Lower basement drains may need a backwater valve to prevent basement flooding. Source: Unknown

Basement fixtures below red line may need backwater valve to prevent basement flooding-Source: City of Worcester

Click on the backwater valve graphics to enlarge them.

What is a Backwater Valve?

Backwater valves are installed to prevent or reduce the frequency of sanitary and storm water backing up on the mainline through the lateral and flooding a basement.  This backing up condition is referred to as backflow.  Backflow can occur due to 1. obstruction and/or 2. the collection system reaching its capacity.

Who may need a Backwater Valve?

If your home is located on a sloping street and your home is at a lower level than other homes on that street, you may be subject to backflow. Folks who are considering a home purchase on a sloped street should carefully look into this location location location consideration, if feasible, in concert with an appropriate expert such as a plumber, home inspector.  Further, the best backwater valves and the backwater valve monitoring system are not inexpensive.  The above graphics, obtained from public sources, indicate a few configurations where backflow may be expected.

Backwater Valve Location

The location of plumbing fixtures in the basement does not always indicate the need for a backwater valve, unless these fixtures drain to the mainline by gravity.  In instances, where basement drains are pumped up to a upper drain and the pump outlet is provided with a check valve, a backwater valve may not be needed.

The backwater valve should be properly located and installed.  The backwater valve is installed as shown in the second figure from the top.  The backwater valve should be located upstream of the foundation drains or downspouts connected to the laterals, otherwise the homeowners may flood themselves.  Also the backwater valves should impact the basement drain only.  Homeowners should always consult an appropriate expert.

Backwater Valve Permit

You may need a permit from your local municipality to install a backwater valve.  I think municipal permitting becomes necessary because an improper installation of a backwater valve could push the backflow into a neighboring basement that did not previously was not subject to backflow.  I reckon there could be instances where residents have installed backwater valves without obtaining a necessary permit; after all, enforcement resources are not unlimited.   Sometimes people blame their local municipality for their basement and residential flooding problems and this is often without a sound and sufficient reasoning.

Other Backwater Valve Considerations

In some instances, basement floor drains may be connected to the municipal stormwater collection system.  This isn’t kosher, within the context of the Clean Water Act and the NPDES Phase 2 Storm Water Program; the basement floor drains should be connected to the municipal sanitary sewer collection system.

The backwater valve is imperfect.  Its prudent to consider avoiding buying a home that is located on a lower level than other homes on a sloping street.  Basements can be versatile additions to a residential property, particularly in north and northeastern parts of the country.  A basement can have multipurpose utility, however, installing drains in a basement may lead to complicated conditions, in some cases.  Its a bit unlikely that you will hear this from your real estate agent.  It would also be fair to expect that location and properties that are prone to flooding due to a gravity challenged location might be subject to higher home insurance rates.  Further, it would be reasonable to expect that properties that have resulted in multiple requests for reimbursements for flooding damage are likely to be flagged.

Does your home need a Backwater Valve? Source: Town of Las Gatos, California