Building a strong record of the backup
You prove a sewage problem with documentation: dated photos and videos of the backup, the affected areas, and discarded items; written reports to your landlord; any plumbing or cleanup records; and receipts for contaminated belongings.
Capture the sewage and the damage right away, before cleanup, because that evidence disappears fast. Photograph affected floors, walls, and belongings, and keep a dated list of everything you had to throw out. Save every text and email with your landlord, since these show what the landlord knew and when.
Keep any plumber or cleanup invoices, and note each recurrence if the problem happens more than once. Medical records help if anyone got sick from the exposure. 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.