If your home or office building is on the top of a sewer system (cul du sac, in the middle of a block at the crest of a hill or in an industrial park) the sewer may be prone to backups. If you are down stream of the backup, your situation does not change. However, if you are upstream of the backup, you may have the misfortune of having all buildings upstream of your building or house discharging sewage to your property.
A Back Water Valve is basically a check valve installed on the residential sewer line. It allows sewage to flow out of the building but does not allow sewage backups from downstream of the back water valve to flow upstream into homes or places of business. When municipal sewer backups occur, the sewage continues to flow from upstream the blockage. However, the sewage cannot flow downstream in the direction it would normally flow if there is a blockage. Therefore, the house or come up through basement floor drains.
The sketch above will be helpful for you to remember. This is a back water valve. It opens from the top and should remain serviceable when installed. If the plumbing does not flow out of the house. Do not open the back water valve unless you are absolutely certain there is not a blockage in the sewer system, downstream of this valve. The result could be that you will flood your home with sewage from your neighborhood. And the flood will not stop until the blockage is free and sewage is able to traverse the municipal sewer system again.
The back water valve has a flapper, check ball or a floatable gate inside. When water rises to the backwater valve, it either rest against a flapper with a rubber seal, causes the buoyance of a plastic ball or a gate to rise to the back side of the upper chamber in the back water valve. This action does not let go until the sewage has subsided from the downstream side of the upper chamber.
This diagram demonstrates the effectiveness of back water valves on the first two homes and how the backup will affect the homes next upstream of the blockage.
When the back water valve is in place, the sewage cannot pass beyond the back water valve and your home is safe from sewage. However, if sewage is not coming up, keep in mind, no sewage will get out, either. In fact, one of the signals your sewer is backing up outside your home is usually the result of having water and sewage from your home, not being able to get out and the pipe becomes full with sewage. On the bright side, you have the ability to stop using water and clean up the mess, without sewage continuing to pour into your home. The next step is to find where the problem is and call a plumber (if the problem is on your property) or the city if the problem is theirs to rid.