The following provisions on backwater valves are from the National Plumbing
Code:
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5.5.1 Where Required.
a. Fixtures and/or drain inlets subject to backflow and flooding from
blocked or restricted public sewers shall be protected by a backwater valve.
Such situations include those where fixtures and/or drains are located above
the crown level of the public sewer at the point of connection thereto, but
are below the level of the curb at the point where the building sewer crosses
under the curb at the property line.
b. Backwater valves shall be installed in branches of the drainage system
which receive flow only from fixtures and/or drains subject to backflow from
the public sewer. Other portions of the drainage system not subject to backflow
shall drain directly to the public sewer.
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5.5.2 Material Standard and Accessibility
Backwater valves shall conform to the standard listed in Table 3.1.3
and shall be installed so that their internal working parts are accessible
for periodic cleaning, repair or replacement.
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