A landlord is bounded by the following covenants to a tenant:
1. Quiet enjoyment of the premises, which means tenant can enjoy the use of the premises without interference.
2. Non-derogation from the grant, landlord must observe all the terms granted to tenant.
3. Property must be fit for habitation at the start of the lease term;
4. Structure and facilities must be in good repair.
5. Good title. He is the owner and has the right to rent.
6. Pay property tax and quit rent if any
7. Exclusive possession of the premises for the fixed term, so long as the fixed term given has not expired.
8. If landlord has sold or transferred his interest, buyer or successor-in-title is bound to observe the terms of the lease granted
Under points 3 and 4 of the landlord's covenant, he must ensure the property is in good condition. As such, the onus falls on the landlord to do the repairs.
You may write to HDB or to the Management Corporation Strata Title (MCST) who will instruct the landlord to do the repairs.
If there is an inter-floor leak, for a private property, the Subsidiary Proprietor and his/her neighbour are required to jointly carry out an investigation to establish the cause of the leak and proceed with the repair works. Both should resolve the cost of repair.
Hope this answers your question.