Where this starts
A laminate worktop is a particleboard core faced on top and front with laminate and usually sealed on the underside. The faces are effectively waterproof. Everything that fails does so where the core has been CUT, because a cut edge is exposed chipboard and chipboard swells when it gets wet — permanently, because the particles are pushed apart and the bond does not recover on drying.
SO MAP THE SWELLING AND IT NAMES THE SOURCE. Around a SINK cutout, the seal between the sink rim and the worktop has failed, or the cut edge was never sealed. At a JOINT between two lengths, the joint sealant has gone or the connecting bolts have loosened. At the BACK, water is running behind the upstand or the tiles. At a TAP HOLE, the tap's own seal or a weeping flexible connector. At the FRONT edge of a dishwasher or washing machine, steam escaping each time the door is opened at the end of a cycle.
THE DISHWASHER CASE IS THE ONE PEOPLE MISS. An integrated machine venting steam upward at the end of its cycle, or a machine whose door is opened immediately while it is still full of vapour, delivers moisture to the underside of the worktop above it hundreds of times a year. The damage appears as a swelling strip along the front edge directly over the appliance, and nothing is leaking.
WHAT CAN AND CANNOT BE DONE. Swollen core cannot be shrunk, filled or sanded back — the material is gone. What can be done is stop the water, and then either live with it, replace the affected length, or in some layouts replace a single section between joints. That is why establishing the source first is not a formality: replacing a worktop without it produces the same swelling in the same place.
ONE CHECK WORTH DOING EVERYWHERE, not only where it has failed: run a finger around every cut edge you can reach underneath. A cut edge that was never sealed when the worktop was fitted is a failure waiting for its first spill, and sealing it now costs almost nothing.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A failed seal at the sink rim
Most commonSwelling in a band around the sink cutout, worst at the front edge, sometimes with the sealant line visibly split or lifted.
How to check: Run a finger round the sink seal and press. A gap anywhere means water has been reaching the cut edge underneath.
A worktop joint that has opened
Most commonSwelling on both sides of a butt or mitred joint, sometimes with the two surfaces no longer flush, often near a sink or a corner.
How to check: Whether the joint is still tight and sealed. Connecting bolts loosen and the sealant in the joint is what keeps water out.
Steam from an appliance below
CommonA swelling strip along the FRONT edge directly above a dishwasher or washing machine, with nothing leaking and no water visible.
How to check: What is under the affected strip. Steam venting upward hundreds of times a year does this without a drop being spilled.
A cut edge never sealed at installation
CommonSwelling at any cut edge — a tap hole, an end, a cutout — with bare chipboard visible when reached from underneath.
How to check: Feel the underside of the cut edges. Bare chipboard has no defence at all against the first spill.
Water running behind the upstand or the tiles
CommonSwelling along the BACK edge, often behind a tap or where the tiling meets the worktop, with the sealant line at the wall failed.
How to check: Whether the seal at the wall is intact. A splashback joint that has opened delivers water straight to the back edge.
A weeping connection under the sink
Less commonSwelling from below rather than above, damp in the cupboard, and often a stained or lifting cupboard base as well as the worktop.
How to check: Whether the cupboard beneath is damp. Damage from below is a plumbing leak and the worktop is downstream of it.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Sink cutout, joint, back edge, front edge over an appliance, tap hole — each names a different source, and the worktop cannot swell anywhere the core is not exposed.
- 2
It is the commonest source and the cheapest to correct, and a seal can look intact while having lifted at one point.
- 3
A dishwasher or washing machine venting steam upward damages the worktop above it without anything ever leaking, and nobody looks for a leak that is not there.
- 4
An unsealed cut edge is bare chipboard, and sealing the ones that have not failed yet costs almost nothing and prevents the next one.
- 5
Damage from below is a plumbing leak, which has to be found regardless, and it usually means the cupboard base is going too.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a worktop length has to be replaced, since removing one without damaging the units, the sink, the hob and the tiling is the skilled part.
- Where the cupboard beneath is damp as well, because there is a plumbing leak and the worktop is only the visible end of it.
- Where the swelling is around a hob cutout, since heat and water together at an unsealed edge is also a safety question about the appliance's clearances.
- Where the worktop is solid surface, stone or a jointed composite, which do not swell and where a similar-looking failure means something quite different.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.