Outside drains
A non-return valve allows waste water to flow out of a property but prevents it flowing back in. It is fitted in a chamber on the outgoing run and protects basements, lower ground floors and properties on shared or overloaded sewers from sewage backflow flooding.
Backflow happens when the sewer surcharges — typically in heavy rain — and the lowest connection on the network becomes the relief point. If that is your ground-floor shower, you have a problem the valve solves.
Valves need to be accessible for inspection and cleaning; they are fitted in a chamber, never buried mid-run.
The same fitting doubles as an effective rat barrier where drainage rat activity is a known local issue.
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