For the problem stated by you, the issue most probably is on the floor of the bathroom above your roof.  I am not sure if you have control over the bathroom above yours but if you can work out some thing with the owner of the property above you, the following causes and repair methods would be useful for you. Good luck

Cause No.1 There is a possibility that the leakage is happening through the joints or floor and walls of the bathroom above yours. In this case when the bathroom is used for bathing etc, the water may leak through the holes or recess into the joints of the floor and walls. This leakage causes the water to accumulate on the roof slab of your bathroom.  Eventually the leakage will start into your bathroom.

How to repair:

Cause No.1 Contact an experienced and good mason to repair the floor of the bathroom above you using the steps below.

a)        First you should try to grout the joints of wall and floor properly with cement mortar 1:3 and should construct raising cement ‘gola’ in the joints. In this treatment first clean the joints of the floor and walls properly with brush and cloth and see whether there are any impurities left in the joints. Then grout the joints with cement mortar 1:3 fully and after that raise pointed triangular cement mortar gola by 1 cm to 2 cm from the floor which may be constructed on all the joints of floor and walls and let it dry.

Precaution should be taken that the upper bathroom should not be used for bathing or washing of floor till the cement mortar applied on the joints is dried/set.

After this the cement mortar gola should be cured or wetted for two or three days.

b) Ask the mason to remove the water of P trap of toilet sheet in case of Indian sheet and from the floor trap. Then apply coat of cement on the joint of P trap and sheet, similarly in the joint of floor trap and let it dry for the setting. By doing these repairs you should watch at least for one week and observe whether the leakage continues or stops. If the leakage does not stop, it is presumed that there is leakage in the joints which is stated in the cause No .2.

Cause No.2 The bathroom above your roof would have a water supply pipe line, waste water sewer line, the joints of toilet sheet with P trap or joints of floor trap etc. There is a possibility that there may be leakage either in the water pipe line or waste sewer line. There may be leakage in the joints of toilet sheet, floor trap and in any other fixtures’ joints etc.

How to repair:

Cause No.2 In this case you should contact good plumber for the repair of joints of pipe and show him your problem. For repair you should dismantle the floor of the bathroom on your roof and watch the leakage in the joints and the pipes. The leakage may be in any joints or it is possible that the pipes are leaking due to rust or breakage. After finding the leaking points, the leakage should be plugged. If you and plumber feel that the pipes laid in the bathroom are old or rusty and they may not work for some years, the pipes should also be replaced by the new ones. After plugging the leakage or replacing the new line, the line should be tested for two or three days by filling in the water and watch if there is any leaking.

When you are satisfy that there is no leakage after the repair, the floor should be cleaned and the pipe should be painted. Required flooring should be laid in the bathroom.

When the upper storey owner is uncooperative: 

If the house owner of upper bathroom is not co-operative for necessary repairs, the leakage can be stopped by doing temporary arrangements which are given below.

1. If the leakage is at one point, temporary plastic pipes can be fixed with the RCC slab and the leaked water may be drained out by jointing this horizontal pipe of roof with vertical pipe in the corner of bathroom. These pipes can be fixed with clamp and be plugged with the help of drilling machine.

2. If there is leakage in many points, GI sheet properly painted or Plastic sheet having a slope in  one corner may be fixed in your bathroom at a height about 7 feet by making recess in the wall. The leaked water collected in low level corner is drained out through a pipe in the corner of bathroom.