If you live in a densely-populated area, chances are good that your home uses a municipal supply line to obtain its water. Commonly known as "water mains," these pipes typically run underneath neighborhood streets and deliver water to individual homes via "spur" lines. Virtually all of the homes that use municipal water supplies also use sewer lines to dispose of their water-soluble waste products. These outgoing lines join up with the municipal sewer lines that generally run parallel to clean-water mains.
Your homeowner's insurance policy is probably set up to cover certain water-related problems that might arise during your tenure as a homeowner. Most of these concern pipe leaks and breaks that affect the inside of your home. In fact, there are virtually no circumstances under which your homeowner's insurance policy will cover any costs related to a break in the water main that runs under your street.
Even if this break causes water to flood your yard and destroy your garden, your insurance company is likely to argue that the offending pipe belongs to the municipality in which you live and isn't covered by your policy. Likewise, a water main break that causes large quantities of water to seep under your home's foundation isn't likely to be covered. Despite the fact that such seepage has the potential to cause serious damage to your home's structure, it's exempt from coverage by the fact that its source is outside of your insurance policy's purview.
In most cases, the spur line that connects your home to the municipal water main will be governed by similar restrictions. Unless you can prove that the break to your incoming water line was caused by malfunctioning or defective equipment or a contractor's gross negligence, any water damage that your home or yard sustains as a result of the breakage is likely to remain uncovered. In fact, your homeowner's insurance policy probably features an "interior damage" clause that limits your insurer's liability for exterior water damage.
If the break occurs inside your home, the insurance calculus will change significantly. Of course, any break in an interior pipe won't technically involve a water main. However, the result will be functionally identical to an external rupture. If one of your home's water pipes sustains a break that can be construed to have occurred suddenly or without cause, your homeowner's insurance policy is likely to cover some or all of the associated cleanup and repair costs.