Are you in Klang valley?

Normally there should be 3 or optionally 4 types of piping segments in a unit of a multi story building:
Toilet (soil waste)
Bath and basin
Kitchen and laundry
Balcony floor trap (optionally)

If there is no plaster ceiling in the bathroom, we should see two sets of pipes that goes out into the service column where it joins their respective down pipe, one for soil waste (WC) and the other for bath.

There's also vent piping (intake).

Sewerage smell is likely due to lack of water seal that trap these gases. Sometimes this points to inadequate seal design that allows the sealing water in the trap having the tendency to be siphoned down when an upper story unit flushes. The effect is made worse if more than one unit flushes down at the same time.

Another possibility, neighbouring unit left vacant that the seal trap dries up allowing sewer gas to intrude and is blown around thus other units can smell it.

I remember during the SARS outbreak in 2002-2003, reports came outs detailing how and why it was easier to spread in the affected HK building than in SG. There was a diagram contrasting the seal trap design of multi unit multi story dwellings of both countries.

In the SG trap design the trunk after the seal trap that connects to the down pipe inclines slightly upwards making it harder for water to flush out unless in large enough quantity and velocity. In the same token, the siphon effect is lessened when an upper floor flushes thus maintaining the seal.

In contrast, the HK building's plumbing design where SARS was found to have spread, the connecting part to the down pipe is relatively level, this making it easier to siphon out the water from the neighbouring seal traps below when flushing or bathing etc.

Not only SARS but various pathogen can spread in like manner of a compromised seal trap ie Norovirus.

If possible, in the problem areas replace the 6" floor trap with one that has an insert with a self closing seal against vermin (cockroaches & lipan) and sewer gas.

This post has been edited by halcyon27: Dec 29 2018, 11:43 PM