Safety first
Water from a sewer backup is considered contaminated: avoid direct contact, wear waterproof boots and gloves, and keep children and pets away until the area has been cleaned and disinfected.
If the water is approaching outlets, baseboard heaters or the electrical panel, cut power to the affected area at the main breaker if you can do so safely — otherwise wait for a professional.
What to photograph and keep
Before moving anything, photograph the scene as a whole, then each affected item up close: furniture, boxes, appliances, flooring. A waterline mark on a wall or door, photographed next to a ruler or other reference object, is especially useful.
Keep damaged items at least until an adjuster has seen them, even if they look unsalvageable — don't throw them out before the inspection, except where there's an immediate health risk.
What an insurer generally asks for
An insurer generally asks for pre-cleanup photos, a detailed list of damaged belongings with their approximate age, and sometimes a plumber's report confirming the cause of the backup — municipal sewer, a blocked lateral, or a failed sump pump.
Every policy is different and coverage for a sewer backup varies from one contract to another; only your insurer can confirm what's covered in your case.
Avoiding a repeat
Once the cleanup is done, the next step is understanding why the water got in. A camera inspection of the lateral drain and a check of the backwater valve (or installing one, if there isn't one) target the cause rather than the symptom.
If the sump overflowed rather than the sewer, a battery backup pump keeps a power outage during the next storm from recreating exactly the same damage.