How to prevent seepage in walls and ceilings of the house

This is disturbing. When I was in an apartment in the year 2000, I faced a similar problem. The leak from the ceiling will be definitely due to the leakage from the apartments that are above our apartment. The biggest problem is the apartment owner will not have any problem even though his flooring is leaking. But the apartment which is just under the leaking room will have the problem. These leaks will not appear immediately. But as time passes we will notice the leaks.
Generally, the water leak from the bathroom of the apartment above our apartment will cause this problem. We have to detect the leakage by removing the tiled on the floors in the bathroom. Once we know where is the leak we can arrest it by applying leakproof chemicals. This chemical can be mixed in the concrete and apply a coat of the same on the ceiling and then do the tiling again.
You have to contact a civil engineer or a good experience mason and show him the leakage. Then you have to convince the owner of the apartment above your apartment. The mason will suggest the remedy. When we faced such a problem when it is shown to a mason, he observed the leaks on two floors and he carried out the repair work. After that, the leak is arrested. If there is an association of all apartment owners you have to discuss this problem in that meeting and convince the apartment owner and get the repair done.

Identification of the source of water and leakage in the old houses is very challenging and a difficult exercise even for the expert technical people. These problems generally require experienced technical persons to pinpoint the source of seepage.
In old houses, concealed water pipes are generally one of the major source for these problems because the old pipes get rusted and start leaking from some places and the leakage is so small that it is very difficult to make it from outside appearance. Another source for seepage is from the vertical walls of the house where small cracks develop and the rain water or other water from storage tanks can slowly seep to inside and then slowly enters the houses in the lower storeys.
In such cases, thourough coating of special paints and chemicals on the outside walls will be one of the main requirement of repairs. Water pipe leakage requires a lot of repair works related to unearthing the hidden pipes by breaking the cement over it and that is a big exercise but once the faulty pipes are replaced the problem of seepage can be mitigated.
In one of the cases I have seen that the small quantity of water was entering below the tiles on the floor through the loose sand underneath and then it was oozing out in the floors of the flats on some levels. It required thorough repair by replacing the tiles one by one and fixing them properly with appropriate sand cement mixture.

In multi storeyed apartments such problems are common after some years. If the seepage is from the ceiling, then the cause source is the immediate upper floor apartment.
The residents of upper floor may not be aware of the leakage or seepage. For them there may not be any leakage or seepage of water,because it occurs below the surface tiles and through the ceiling concreting/cementing. It can also occur from the cracked water pipes also. The majority of cases of such seepage is from the bathrooms and toilets on the upper floor.
So it needs co-operation from the upper floor residents. There are experienced people in the construction field who by their experience can easily locate the source and rectify using adhesive chemicals and/or re plastering.
If there is an apartment association in your apartment, inform the matter to the association committee. There may be more such cases and there may be some other repair works also. So the repair works may be taken as a package and taken up to minimise interruption and difficulties. Otherwise taking concurrence from the upper floor resident get a proper person to locate the source and get the repair done. Come to understanding about cost burden.
Sometimes it may just need removing a couple of tiles and putting the leak proof chemicals and replace the tiles. It may also need repairing or replacing the broken closet pipes or drain pipes. It may need a more repair and total relaying of tiles.

You have not indicated whether your residential unit is a part of a multi-storeyed apartment or It contains five houses in a row. These houses were constructed twenty years back and so this requires some treatment to arrest seepage. You can take up the following points to rectify the same.
1) First of all, you need to call up an experienced civil engineer for his guidance and request him to visit the spot from where there is constant inflow of water. He would assess the severity of the problem and then accordingly he would suggest you a right step.
2) If you want to undertake the job from your end, you would be required to engage an experienced mason for the filling up the cracked holes. This would also involve shifting of the tiles to see the underneath conditions.
3) If cracks have appeared beneath the structure, you need to scrub the wall gently to eliminate such deposits.
4) Allow it to dry for at least eight hours before using mortar materials. Mortar is a dense and cohesive material having waterproof characteristics. It could seal the damaged area including the roof or wall with the proper application.
5) Once the job is over, you need to apply exterior water proofing coat making the area for the further absorption of moisture.
6) Identify the vulnerable areas especially the bath - rooms which are the most vulnerable part of seepage. If you instal exhaust fan on such locations, this will take care of expelling the moisture at a faster rate. So the chances of seepage are greatly eliminated.

It is common problem in old apartments. As mentioned above, your apartment is 20 years old then there would be more chances to seepage. I have also experienced this kind of problem staying in old houses. Generally, when the house becomes old then crack problem and de- paint is common. Every house needs to be painted and repaired every 5 years. If you are facing such a problem then first call a civil engineer and try to find the cause of the problem. It has been seen that mostly happens this problem due to upper bathroom and toilet cracked pipe which can be repaired by using adhesive chemical. You can also apply waterproof coating because of that your wall and ceiling would not wet. If any crack hole is found then try to fill it.