Venting

Notes

• **Two rules, and people only remember the first.** A vent is never smaller than half the drain it serves and never below 1¼″ — that gives 1¼″ here. The second rule is developed length: friction grows with the run, so a long vent has to be larger. At 50 ft that pushes this one to **1½″**.
• **Developed length is measured along the pipe.** Every elbow, every offset and the whole vertical rise count. It is not the distance from the fixture to the roof measured on a drawing — running 60 ft of 1½″ because the roof looks close is exactly how a trap at the far end gets siphoned dry.
• **What the vent actually does is protect the trap seal.** Water falling down a stack drags air with it. Without a path for replacement air, that suction pulls the water out of the nearest trap, and then sewer gas has an open door into the room. A vent that is undersized does not fail loudly — it fails as a smell nobody can place.
• **A dry trap smells the same as a broken one.** Floor drains and guest bathrooms that go months without use lose their seal to evaporation, not to venting. Before re-plumbing anything, pour a bucket of water down and wait a day.
• Figures here follow the IPC. UPC jurisdictions differ on several points — wet venting and island fixtures especially — and the local inspector decides. Use this to size a plan and to read a quote, not to argue at inspection.