The errors you don’t see until it’s too late
A bathroom renovation can look flawless on handover day. Perfectly aligned tiles, gleaming fixtures, taps running like clockwork. Then six months later, a damp stain appears on the downstairs neighbour’s ceiling. Or the shower tray starts pooling water in one corner. Or a circuit breaker trips every time you use the hairdryer.
Technical errors in bathroom renovations are silent, slow, and brutally expensive to fix. Because fixing isn’t just repairing the fault — it’s demolishing whatever was built on top of it, correcting it, and putting everything back together.
At Bathscape, we’ve been renovating bathrooms in Valencia long enough to have seen each of these errors at least a dozen times. Not in our own work — in the jobs we come in to repair afterwards. Which are, sadly, far too many.
Here are the 7 most serious technical errors, with their repair costs, how to detect them, and above all, how to prevent them.
Error 1: Poor or non-existent waterproofing
Repair cost: 3,000-5,000 EUR
This is the king of errors. Waterproofing is the barrier that prevents water splashing in the shower or condensing on walls from seeping into the slab, the structure, or the downstairs neighbour’s flat. And it’s the first thing that gets skipped or done poorly when someone tries to cut costs.
What goes wrong: A single coat of membrane is applied instead of two. Waterproofing in the shower zone is omitted (“if I seal the tiles properly, it’s not needed”). An unsuitable product is used. Floor-to-wall joints aren’t finished with elastic sealing tape.
When you notice: 3 to 18 months later. Damp stains on the ceiling below, tile detachment, mould smell, white efflorescence on grout lines.
How to avoid it: Insist on waterproofing across the entire wet zone (floor + walls up to 2.20 m in the shower area, 1.20 m elsewhere) with a complete system: primer + two coats of liquid membrane or sheet membrane + reinforcement tape on corners and junctions. Under the CTE (Technical Building Code, Basic Document HS: Healthiness), waterproofing in wet zones is mandatory.
We have a complete technical guide on waterproofing if you want to dive deeper.
Error 2: Incorrect drain slope
Repair cost: 1,500-3,500 EUR
Water has to go somewhere. And if the shower floor or drain slope isn’t properly calculated, the water just sits there. It seems unbelievable that something so basic fails so often.
What goes wrong: The slope of the shower tray or wet-room floor is below 1.5% (the recommended minimum). Or worse: there’s a reverse slope that pools water in the corner opposite the drain. It also fails when the waste pipe has sections with no gradient or with dips and rises (accidental traps).
When you notice: Day one. Water doesn’t drain well, pools form, and it takes minutes to clear. It seems minor, but it generates permanent moisture, fungal growth, and accelerated deterioration of grout lines.
How to avoid it: Check the slope with a level during construction. The minimum is 1.5% towards the drain (1.5 cm per linear metre). For flush-floor showers, 2% is safer. In waste pipes, the gradient must be constant between 1% and 4%, with no low points. This is how our process works: we verify slopes with a laser level before tiling.
Error 3: Undersized pipe sections
Repair cost: 2,000-4,000 EUR
When a plumber installs pipes with a smaller bore than required, the problem isn’t evident until two simultaneous uses coincide: someone turns on the kitchen tap while you’re in the shower and the flow drops. Or you flush the toilet and the basin loses pressure.
What goes wrong: 16 mm outer diameter pipes are installed where 20 mm is needed, or 20 mm where 25 mm is required. It also happens with waste pipes: a 40 mm drain for a toilet that needs 110 mm is a guaranteed blockage.
When you notice: In daily use. Insufficient pressure, gurgling drains, frequent blockages. And after a year, the daily frustration forces you to rip everything out and replace the pipes.
How to avoid it: Minimum pipe sizes are specified in CTE DB HS4. For supply: 20 mm on risers, 16 mm on individual branches (basin, bidet), 20 mm for shower. For waste: 40 mm on basin, 110 mm on toilet, 50 mm on shower. Don’t accept “this will do, it works the same”. Technical codes exist for a reason.
Error 4: Electrical installation in the wrong IP zone
Repair cost: 1,000-2,500 EUR (plus risk of electrocution)
This error doesn’t just cost money — it can cost a life. The bathroom is divided into electrical safety zones based on proximity to water, and each zone has restrictions on what can be installed and at what level of protection.
What goes wrong: A socket is installed 30 cm from the shower (Zone 1, where it’s forbidden). A luminaire without adequate IP rating is placed above the bathtub. A conventional switch is used inside Zone 2. No 30 mA RCD is installed for the bathroom circuits.
When you notice: If you’re lucky, when the RCD trips constantly. If you’re not, when someone gets a shock.
How to avoid it: The REBT (Low Voltage Electrotechnical Regulation, instruction ITC-BT-27) defines the zones with absolute clarity:
- Zone 0 (inside bathtub/shower): nothing electrical, only SELV 12V if strictly necessary
- Zone 1 (above bathtub/shower, up to 2.25 m): only water heater with IP X5
- Zone 2 (60 cm around Zone 1): luminaires IP X4, sockets with isolation transformer
- Zone 3 (rest of the bathroom): sockets and switches with 30 mA RCD protection
Any competent electrician knows these zones. If yours doesn’t, change electrician. Now.
Error 5: Missing expansion joints
Repair cost: 800-2,000 EUR
Materials expand and contract with temperature changes. In a bathroom, where the temperature can swing 15-20 degrees C between a hot shower and the overnight ambient temperature, this expansion is significant. If the tiling has no joints to absorb that movement, something is going to crack.
