I'm not sure about all states, but in my state, sewer backup is not covered by default on most homeowners policy. You have to add an extra endorsement that's usually refers to something like "backup of sewer and sump-pump failure". Basically, insurance policies cover no water that comes in through your basement by default, but you *can* get your sewer/sump pump lines covered if you pay extra. Theres nothing you can do to get water coverage for water seeping in through cracks in the basement though. Theres a reason that every basement in existence has a drain in the floor, they're basically expected to leak sooner or later.
The main things NOT covered by homeowners insurance that a lot of people assume is covered:
1) Flood water - this is any water that enters the home from outside, at ground level. There is no endorsement that you can add to cover this, the only available coverage is through FEMA flood policies via the US government. They're expensive, and the coverage sucks, but it's available.
2) Anything that is old and falling apart(roof, deck, siding, etc). Insurance only covers damage that can basically be pinned to a singular event. Lighting hits your house, hail hits your house, a tree hits your house, a fire happens, etc. If something just erodes or falls apart over time, there's no coverage for that. So keep your shit in good shape and do preventative maintenance. Insurance isn't going to buy you a new deck or fence just because you haven't taken care of yours and it rotted away. Insurance is for catastrophic events, it's not maintenance.
3)Earthquake/Volcano (this is usually available via an extra endorsement)
3)No coverage for anything concerning a car. Your house burns down with your car in the garage? Better have full coverage car insurance because your house does not cover ANY vehicles(cars, trucks, recreational vehicles, etc) The lone exception to this are vehicles used in the regular upkeep of the property(i.e. a riding lawnmower or tractor used to mow, but not tractors used to do work like farming, has to be just for home upkeep)
4)No coverage for personal belongings(personal property) that just break. Your new TV falls off the wall and breaks? Not covered. The only breakage covered is vandalism if someone breaks into your home and tears it up. Your kid spills paint all over your carpeting? No coverage. Residents of the home causing accidental damage to personal property is not covered. Now, accidental damage to the structure of the home may be covered depending on the circumstances, but there's just no coverage for breaking your personal belongings.(The only exception being some types of items that you can specifically schedule coverage for, like jewelery, antiques, etc, but your blanket general coverage does nothing for breakage)
5)Underground pipes/wires in your yard. Insurance is only covering damage that occurs to your home, inside your home, or to a detached structure(like a shed or garage). If your drain line in your yard gets clogged from years of tree roots burrowing into it, insurance isn't paying anything to have that fixed, it's not damage to your home or a structure of any kind.. Related to this is also trees. If a tree falls on your property, or just branches or a partial tree, insurance will not pay to remove it unless it damaged some structure, like your home, a shed, or your fence. If a huge tree just drops right in the middle of your yard and hits nothing on the way down, then that's just all on you.
Water is always the biggest problem area, especially major storms like hurricanes, because you could be dealing with several different types of water. If your roof gets damaged, shingles fly off, and water leaks in that is covered. The water came in due to damage to the roof. However, if at the same time the creek at the end of your street floods and 6 inches of water flows into your home from the storm, that isn't covered, that's a flood. Basically if you are going to have water damage, make sure it doesn't come into your home at ground level(or below), or you are boned. LEaky roof is covered, leaky window is covered, broken pipe is covered, anything that comes in at the ground or foundation level = not covered.
That was the huge issue with Katrina in New Orleans, the actual storm wind/rain damage was bad, but not horrible. Insurance companies paid for damage to roofs and such. The problem was the flood water that was 10 feet deep and not covered at all unless people had additional FEMA flood policies. Insurance companies got crucified in the media over that, but the US government flat out doesn't allow private insurance companies to offer flood coverage. FEMA handles all of that. State Farm or Allstate or whoever couldn't give you a flood policy of their own if they wanted to. All of us have to broker out flood coverage through FEMA.
