How to Eliminate and Prevent Mold Under the Bathroom Sink

Mold beneath a bathroom sink is almost always a symptom of a hidden plumbing struggle rather than a lack of cleanliness. When stagnant moisture lingers in the dark, unventilated space of a vanity cabinet, wood fibers absorb the water and provide a perfect, nutrient-rich environment for fungal spores to colonize. If you catch it early, it is a simple matter of drying, cleaning, and sealing. Done well, this project transforms a musty, potentially structural hazard into a dry, sanitized storage area. The goal is to move from reactive cleaning to preventative maintenance. Once you identify the source of the dampness and eliminate the growth, your cabinet will stop acting like a sponge and start functioning as a reliable home for your toiletries again.

  1. Empty and Protect. Remove every item from under the sink and discard anything that shows signs of mold growth. Place reusable items in a dry, ventilated area to ensure they are fully sterilized before returning them later.
  2. Find the Culprit. Dry the cabinet floor completely with a cloth, then run the sink faucet while watching the pipes underneath with a flashlight. Look specifically for beads of water forming at the p-trap connections or the supply line valves.
  3. Attack the Root. Spray a liberal amount of white vinegar or a commercial mold-killing solution onto the affected surface. Let it sit for at least an hour to penetrate the pores of the cabinet material before scrubbing.
  4. Scrub It Away. Use a stiff-bristled nylon brush to scrub the area, moving in circular motions to lift deep-seated mold. Wipe away the loosened debris with a fresh rag, rinsing the rag frequently in a bucket of clean water.
  5. Eliminate All Moisture. Place a portable box fan or a high-powered desk fan facing directly into the open cabinet. Run the fan for at least 24 hours to ensure the wood is bone-dry before you consider any repair or sealing work.
  6. Lock Out Future Growth. Once dry, apply a bead of clear silicone caulk to any gaps where the drain pipe meets the floor or cabinet bottom. If the vanity base is bare wood, apply a thin coat of water-resistant polyurethane or shelf liner to deter future moisture absorption.