That is why we keep the membrane exposed until it has been checked, and you are welcome to see it before it is covered. A careful contractor should be able to tell you which membrane system is used, how corners and drains are detailed, and show you the layer before tiling starts.
In an existing bathroom, signs of trouble include grout that stays dark long after showering, loose or hollow-sounding tiles near the shower, a musty smell from the floor and, in apartments, stains on the ceiling below. These point to water getting behind the finish; they do not on their own prove where it comes from.
What are your obligations to the neighbours?
In an apartment block your bathroom sits above someone else’s home. Spain’s Horizontal Property Act lets each owner modify the installations of their own unit, as long as the work does not damage the building’s security, structure or exterior, or harm another owner’s rights, and requires the owner to notify the community’s representative beforehand. A leak from a poorly built shower can harm the neighbour below, which is one more reason to treat waterproofing as essential rather than optional.
If the shared soil stack or pipes running through your bathroom are themselves in poor condition, that is a matter for the community, and we will point it out so you can raise it.
What should you ask before signing a bathroom quote?
- Is there a separate waterproofing stage, and which areas does it cover?
- How are the drain, pipes and corners detailed?
- Are falls formed in the base or in the adhesive?
- Will I see the membrane before it is covered?
- Who carries out the electrical work near the shower?
We answer all of these in our itemised quote for a bathroom renovation or a walk-in shower.
Official sources
- REBT, Real Decreto 842/2002 (ITC-BT-27, rooms with a bath or shower) – BOE consolidated text
- Ley 49/1960 de Propiedad Horizontal, art. 7.1 – BOE consolidated text
Bauman Raymond Attila · Reforma Baños Málaga · Avenida Antonio Belón 3, Marbella. Last reviewed: 05/11/2026.
