Water damage
Why Under-Sink Water Filters Leak
It is a pressurised plastic housing with a rubber seal, sitting on a cabinet floor nobody looks at, usually fitted once and then forgotten. Here is how one fails, and what it does when it does.
There is a good chance you have one of these and have not thought about it in years: a cylindrical plastic canister under the kitchen sink, plumbed into the cold supply, holding a replaceable filter cartridge. It was probably fitted by somebody else. It has a rubber seal, it is under mains pressure every second of every day, and it sits on a cabinet floor nobody looks at.
When one fails it rarely bursts. It weeps — slowly, downward, into the cabinet base, then into the subfloor, then sideways along whatever the floor is made of. By the time it is visible in the room it has been going on for a while.
The damage from these is almost never about volume. It is about time. A fitting losing a few drips an hour into a closed cabinet can do more harm over three months than a burst line does in an afternoon, because nothing about it gets noticed.
Why a slow leak is the expensive kind
A burst supply line is loud, obvious and dealt with in an hour. Everybody knows what happened and roughly where the water went.
A weep is the opposite. It is quiet, it is behind a cabinet door, and the water goes where water goes — down through the cabinet base, into the subfloor, under whatever flooring runs out of the kitchen. Continuous flooring makes that worse: a laminate floor running unbroken from the kitchen through the rest of the house gives water a path with nothing to stop it, which is why a leak under one sink can end up being measured two rooms away.
That is exactly what happened onthe Richmond kitchen we documented. The failure was a filter canister. The readings ran past the kitchen into the dining room, because the laminate was continuous through the house.
Five ways these fail
None of them are dramatic, and all of them are gradual. That is the pattern worth recognising.
The O-ring perishes
The housing seals against the cap with a rubber O-ring. Rubber hardens with age, heat and time under compression. It does not fail suddenly — it stops recovering its shape, and the seal goes from watertight to very nearly watertight, which is the dangerous version.
The housing was over-tightened, or under-tightened
These are plastic threads. Over-tighten and you deform the seat; under-tighten and the O-ring never fully seats. Both leaks look identical from outside, and both usually start at the next cartridge change rather than at installation.
The plastic itself ages
A clear or translucent housing spends years under constant pressure. Crazing — a web of fine surface cracks — is the visible stage of that, and it is a reason to replace the housing rather than reseal it.
Pressure spikes nobody felt
Household water pressure is not constant. Thermal expansion and sudden valve closures produce brief spikes well above the static pressure, and a seal that holds at rest can weep under them.
The cartridge was never changed
A clogged cartridge raises the pressure differential across the housing and makes the unit work harder than it was designed to. The interval is on the manufacturer’s instructions, and it is the one maintenance task on the whole assembly.
Five things to check under your own sink
None of this requires tools and none of it involves taking anything apart.
Look at the cabinet floor, not the fitting
A slow leak shows up on the deck below before it shows on the housing. Dark staining, a lifted or swollen edge where two boards meet, a ring you do not remember, a cabinet floor that feels soft under a thumb — those are the early signs. The fitting itself often looks fine.
Put a dry paper towel under it overnight
The simplest test there is. A weep too slow to see is not too slow to blot. If the towel is marked in the morning, something above it is leaking.
Check whether anything is catching it already
A folded towel, a tray, a takeaway container under the housing means somebody already knew. On the kitchen we documented, there was a towel under the canister and it had been there a while.
Find out what your water pressure is
A gauge that screws onto a hose bib costs very little. California’s plumbing code requires a pressure regulator where static pressure exceeds 80 psi, so a reading above that is both a plumbing issue in its own right and a reason the fittings downstream are working harder than they should.
Know where the valve is before you need it
There is usually a shut-off on the supply feeding the filter, and a main for the house. Knowing both, before anything is wrong, turns a flood into a puddle.
If you find water: close the valve feeding the filter, then deal with the fitting. In that order. The point of stopping the supply first is that it turns an urgent problem into a scheduled one.
What we are not going to tell you to do
Plenty of pages will walk you through dismantling the housing and replacing the O-ring. We are not going to, for one reason: the torque figures, the seal sizes and the lubricant that is safe on that particular plastic are specific to the unit, and they are in the instructions that came with it. A guessed-at reassembly of a pressurised fitting under a cabinet you cannot see into is how a weep becomes a flood while you are out.
If the unit is old, the honest answer is usually to replace the housing rather than reseal it — and if you do not know how old it is, that is itself an answer.
If it has already got into the floor
Staining on the cabinet base, a soft spot, a floor edge that has lifted, or a baseboard that has darkened all mean the water is no longer only in the cabinet. At that point the question stops being about the fitting and starts being about how far it travelled — which is a question you answer with a moisture meter, not by looking.
What should happen next is: find the extent before anything is opened up, contain the area, remove what is wet, and dry the structure to a measured standard witha drying log to show it got there. If you are in Contra Costa County,that is the work we do — and we would rather tell you on the phone that you do not need us than turn up and find out.
Sources
- California Plumbing Code § 608.2, excessive water pressure — a pressure regulator is required where static service pressure exceeds 80 psi. Summarised for homeowners by the City of San Diego Public Utilities Department,Water Pressure. Checked 23 October 2026.
- Cartridge replacement intervals, housing service life and reassembly torque are specific to each unit and are stated in its own manufacturer instructions. This page deliberately names no figure for them.