Bathroom Drain Odour HK — Sink, Shower, Floor (2026)
You walk into the bathroom and immediately know — something smells. Not always sewer-strong, sometimes mild but persistent, sometimes only after showers. Most people try a single drain cleaner and hope. That works ~30% of the time. The other 70%, the smell comes from a drain you didn't expect.
This guide gives you the 4-source diagnostic that pros use — narrow down the source first, then fix targeted.
Want a pro to diagnose + treat in one visit?
4-source check + targeted clearing + 30-min retest. We don't quote until we know which source.
The 4 sources of HK bathroom odour
1. Sink P-trap — biofilm + hair (most common, ~40%)
The U-bend under your bathroom basin holds water that seals against sewer gas — but also catches hair, toothpaste residue, dead skin. Over months, biofilm grows. Bacteria produce hydrogen sulfide (rotten-egg smell). Worst right after using the basin.
2. Shower drain trap — hair + soap scum (~25%)
Hair + soap scum + shower-gel surfactants combine into a slimy clump in the shower drain trap. Same biofilm chemistry but slower than the sink because shower drain sees less concentrated waste. Often the smell rises during/after a shower run.
3. Floor drain — dry P-trap or biofilm (~25%)
Covered in our floor drain smell guide. Quick recap: 70% dry P-trap (1L water fixes), 25% biofilm (enzyme cleaner), 5% broken seal.
4. Basin overflow channel — overlooked source (~10%)
The small hole near the rim of your basin is the overflow channel. It connects to the basin drain via a hidden passage that collects soap+water residue over time. The overflow tube itself biofilms. Smell rises through the overflow when water fills the basin.
The cling-film diagnostic (60 minutes, free)
Cling film + flat weight over the sink drain. Leave overnight. If next morning the smell is gone → your sink is the source. If smell persists → sink isn't it; uncover it, move to night 2.
Cling film + weight over shower drain. Note: shower has to be dry for this. Leave overnight. Smell gone → shower drain is it. Persists → uncover, move to night 3.
Same protocol. Floor drain is the easiest to forget but often the culprit, especially in HK summer (dry P-trap from AC).
If 1-3 didn't fix it: tape over the basin overflow opening (the small hole). If smell stops, the overflow channel needs cleaning.
Targeted fix by source
Sink P-trap (biofilm)
- Remove hair with a wire hook (cheap, $20 at any HK hardware store)
- Pour 250ml enzyme cleaner (Bio-Clean, Drainbo) — 6h dwell
- Flush hot water 60 sec
- Repeat 3 nights for stubborn cases
- Avoid caustic — kills the bacteria you want active
Shower drain (hair + soap)
- Lift drain cover (some pop out, some unscrew)
- Hair removal — use disposable plastic snake ($25 at hardware stores)
- Enzyme cleaner + hot flush
- Install a fine-mesh hair catcher ($30-50) to prevent recurrence
Floor drain
See our dedicated guide — 1L water for dry P-trap is the 60-second fix.
Basin overflow channel
- Inject 100ml enzyme cleaner into the overflow hole using a thin-nozzle bottle
- Wait 30 min
- Run hot water with sink basin closed, then release — flushes through overflow
- Repeat weekly for 3 weeks; usually clears stubborn biofilm
What we do on a "bathroom odour" call
- Smoke + ammonia test — localise to one source in 15 min
- Targeted clearing of identified source
- 30-min retest
- Before/after WhatsApp photos
- Prevention briefing — 2-min walkthrough on habits to keep it from coming back
Honest pricing
Starting price HKD 800 for residential bathroom drain odour diagnostic + treatment (HK Island / Kowloon / urban NT). Includes:
- 2-hour typical response window
- 4-source diagnostic (smoke + ammonia)
- Targeted treatment (enzyme + mechanical)
- 30-min retest + WhatsApp photos
- Receipt + on-site retest before we leave
- Prevention walkthrough
Add-ons:
- +HKD 200 if 23:00–06:00
- +HKD 50 NT districts
- Parts at cost: P-trap replacement HKD 150-500 if cracked, hair-catcher install HKD 30-50