What goes wrong: The entire floor and wall surface is tiled without perimeter joints or intermediate expansion joints (mandatory every 3-4 metres or at junctions between surfaces). Tiles lift, crack, or debond from the substrate.
When you notice: 6 to 24 months later. A tile lifts, several sound hollow when tapped, fine cracks appear in the grout. One morning you wake up and you’ve got a ceramic minefield on the bathroom floor, just like that.
How to avoid it: 5-8 mm perimeter joint at all floor-to-wall and wall-to-wall junctions, filled with flexible silicone (not cement grout). Intermediate expansion joints every 3 metres in flooring and every 4 metres in vertical tiling. This is not optional — it’s regulation.
Error 6: Tiling on an unstable substrate
Repair cost: 2,000-4,500 EUR
“I’ll just stick the tile on top and done.” If the substrate (the wall or floor the tile is bonded to) isn’t stable, flat, and adhesion-ready, the tile is going to come off. It’s only a matter of time.
What goes wrong: Tiles are laid directly on old paint without chipping or priming. Porcelain stoneware is placed on a cement mortar that hasn’t cured. Tile is laid over tile with no adhesion treatment. Tiling is done over plaster (prohibited in wet areas).
When you notice: 3 to 12 months. Tiles that sound hollow, that move under pressure, that detach from a corner and end up falling to the floor. Sometimes in a cascade: one takes its neighbours with it.
How to avoid it: The substrate must be cement mortar or cement board (never plaster in wet areas). It must be clean, dry, flat (maximum deviation of 3 mm per metre) and free of paint residue, dust, or grease. If tiling over existing tiles, a specific adhesion primer must be applied and flexible tile adhesive used (C2TE per EN 12004 standard).
Error 7: Poor ventilation design
Repair cost: 500-1,500 EUR (but moisture damage can multiply that figure)
A bathroom without adequate ventilation is a mould incubator. And mould isn’t just unsightly — it’s a health risk, especially for people with allergies or respiratory conditions.
What goes wrong: No mechanical extractor is installed in interior bathrooms (without a window). An extractor with insufficient airflow is installed. The extractor is connected to a shared duct without a non-return valve (your neighbour’s smells reach your bathroom). No air inlet is provided via the door (bottom gap or transfer grille).
When you notice: Within weeks. The mirror steams up and takes an eternity to clear. Silicone joints blacken rapidly. Mould appears in corners and on the ceiling. The towel never fully dries.
How to avoid it: CTE DB HS3 specifies minimum ventilation rates: 15 l/s per wet room. For a standard bathroom, that requires an extractor of at least 54 m3/h. If the bathroom has no window, the extractor is mandatory (not optional, mandatory). In addition, the door must allow air entry: a 1-1.5 cm bottom gap or a transfer grille.
At Bathscape we install humidity-sensing extractors: they activate automatically when humidity exceeds 70% and switch off when it drops. No switches, no forgetting, no mould. Discover these systems in our Smart Tech designs.
The common denominator: haste and false economy
If you look across all 7 errors, they all share something: they occur when someone tries to go faster or cheaper than the engineering allows. Skipping waterproofing saves 300-500 EUR in materials. Repairing the leak costs 3,000-5,000 EUR. The ratio is 1 to 10.
Our internal data confirms it: 68% of the remediation projects we carry out at Bathscape stem from technical errors in a previous renovation done by another company. Not defective materials, not natural wear and tear — preventable errors. Each of those repairs costs the homeowner an additional 2,000 to 6,000 EUR on top of what they originally paid.
We believe the industry needs more technical training and less salesmanship. And that clients need tools to distinguish the professional from the cowboy before signing anything. Our evaluation checklist is a good starting point.
Our guarantee against these errors
At Bathscape we offer a 3-year guarantee on all our renovations. But our real guarantee isn’t a piece of paper — it’s the process: prior technical inspection, execution with documented protocols, verification of each phase before moving to the next, and complete photographic documentation.
If something goes wrong (because in real life, things sometimes do), we detect it early and correct it at no additional cost. That’s what a fixed price with a real guarantee means.
Configure your renovation in our configurator and compare with what others offer you. And if you’re going to hire someone else, at least take this list of errors as a reference. Because the tears end up costing more than the tiles.
Frequently asked questions
How do I know if my current renovation has any of these errors?
Warning signs: tiles that sound hollow when tapped, water that doesn’t drain well in the shower, damp stains on the ceiling below, persistent mould smell, circuit breakers that trip frequently. If you detect any of these symptoms, request a technical inspection.
Can I claim against the contractor who made the error?
Yes, within the legal warranty periods: 1 year for finishing defects, 3 years for habitability (waterproofing, installations). You need to document the defect and demonstrate it’s due to poor workmanship. A surveyor can help you with the report.
How much does it cost to prevent these errors versus repair them?
Prevention (doing things properly from the start) adds between 10% and 15% to the cost of a budget renovation. Repair costs between 30% and 80% on top of the original renovation cost. The maths are overwhelming.
Are technical errors covered by home insurance?
It depends on the policy. Water damage is usually covered, but many insurers exclude construction defects or recent renovation work. Check your policy before assuming you’re covered.
Next step
If you’re going to renovate your bathroom, choose a professional who understands these 7 technical points and can demonstrate how they prevent them. Our configurator generates quotes that include all prevention items — because we don’t cut corners on what matters.