Domakihub Clean Your Sink Overflow Hole in 5 Simple Steps
The hidden port near your basin rim harbours biofilm and limescale — here is how to clear it properly.
Reviewed by Domakihub cleaning pros · 2026-06-23 · 4 min read
The overflow hole sits just below the rim of most bathroom sinks — a small slot designed to prevent flooding when the tap is left running. Because water rarely passes through it at full force, the channel inside builds up a dense layer of biofilm, mildew, and limescale that no standard surface wipe will ever reach. The video above shows exactly how the grime accumulates and what a clean overflow looks like afterward; the steps below give you the tools and method to do it yourself in under fifteen minutes. Whether you manage a family home in Athens, a holiday rental in Crete, or a hotel property on the coast, this quick routine belongs in every bathroom-cleaning schedule.
Why the Overflow Hole Is So Easy to Neglect
Most bathroom cleaning routines focus on visible surfaces — the basin bowl, the taps, the drain. The overflow slot is tucked away at eye level on the inside curve of the sink, which means it escapes the sponge on every pass. Over weeks, the residual moisture and soap film inside the narrow channel create ideal conditions for black mould and slime-forming bacteria.
In Greek homes and rental properties, where summers bring high ambient humidity, the problem compounds quickly. A neglected overflow hole is also a common source of musty bathroom odours that persist even after a thorough surface clean — because the odour source is hidden inside the channel itself.
What Makes This Spot Hard to Clean
The inner channel connecting the overflow slot to the drain pipe is too narrow for a standard cleaning brush and too curved for a straight pipe cleaner to navigate without bending. The slot opening on the basin face is also too small for most nozzles, which is why most people skip it without realising they have skipped it.
The right combination of a flexible bottle brush or a small interdental-style drain brush, a foaming or enzymatic cleaner, and a short soak solves the access problem entirely. No specialist equipment is required — items already in most Greek households or available at any pharmacy or hardware shop will do.
How to Clean the Sink Overflow Hole
- 1
Gather your tools before you start
You need a small flexible bottle brush or a pack of pipe cleaners, white vinegar or a bathroom descaler safe for ceramic and enamel, a squirt bottle or a funnel, baking soda, and an old toothbrush. Lay everything beside the sink so you do not need to stop mid-process. If the property has a chrome or enamel basin, confirm the descaler label states it is safe for those surfaces — most standard bathroom sprays and diluted white vinegar are fine.
- 2
Apply a foaming cleaner into the slot
Mix one tablespoon of baking soda with enough white vinegar to create a foam, or use a squirt of enzymatic drain cleaner. Using the squirt bottle or a funnel, push the solution directly into the overflow slot on the front of the basin. The fizzing action helps lift the biofilm from the walls of the inner channel. Leave the solution to soak for at least five minutes — ten is better for heavier build-up.
- 3
Scrub the channel with a flexible brush
Insert a small flexible bottle brush or a thick pipe cleaner into the overflow slot and work it back and forth with short strokes. Rotate the brush as you push it in to reach all sides of the curved channel. You will likely see dark brown or black residue come out on the first pass — this is the biofilm. Rinse the brush under the tap and repeat until the brush comes back visibly clean.
- 4
Flush the channel thoroughly with hot water
Fill a cup or jug with the hottest tap water available and pour it steadily and directly into the overflow slot. This flushes any loosened debris and remaining cleaning solution down through the channel and into the drain below. Repeat two or three times. If you have a handheld shower head, a short directed burst works very well here.
- 5
Finish with a limescale treatment if needed
In areas of Greece with hard water — much of Attica, the Aegean islands, and parts of the Peloponnese — white calcium deposits can build up around and inside the overflow slot. After flushing, apply undiluted white vinegar or a citric-acid solution to the slot opening with the squirt bottle, leave for five minutes, then scrub the opening itself with the old toothbrush before a final hot-water flush. Doing this monthly prevents the scale from hardening into a deposit that requires mechanical removal.
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FAQ
- How often should I clean the sink overflow hole?
- For a family bathroom in regular daily use, once a month is a reasonable minimum. Holiday rentals and hotel bathrooms benefit from a quick clean between every guest stay, since heavy short-term use accelerates biofilm growth. If the bathroom develops a musty smell that persists after a full surface clean, the overflow hole is usually the first place to check.
- Is bleach safe to use in the overflow channel?
- Diluted household bleach (one part bleach to ten parts water) is effective at killing bacteria and mould inside the channel and is generally safe for vitreous china and most enamel basins. However, avoid leaving it in contact with chrome fittings around the overflow slot opening for extended periods, as concentrated bleach can discolour chrome over time. Rinse thoroughly with hot water after any bleach application. White vinegar or citric acid are gentler alternatives that handle both biofilm and limescale without risk to metal finishes.
- Can a blocked overflow hole cause the sink to drain slowly?
- Not directly — the overflow channel connects to the drain pipe rather than the main drain pathway, so a blocked overflow does not itself slow the basin drain. However, a completely blocked overflow is a safety issue because the sink can no longer shed water safely if the main drain becomes blocked. Heavy slime build-up inside the channel can also contribute to persistent drain odours even when the drain itself is clear, because the smell travels up through the connected pipe.