I'm about to head north to put an addition on a friends cabin and at some point down the road were going have to tie the house plumbing system into a septic system. What size pipe should I be running out of the addition to ultimately hook into the system ?

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What size pipe should I be running out of the addition to ultimately hook into the system ?

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No brainer! 4"PVC Schedule #40 from cabin to septic tank. Good luck, Tom

Tom is on the money, 4" its is. Where are you doing this work that an inspector won't be involved?

Schedule 40 inside. SDR35 outside.