Legal Question in Construction Law in California

That's not a legal question. That is a factual question. Whose construction work is defective is not something the law can tell you. Only a contractor can tell you that, after identifying the exact cause of the leak and reviewing which contractor did the work that causes the leak.

I don't know for sure, it might require looking at the plans each worked from and/or their contracts, but it also seems possible that responsibility might lie with whichever of them finished their work last.