Under-Sink Kitchen Cabinets: Why They Swell and How to Protect Them

In most kitchens, the first part to fail is the under-sink cabinet: its floor swells, its edges lift and it starts to smell damp. The reason is simple: it is the only cabinet with water supply and drain pipes running through it. The good news is that most of this damage can be prevented with the right materials and a few simple details in how the cabinet is built and installed.

Where does the water in a sink cabinet come from?

A large, sudden leak is usually spotted quickly. What ruins the cabinet over time is small, constant moisture:

• A slow drip from the trap joint or tap hoses that goes unnoticed for weeks.
• Dishwasher and water filter connections that add more joints to the same space.
• Water seeping past the sink rim and worn silicone onto the top edge of the carcass.
• Condensation on the cold water pipe in summer.
• Wet sponges, cloths and detergent bottles left on the cabinet floor.

When MDF or particleboard takes up water through a cut edge it swells, and it does not return to its original shape once dry. That is why repairing a swollen floor usually means replacing it.

The right materials for a sink cabinet floor and carcass

No common wood-based board is truly waterproof; the difference is how much later and how much less it is damaged. The usual options:

• Moisture-resistant MDF (MR): usually has a green core and absorbs less water than standard MDF. A sensible choice for the sink cabinet carcass, but it will not tolerate standing water either.
• PVC or composite board: does not absorb water and is a reliable choice for the sink cabinet floor, although it looks different and holds screws differently.
• Aluminium or plastic protective tray: sits on the floor with a raised lip and holds leaking water so it can be seen. It is the simplest and most effective layer of protection.

For a general comparison of these boards across the rest of the kitchen, read our guide to MDF, plywood and solid wood.

Build the sink cabinet so a leak shows itself, rather than staying hidden and eating the floor from inside.

Build and installation details that make the difference

• Edge banding on every edge: including the back and bottom edges nobody sees. In a sink cabinet, an unbanded edge is where swelling begins; edge banding is covered in our cabinet edge banding guide.
• Sealed pipe holes: wherever a pipe passes through the floor or back, the cut edge should be closed with silicone or a grommet.
• Legs and plinth: a cabinet on legs does not get wet from below if water spills on the kitchen floor.
• Silicone around the sink: the joint between sink and worktop must stay intact; replace cracked silicone promptly.
• Access to valves: place the shut-off valve and joints where they can be seen and checked without removing the floor or a shelf.

Simple care that extends cabinet life

Every few months, check under the joints and across the cabinet floor with a torch; a dry paper towel on the floor reveals a hidden drip early. Replace tap hoses that show cracks or rust before they burst. Keep wet sponges and cloths out of the cabinet and stand detergent bottles on the tray. If the cabinet floor has started to swell, find the source of the leak first; replacing the floor without fixing it brings the same result a few months later. For cleaning tips for the rest of your cabinets, see our kitchen cabinet cleaning and care guide.