Guides · Maintenance

A tap leaking at its base: what gets damaged, and in what order

In short. A leak at the base of a tap does not damage everything at once: it follows an order, and the order is the only thing that tells you how much time is left. The stone is almost always the last to suffer and the first to be noticed; the board underneath is the first to go, and the only one that, once it goes, forces the top to be lifted. Here is the sequence, how to read it from the outside, and what can still be saved at each step.

Why the water is never visible where it gets in

A tap does not sit on the stone: it goes through it. It sits in a hole, held by a stud or a bracket that presses a washer against the upper face of the top. When that washer flattens, or the stud works loose through vibration and the movement of the flexible hoses, the water does not stay on top: it runs down the body of the tap, into the hole and out underneath, inside the cabinet. That is why the fault always arrives late: the top looks dry, and then one day the side of the cabinet is swollen. We have written about the four routes water takes to end up under a worktop in the guide to undermount sinks; here we follow one, because it is the only one that wets the stone from the inside.

First: the sealant joint

The first thing to let go is the bead: the one around the tap’s base plate and, in many installations, the one between stone and cabinet. It is not a cosmetic detail. Joints exposed to non-pressurised water are the subject of a dedicated European standard, EN 15651-3, which sets definitions and requirements for sealants used in joints in indoor sanitary areas of buildings — bathrooms, showers, domestic kitchens and prefabricated sanitary units.

A sealant, though, is a seal: it is not a fixing and it holds nothing in place. When it lifts along one side, that is the warning, not the cause. Redoing it before the leak has been found keeps the moisture in and removes the only visible clue.

Second: the board under the stone

A stone top does not float. It rests on the cabinet, on a sub-top or on rails, and that bearing surface is almost always particleboard or MDF. The European standard on particleboards, EN 312, classifies them by intended use: among the types it sets out are P3, non-load-bearing for humid conditions, and P5, load-bearing for humid conditions, alongside the types meant for dry conditions. Under a stone top, nobody checks which one was fitted until it swells.

And swelling is the point of no return, for two reasons. It does not reverse on drying: the board does not come back flat, it grows in places and delaminates in others. And the stone rests on it: a bearing surface that loses its flatness puts the slab out of plane, and it then works in bending where it should not — the mechanism behind the cracks described in the guide to cracks at cutouts. A leak of a few millilitres a day, ignored for months, does not ruin the stone: it ruins what keeps the stone flat.

Third: the stone, which absorbs less than people fear

Natural stone absorbs water, and how much is a measured property with its own test method: EN 13755, natural stone test methods — determination of water absorption at atmospheric pressure. For marbles the figures are low, but here the figure is not what matters: time is. A slab wetted from below can only evaporate through the exposed face, because the other one is bedded down, and it stays damp far longer than anyone expects.

The visible effect is a dark shadow around the hole, often crescent-shaped. It is not a stain in the sense of the guide to stains and etching: it is water in the pores, changing the way light enters and leaves the stone, and it clears as the stone dries. If it is still there after weeks, the marble is not being slow: water is still arriving, or it has carried something in with it.

Fourth: efflorescence, and then the metal

Water crossing an adhesive, a mortar bed or a board does not travel clean: it dissolves the soluble salts it meets and carries them to the surface, where it evaporates and leaves them behind. The result is a whitish deposit at the edge of the hole or along a joint: it comes back every time, because the water comes back, and it is a signal to read rather than a cleaning problem. The last thing to go is the metal — bracket, plate, screws: rust at the bottom of a hole is the signature of an old leak, and from then on it stains the stone from within, with an iron halo that will not wash out.

How to read it from the outside, in five minutes

• run a hand over the board under the hole and over the cabinet side nearest the tap: swelling can be felt before it can be seen;
• look at the underside of the stone with a light held low and raking: damp stone is darker, and the outline tells you how far the water has travelled;
• move the tap: if it turns in its hole or rocks, it has lost its clamping, and the clamping is what holds the washer;
• press the sealant bead with a fingernail: if it has come away along one side, it is no longer sealing;
• look for the white deposit at the edge of the hole and rust at the fixing: both mean “for a long time”.

What can still be saved, step by step

The bead is always remade, and it is the only item on the list that can be remade without help — once the leak has been found and the surface is dry, not before. The board survives only if it has not swollen: if it has, it must be replaced, and the top has to be lifted to do it, which is not a two-person job, as we explain in the guide to delivery and fitting.

The stone, in the great majority of cases, recovers: remove the source, let it dry, and it returns to what it was. If a shadow remains, it is treated with a poultice; if the finish has lost its gloss in the wetted area, the route is the one in the guide to re-polishing in place.

How we work

We select the block and work the stone in house, and holes are cut in the workshop rather than on site: a hole cut with the slab held still, radiused and checked, is the first reason a tap stays clamped where it should. We do not use resin to reinforce slabs: across a large surface what is needed is continuous support, and the mesh on the back is already a good backing. It shows here too, because an unresinated slab, once dry, can be assessed for what it is. For a project supply the route is the Contract section; the contacts reach the people who follow the work, and the tables show how we work when there is only one piece.

What to ask whoever comes to put it right

• what they found loose, and what they tightened it with;
• whether the board under the hole was looked at and touched, not merely dried;
• which type of board goes back in, and whether it is intended for humid conditions;
• if the top has to be lifted: who lifts it, how many of them, and who answers for a breakage.

Frequently asked questions

Will the dark shadow around the hole go away?

As a rule yes, because it is not a stain: it is water in the pores. Once the source is removed it clears as the stone dries, slowly, because the slab can only evaporate through the exposed face. If it is still there after weeks, the leak has not been closed or the water carried something in.

Can I just redo the silicone?

Only after the leak has been found and closed. A new bead over an open leak hides the clue and keeps the moisture in: the damage carries on where it cannot be seen. Silicone seals; it does not clamp.

How can I tell whether the board underneath has swollen?

It can be felt before it can be seen: the surface turns uneven and the edge flakes under a fingernail. Another sign is a top that starts bearing badly at one point: when a top fitted years ago begins to rock, it is usually what sits beneath it.

Does the top have to come off to repair a leaking tap?

For the tap, no: the work is done from inside the cabinet. The top is lifted only if the bearing board has to be replaced, and that is a decision taken after looking at it.

Request a quotation The contract division

This page describes assessment and inspection criteria; it quotes no prices, no response times and no company case histories. The three standards are cited for their subject matter and without reproducing any numerical values from them: EN 13755 (water absorption of natural stone at atmospheric pressure), EN 312 (requirements for particleboards, classified by intended use) and EN 15651-3 (sealants for joints in indoor sanitary areas). Assessing an individual case is for whoever can see the top and open the cabinet.