Chungking Mansions Drain Services

The Licensing Authority's register shows licensed guesthouses on every floor from the 3rd to the 17th across all five blocks — 235 licences in all — and several on one floor is unremarkable: the 10th floor of Block B alone holds rooms B3, B4 and B7. Inferring from design, once a 1961 floor plate has been divided into that many rooms, several rooms' toilets, basins and shower trays join one lengthened horizontal branch pipe before reaching the stack; with too little fall and too many bends, tissue, hair and soap scum settle at the bends. The classic symptom is a floor drain that gurgles or wells up when the room next door flushes, or the two rooms at the far end of the corridor draining noticeably slower. In practice, run water in each room in turn, note which rooms fail together, and only then decide between clearing the branch pipe, clearing the floor's horizontal drain, or asking the owners' corporation to inspect the stack.

English Wikipedia describes most rooms here as small, with the toilet and shower sharing a single compartment. Inferring from design, that layout sends shower water, hair and soap scum through one floor drain, which is wet almost every day, so the trap and the short branch pipe soon line themselves with soap scum holding hair. The first symptom is standing water over the top of your foot in the shower and a minute or more to drain away; the smell comes later. For maintenance, a strainer over the floor-drain outlet, cleared of hair daily, is the cheapest step there is — and once draining has started to slow, do not wait, because the longer it is left the harder the scum sets. Inferring from the use, a let room's shower is used more heavily than a home bathroom, so deposits build up faster; a room that stands empty for a long time has the opposite problem, a dried-out water seal.

English Wikipedia notes that many rooms have no window, or only a small one facing a dim light well. Inferring from that, ventilation in such a room depends on an extractor fan. The Buildings Department explains that a trap holds water to form a seal that keeps the smell inside the drainage pipe out of the room; once that seal is inadequate, switching the fan on with the door shut draws sewer air back through the outlet. In rooms like these the smell tends to be constant rather than occasional. Start by pouring about half a litre of water into each floor-drain outlet weekly, and do so regularly in rooms that stand empty for long periods. If the smell survives that, check whether the trap is too shallow, damaged or altered, and whether the pipes out in the light well are leaking. Note that altering a trap or an anti-syphonage pipe is minor works and must be carried out by a registered minor works contractor.

The Chinese Wikipedia article records 20 to 30 food premises above the third floor. The Environmental Protection Department requires a grease trap to sit along the waste discharge pipe with enough capacity to hold about twenty minutes of flow, sets a minimum of 250 litres for a trap serving one sink, requires every trap to have a vent, and states that grease must be removed once it builds up to 30% of the capacity. Its maintenance guidance goes further: as a general rule, inspect the trap at least every three days, clean it immediately when grease exceeds 30% of the liquid in the trap, never dispose of trap waste into a toilet, a rainwater inlet, an open channel or a manhole, and keep the cleaning records on file. Inferring from design, kitchen waste water leaving an upper floor here travels a long way before it reaches the manhole at ground level, cooling and setting as it goes — so grease blockages in this building usually sit not in the kitchen but in the horizontal drain and the length feeding the manhole. Hot water or a degreaser rarely touches that; it takes mechanical cleaning plus a disciplined trap-cleaning cycle.

There is a dated, documented case here that shows exactly why a break in an old pipe matters. on.cc reported on 30 September 2015 that an acting chief inspector (crime) of Yau Tsim District said officers had found minor renovation works in progress on the external wall of an upper-floor unit at Chungking Mansions, during which someone poured corrosive liquid into a drain; it was believed to have escaped through a break in the pipe and splashed seven passers-by aged 22 to 46, and the case was handled as one of an object falling from height. The report added that, as understood, the drain at the person's premises was blocked and drain cleaner was poured in as a result. At about 8 pm that evening police carried out a leak test at the scene, laying white paper on the pavement and pouring coloured water into the building's drain, confirming that it sprayed out of the pipe onto the footway. What that means in practice for this building is simple: where a sixty-year-old pipe on an external wall or in a light well has a break, a chemical poured in never even reaches the blocked length — it comes back out at the break, and what is under the break is the Nathan Road pavement. So the first step here is not more chemical, but establishing which floor and which length is blocked, and whether that length is leaking.