Building a strong record of the problem
You prove a plumbing problem with documentation: dated photos and videos of the issue and any damage; written reports to your landlord; any plumber or repair records; and receipts for damaged belongings or a spiked water bill.
Capture the problem clearly — a fixture that will not work, water where it should not be, or a drain backing up — and photograph any resulting damage. Save every text and email with your landlord, since these show what the landlord knew and when.
Keep records of repeated repairs and note the dates you were without water or a working fixture. A jump in your water bill can even help show a hidden leak or a running toilet. You do not need an expert to have proof; a clear, dated record usually tells the story. A free review can help you understand which evidence matters most in your situation.
This is general information about California tenant rights, not legal advice. Every situation is different.