Rewaterproofing a balcony in an apartment block can be a complex process, as it involves both technical and safety considerations. Here are the general steps for re-waterproofing a balcony in an apartment block:

1. Inspection: The first step is to conduct a thorough inspection of the balcony to identify the source of the water leaks and the extent of the damage. This should be done by a professional waterproofing contractor.
2. Surface preparation: The surface of the balcony must be cleaned and prepared before any waterproofing work can begin. This typically involves removing any loose or flaking paint, dirt, and debris. Any cracks or holes should also be filled and repaired.
3. Waterproofing: The next step is to apply a waterproofing membrane to the surface of the balcony. This can be done by either painting on a liquid-applied membrane or installing a sheet membrane. Both options have their own advantages and disadvantages, so it’s important to choose the best option for your specific needs.
4. Drainage: It’s important to ensure that any water that does penetrate the membrane can easily drain away. This can be achieved by installing drainage channels around the edge of the balcony and ensuring that there is adequate slope to allow water to flow away.
5. Final touches: Once the waterproofing and drainage work is complete, the balcony can be finished with tiles or other suitable surfacing materials.
6. Maintenance: Proper maintenance is also important to ensure the longevity of the waterproofing system. Regular inspections and cleaning should be done to ensure the membrane is functioning properly.

It is important to note that rewaterproofing a balcony in an apartment block can be a complex process and should only be done by professional waterproofing contractor, who has the expertise and experience to handle the job safely and efficiently.