Looks perfect — until water finds a path

The tiles are spotless, the grout lines look crisp, and the shower tray falls perfectly to the waste. Yet waterproofing systems can still fail quietly, because leak migration doesn’t care what the surface looks like. In bathrooms, balconies and wet rooms, water is patient: it only needs one path, and it will take it-often somewhere you won’t think to look.

You usually notice it late. A faint blister in paint on the hall side of the wall, a sweet, damp smell that comes and goes, a skirting board that starts to lift at one end. The space looks fine-until it doesn’t.

The lie we tell ourselves about “waterproof”

Most people imagine a leak as a drip straight down, a neat little crime scene with a clear culprit. In reality, water travels sideways, rides along junctions, and settles where gravity and capillary action agree. That’s leak migration: the reason the stain rarely appears directly under the failure.

A shower can leak at the screen fixings and show up at the doorway. A balcony can fail at the threshold and present as damp in the ceiling below, two metres away. The building doesn’t read your plans; it follows the easiest route.

And when you finally open something up, the surprise is almost always the same: the “wet bit” isn’t that wet, but the timber next to it is soft, or the plasterboard edge has been drinking for months.

Where waterproofing systems actually win (or lose)

Waterproofing systems aren’t just a product you roll on. They’re a chain: substrate prep, falls, junction detailing, compatible adhesives, penetrations sealed correctly, and a membrane that’s continuous. Any weak link becomes a doorway.

The failure points are boring, repetitive, and that’s why they get missed:

• Corners and junctions: wall-to-floor changes of plane that move slightly over time.
  
• Penetrations: pipes, mixer bodies, shower wastes, floor drains-anything that punches through the membrane.
  
• Thresholds and upstands: doorways, balcony sliders, the tiny vertical returns that stop water escaping.
  
• Fixings after the fact: shower screens, grab rails, vanity brackets installed through finished waterproofing.
  

A system can be “to spec” on paper and still fail in the handover rush when one trade assumes the other has sealed the last 10%.

The part nobody sees: continuity and compatibility

Membranes don’t fail only because they’re thin. They fail because they’re interrupted, contaminated, or asked to bond to something they weren’t designed for. Dust, primer skipped, damp substrate, wrong cure time, incompatible sealant-each one turns a continuous barrier into a patchwork.

It’s also where DIY and even good-intentioned repairs go wrong. A bead of silicone feels decisive, but silicone over a moving gap without proper backing can peel like a sticker. A “quick paint-on coat” on top of existing residue can delaminate, trapping water rather than stopping it.

“If water can get behind it, it will live behind it.”

That’s why reputable systems insist on the unglamorous stuff: correct primers, reinforcing tapes at corners, puddle flanges at drains, and specified cure windows before tiling.

A short, practical way to think about leak migration

When you’re trying to diagnose a problem-or prevent one-stop hunting for the stain and start tracing the paths. Water tends to:

1. Enter at a high-stress detail (a corner, a penetration, a threshold).
   
2. Travel along a layer (back of tile adhesive, along cement sheet joins, under screeds).
   
3. Show up where it can evaporate (skirtings, paint lines, ceiling junctions), not where it entered.

Two small habits help more than most “miracle sealers”:

• Take photos of waterproofing before tiles go on, especially corners, drains and thresholds.
  
• Keep a simple map of what’s behind each wall: plumbing runs, niches, screen fixings, and the nearest escape routes.

Delay turns a minor breach into a slow spread. Not dramatic, just expensive.

The quiet checklist that prevents the loud repair

If you’re building, renovating, or signing off work, you don’t need to become a waterproofing inspector. You just need to ask the questions that force clarity.

• What system is being used, exactly? (Not “a membrane”, but the named system and compatible components.)
  
• How are penetrations sealed? (Collars, flanges, proprietary grommets-something more than optimism.)
  
• What’s the plan for post-waterproofing fixings? (Screens and rails should be designed into the detail.)
  
• Were the falls checked before waterproofing? (A membrane won’t fix a flat floor that holds water.)
  
• Was it allowed to cure before tiling? (Rushing traps solvents and compromises adhesion.)

None of this is about mistrust. It’s about recognising that water is relentless and humans are tired.

Risk area	What to look for	Why it matters
Drains & wastes	Proper flange/collar, clean bond	Most common entry point for hidden leaks
Thresholds	Upstand detail, continuity under frames	Stops sideways escape into adjacent rooms
Screens & fixings	Planned anchor points, sealed penetrations	Prevents “perfect finish, punctured membrane”

When “it’s probably the grout” is the wrong answer

Grout discolouration and surface mould are annoying, but they’re not proof of a membrane failure. The danger is the opposite assumption: that a persistent damp smell or recurring stain is “just condensation”.

A good rule: if moisture appears in a dry zone (hallway wall, bedroom ceiling, skirting outside the bathroom), treat it as leak migration until proven otherwise. Surface resealing buys time, but it can also hide the evidence long enough for damage to spread.

Water rarely announces itself. It negotiates.

FAQ:

• What’s the biggest misconception about waterproofing systems? That tiles and grout are the waterproof layer. They’re a wear surface; the waterproofing is behind them, and it must be continuous.
• How can leak migration show up away from the bathroom or balcony? Water can travel along boards, adhesive beds, joists or screeds and emerge where it can evaporate-often at edges, doorways, or ceilings below.
• Is silicone enough to stop a leak around a shower screen? Sometimes it masks symptoms, but if water is getting behind the screen fixings or into an unsealed junction, silicone alone often fails with movement and time.
• What should I document during a renovation? Photos of corners, drains, thresholds, niches and any penetrations before tiling. If an issue arises later, this saves guesswork and arguments.
• When should I stop patching and investigate properly? If stains recur, skirtings swell, paint blisters, or a damp smell persists, assume the problem is behind the surface and get a targeted inspection before damage spreads.