Redundant drain lines are the most likely cause for rat intrusion, these are most commonly left after building works or extentions. Water testing plugs are fitted to block off these drain lines and are a better solution than just filling with concrete as rats can dig their way through.
Standard manhole without a trap, is a frequent cause for rat entry into your private drainage system. The main sewer is often left openly connected to your drains.
The permenant solution for your little friends is a non-return valve (single check/ double check) this allows down stream flow from your property but no more rodents entering from the main sewer.
In a short day your rat problem is permenantly solved and flood protection comes with your install. Single check valves' main function is rat protection but will save you from small sewer surges and flooding. Double check valves' main function is for flood protection.
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