What constitutes a legal separation (as per Indian laws) between a husband and a wife living under separate roofs?
A couple could be living be under the same roof but not living together as husband and wife.
So non-performance of martial obligations towards each other. Not have conjugal and sexual relationship with each other would constitute as "legal separation".
There is some called as "judicial separation" as well, which is basically the Court passing an order to he effect that the couple shall not be under an obligation to cohabit with each other.