Why your bathroom vanity rots (and what architect-spec joiners do instead)

Last updated: 3 October 2026

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A bathroom vanity is the most abused piece of joinery in a house. It lives in the wettest room, it takes a splash every time the tap runs, its drawers are opened wet-handed, and it is usually fixed by four screws into plasterboard. Most units sold as "bathroom furniture" are simply kitchen carcasses with a different door.

That is why vanities swell, sag, discolour and smell. Not because the homeowner did anything wrong, but because the board, the edge sealing and the hardware were chosen for a dry room.

This guide sets out the build-up we use on London projects — the carcass material, the sealing detail under the basin, the hardware tier and the toe-kick lighting that makes a vanity read as built-in rather than bought. For where this sits in the overall room budget, see our bathroom renovation cost guide.

The build-up we specify

1. Carcass material: moisture-resistant MDF or marine ply

Standard MDF is a sponge. Its cut edges absorb water quickly, and once water enters the core the board swells permanently — it does not return to its original dimensions when it dries. Particleboard behaves the same way, and worse at the screw fixings.

Moisture-resistant MDF is the sensible default for a vanity that is not directly in the shower spray. It uses a water-repellent resin in the core and resists intermittent moisture far better than standard board, while still machining cleanly for hinge bores and drawer rebates. Where the unit sits within the splash zone of a shower, or where the room has poor extract ventilation, marine ply is the more robust choice: it holds its shape when wet and recovers better.

The material is only half the answer. Every cut edge, hinge bore, drawer-runner rebate and service cut-out should be sealed with a moisture-cure lacquer or edge-banding before assembly. This is the detail that separates a vanity that lasts fifteen years from one that swells in the first winter, and it takes minutes at the bench.

2. Runners and hinges: what actually fails

Drawer runners fail long before doors do, because drawers are opened more often and carry more weight. The basic soft-close runners fitted to many off-the-shelf bathroom units use lightweight zinc-alloy components. In a family bathroom with daily use they commonly sag, become stiff at the end of travel or stop closing flush at around eighteen months.

The specification we use is a steel ball-bearing runner with a soft-close mechanism and a 5–10 year warranty for owner-occupied homes, stepping up to a full-extension undermount runner with a 10 year or longer warranty where the drawers are used heavily — a double-basin vanity with two deep drawers, for example. Undermount runners also hide the mechanism beneath the drawer box, which looks better when a drawer is open.

The one rule that matters more than the tier: never mix runner tiers within the same run. A heavier door or drawer on lighter hardware announces itself immediately — it closes differently, sounds different and misaligns over time.

Hinges follow the same logic. Concealed-fix hinges, with no visible screws on the door face, belong on mid and premium specifications. Exposed-screw hinges are acceptable only on basic specification or where a period property genuinely calls for a visible butt hinge.

3. Handle finish must match the brassware

The vanity handle is the closest metal object to the tap, and the two are read together every single time the basin is used. Chrome handles on brushed brass taps look like a mistake, because they are one.

Choose a single metal finish family for the room — brushed brass, brushed nickel, matte black or polished chrome — and apply it consistently to the tap, the shower valve and head, the vanity handles, the towel rail, the toilet-roll holder, the robe hooks and the door ironmongery. If you want a second finish, place it physically apart, on an object that is not read at the same moment.

Finish consistency is also a cost-control measure. It prevents the slow accumulation of mismatched accessories that makes an expensive room look budget, and it makes future replacement straightforward because everything is chosen to the same palette.

4. Sealing the end grain under the basin cut-out

The basin cut-out is the single most vulnerable detail in the whole unit. Water sits around the bowl rim, and capillary action draws it into the exposed end grain of the board. Once water is inside the core, the board cannot dry out, and the damage progresses invisibly under the worktop.

The correct sequence is: form the cut-out, seal the exposed grain with two coats of a moisture-cure lacquer or an epoxy sealer, allow it to cure, then bed the basin on a continuous bead of sanitary-grade silicone — never four dabs at the corners. Where the worktop is stone or a solid surface, seal the underside of the cut-out edge as well, and use a closed-cell foam gasket under a stainless steel or ceramic basin to isolate the two materials.

If the vanity has a service cut-out at the back for pipework, seal that too. It is the same problem in a place nobody looks.

5. The toe-kick recess and hidden LED

A vanity that runs flush to the floor looks like a box. A vanity with a 60 mm recessed plinth, with a hidden warm LED strip running along the underside of the unit, appears to float on a line of light. It is one of the cheapest details that separates a fitted look from a flat-pack look.

Specify the strip at 2700 K with a diffuser, driven from a stable constant-voltage supply, and make sure the driver is accessible — normally in an adjacent cupboard rather than sealed behind the plinth. The recess should be deep enough to hide both the strip and the driver from normal viewing angles, and the floor below should be finished before the strip is set so the light has a clean surface to graze.

The same lighting logic applies inside the unit: a strip at the underside of a shelf, or above a drawer that is being opened, reads well in an evening and is genuinely useful at night.

What it costs

A mid-specification off-the-shelf vanity unit in a London bathroom typically runs £250–£700 for the carcass and doors, before the basin, tap and waste. A bespoke, correctly detailed vanity with a moisture-resistant or marine carcass, sealed edges, mid-tier hardware, a stone top and integrated lighting typically runs £1,400–£3,500 depending on width and the worktop material — and it is measured to the millimetre, which in a period London bathroom with out-of-square walls is worth a great deal on its own.

We build vanities through our fitted joinery service and install the plumbing through our bathroom team, working across Hammersmith and Fulham and the rest of Greater London.

Frequently asked questions

What board should a bathroom vanity carcass be made from?

Moisture-resistant MDF with a sealed, edged core, or marine ply where the unit sits in a heavy splash zone. Standard MDF and particleboard absorb water at cut edges and swell permanently, so every cut edge, hinge bore and runner rebate must be sealed before assembly.

How long should vanity drawer runners last?

A mid-tier steel ball-bearing soft-close runner should give 5–10 years of daily use; a premium undermount runner 10 years or more. Basic zinc-alloy runners commonly fail or sag at around eighteen months in a family bathroom.

Should vanity handles match the taps?

Yes. They sit in the same sightline and are read together every time the basin is used. Pick one metal finish family and carry it through taps, shower, handles, towel rail and accessories.

Why does the joinery under my basin discolour?

Water wicks into unsealed end grain around the basin cut-out and cannot dry out. Seal the cut-out with two coats of a moisture-cure lacquer, then bed the basin on a continuous bead of sanitary silicone.

Is a floating vanity safe on a stud wall?

Only when fixed into structure. A wall-hung vanity carries the unit, a stone top, a basin of water and a person leaning on it. Fix into timber studs or a ply pattress sized to the unit; plasterboard fixings alone are not adequate.

Related reading

• Bathroom renovation cost: the complete 2026 guide
• House renovation cost per square metre
• Plumbing and heating services
• Fitted wardrobes and bespoke joinery
• Bathroom design and build