Why bathrooms leak months after completion

You finish a refurbishment, move the towels in, and tell yourself you can finally stop thinking about it. Then the ceiling below shows a brown halo, or the silicon line turns black, or the bath edge starts to “give” when you step in. Bathroom sealing is meant to keep water where it belongs, but it often fails months later because material movement keeps happening long after the last tile is wiped down.

Most leaks aren’t dramatic bursts. They’re slow, repeatable micro-failures: a hairline gap that opens on a cold night, a tray that flexes under weight, a grout line that looks intact until the shower hits it twice a day. The frustrating part is that the bathroom can look perfect right up to the moment it doesn’t.

The leak that starts as a millimetre

Water doesn’t need much. It only needs a route.

A shower enclosure can shed litres a minute, but the leak often begins with a gap you’d struggle to push a credit card into. Capillary action draws moisture along corners. Repeated wetting and drying pumps it deeper. Once water gets behind tiles or under a tray, it stops behaving like a spill and starts behaving like weather: persistent, patient, and hard to pin down.

That’s why “it didn’t leak at handover” isn’t the reassurance people think it is. A bathroom might survive the first few weeks simply because nothing has stressed it yet.

Why bathrooms move after they’re “finished”

A new bathroom is a stack of different materials bonded together: timber joists, plasterboard, tile adhesive, porcelain, acrylic, aluminium frames. Each one expands, contracts, flexes, and settles at its own pace.

Humidity swings do the rest. Steam gets into voids. Extractor fans don’t always keep up. Timber and boards take on moisture, then dry out. Heating goes on, then off. The structure isn’t collapsing; it’s just doing what buildings do: shifting slightly.

The key point is simple: most failures show up where a rigid layer meets something that moves. Tiles and grout are rigid. Trays, baths, stud walls, and floors are not.

Common “movement points” that open up

• Bath-to-tile and tray-to-tile junctions
  
• Shower screen brackets screwed through tile
  
• Tile corners (internal angles) that were grouted instead of sealed
  
• Floor perimeters where tiles meet skirting or walls
  
• Around wastes and traps where parts were tightened once, then never re-checked
  

If your leak appears at a junction rather than the middle of a wall, movement is usually the story.

The hidden trap: sealing that looks neat but isn’t doing the job

A clean bead of silicone is aesthetically convincing. It can also be mechanically useless.

Two mistakes show up again and again. First: sealing over dust, soap residue, or damp substrate, which prevents proper adhesion. Second: relying on a thin surface bead where the gap underneath is too deep or irregular, so the sealant stretches, tears, and lifts over time.

Even good sealant can fail if it’s asked to act like a structural component. Silicone is flexible, not magical. It wants a stable joint with the right depth, the right backing, and clean, dry edges to bond to.

A small tell: if you can press the bead and feel it “hollow” behind, it may not be bridging a well-formed joint. It may just be decorating the edge.

Wet-room reality: waterproofing is a system, not a product

In showers and wet rooms, tiles are not the waterproof layer. They’re the wear layer.

The waterproofing is the tanking membrane, the sealed boards, the correctly formed falls to the drain, the collars around pipes, the taped corners, and the detailing you never see once the tiles go on. If any one of those steps is skipped or rushed, the bathroom can perform fine until repeated use finds the weak link.

A classic failure is water tracking through grout and adhesive, then collecting at the first break in the membrane or at an unsealed fixing. It’s not that grout is “leaking” like a hole in a bucket. It’s that water is getting where it should never have been allowed to go.

The “it only leaks when…” clues that point to movement

Intermittent leaks are maddening, but they’re also diagnostic. They often tell you what’s moving.

If a leak appears only when someone is in the shower, a tray or waste connection is flexing under load. If it shows up after a long hot shower, thermal expansion and steam are involved. If it’s worse in winter, shrinkage and heating cycles may be opening joints.

Here are patterns that commonly match the cause:

• Leaks after stepping in the tray: tray flex, waste compression joint, or inadequate support
  
• Leaks during heavy spray at one corner: enclosure seals, mastic breaks at junction, screen alignment
  
• Leaks hours later: water pooling behind a wall, slow migration along timber or plasterboard
  
• Staining directly below the shower valve: pipework connection or failed penetration seal
  

None of these require a flood. They require repetition.

How to reduce the odds of a “month six” leak

You don’t need to become a bathroom inspector. You do need to respect that bathrooms are hostile environments: water, heat, movement, chemicals.

If you’re planning a new install, or you’re trying to figure out why yours is failing, focus on fundamentals rather than cosmetics.

• Prioritise proper substrate preparation: rigid floors, adequate noggins, boards rated for wet areas.
  
• Treat junctions as movement joints: silicone where movement is expected, not grout.
  
• Use the right sequencing: tanking and detailing first, tiling second, sealing last.
  
• Make ventilation non-negotiable: a correctly sized, properly ducted extractor reduces the wet/dry extremes that drive movement.
  
• Avoid “one bead solves all”: joints may need backing rod, correct gap sizing, and full cure time before use.

And if you’re resealing: remove the old sealant fully. Sealing over failed silicone is a short delay, not a fix.

A quick checklist before you start ripping things out

A leak months later often triggers panic demolition. Slow down first. A few observations can save you thousands.

1. Check whether the leak correlates with weight in the tray or bath.
   
2. Inspect the tray/bath edge: any gaps, lifting, or mould returning in the same spot.
   
3. Look for cracked grout at corners (a strong movement signal).
   
4. Check around penetrations: shower bar fixings, pipe sleeves, screen brackets.
   
5. If accessible, inspect the waste from below for staining and drips during use.

The goal isn’t to “catch the drip” once. It’s to understand the pattern. Bathrooms leak in patterns because bathrooms move in patterns.

Sign you notice	Likely culprit	What it suggests
Crack at internal corner grout	Rigid corner detail	Movement not accommodated
Leak only under load	Tray/bath flex or waste joint	Support or compression issue
Mould returns on one junction	Sealant adhesion failure	Surface prep or joint design

FAQ:

• Why would a bathroom leak months after completion if it passed a water test? Early tests are brief and don’t replicate weeks of heat, humidity, cleaning, and daily flexing. Movement and repeated wetting often reveal weak junctions later.
• Is grout supposed to be waterproof? Grout is water-resistant, not a waterproof barrier. In showers, waterproofing should come from tanking/membranes and correct detailing behind the tiles.
• Should corners be grouted or sealed? Internal corners and changes of plane are typically treated as movement joints and sealed, because rigid grout is prone to cracking as surfaces shift.
• Can I just reseal the visible edge and stop the leak? Sometimes, if the leak is purely at a junction. But if water is already getting behind tiles due to missing tanking or a moving tray, resealing may only mask it temporarily.
• What’s the fastest way to confirm tray movement? If you can access below, watch the waste while someone carefully steps into the tray. Movement at the trap or a brief drip under load is a strong clue.