Why Is the Bottom Corner of a Bathroom Cabinet Often the First to Swell?

The best waterproof material for bathroom cabinets is PVC foam board or marine plywood, because both barely swell in water. Moisture-resistant MDF swells roughly half as much as standard MDF, so moisture-resistant MDF remains the best value for most bathrooms.

Most flat-pack and fitted bathroom furniture in the UK is built from medium density fibreboard or chipboard, wrapped in melamine, foil or lacquer. The face is genuinely water resistant, but the core is not, rather like a biscuit dipped in tea that holds together perfectly until the liquid finds a broken edge. Moisture-resistant MDF (you'll spot its green-tinted core the moment you cut it) contains extra resin and wax, and in my experience it buys you years rather than months when a slow leak goes unnoticed. Solid hardwood sounds like the luxury answer, yet oak and walnut move with every steamy shower, so I only recommend them for well-ventilated rooms with a good hardwax oil finish.

For a busy family bathroom, a well-made engineered wood unit is a sensible middle ground, provided every cut edge is sealed and the base sits off the floor.

A three-piece bathroom furniture set in Sonoma oak gives you matching storage with a melamine finish that wipes clean in seconds, while a compact 65 x 33 x 60 cm Sonoma oak sink cabinet suits cloakrooms where a full vanity would crowd the door swing. PVC and acrylic-faced units cost more (sometimes double), though I've fitted them in wet rooms where nothing else survived beyond three winters.

Swollen MDF caused by water damage can be repaired only when the swelling is under 2 mm and the board still feels firm. Swollen MDF that crumbles, flakes or has lifted more than 2 mm needs replacing rather than repairing.

Here's the honest truth that most quick-fix videos skip: MDF fibres don't shrink back once they've swollen. You can dry the board and stabilise it, but you're managing the damage rather than reversing it.

Start by drying the panel for 48 to 72 hours with a fan or dehumidifier, then sand the raised lip flat with 120 grit paper before brushing in a wood hardener or thin epoxy consolidant to lock the loose fibres together. Once that has cured overnight, a two-part wood filler levels any dips, and an oil-based primer or iron-on edge banding seals the repair so the same corner doesn't drink again. I once saved a client's vanity in a Victorian terrace in York this way after a dripping trap had lifted the plinth by about 1.5 mm, and three years on it still looks factory fresh (the trap, needless to say, was replaced first!).

How Do Common Cabinet Boards Compare When Wet?

The table shows that the cheapest boards swell the most and recover the least, with figures varying by thickness and manufacturer. If your cabinet is standard chipboard and the corner has burst open, replacement will almost always save you time and frustration.

A water damaged bathroom cabinet is fixed by stopping the leak, drying the carcass to below 12% moisture, then sanding, hardening and sealing the swollen edges. A bathroom cabinet base panel with damage beyond 2 mm should be replaced.

This checklist lists the steps for repairing a water damaged bathroom cabinet.

1. Stop the leak source by checking the waste trap, tap connectors and shower seals before touching the cabinet.
2. Empty the bathroom cabinet completely and remove the doors, drawers and kickboard.
3. Measure the swelling with a steel rule, treating anything over 2 mm as a replacement job.
4. Dry the cabinet carcass with a fan or dehumidifier for 48 to 72 hours.
5. Check the board with a moisture meter until the reading falls below 12%.
6. Sand the raised edge flat with 120 grit paper, finishing with 180 grit.
7. Brush wood hardener into the softened fibres and leave the repair to cure overnight.
8. Fill any remaining dips with two-part wood filler and sand flush once hard.
9. Seal every cut edge with oil-based primer or iron-on edge banding.
10. Refit the cabinet on adjustable legs at least 10 mm above the floor.

I learned step five the hard way early in my career, sealing a corner that felt bone dry but read 19% on the meter, and a fortnight later the primer had blistered off like sunburnt skin. Patience with the drying stage is genuinely half the repair.