roof vent

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3" roof vent is too small for that cold climate. I'd bet when when climate conditions are right, youre building up frost inside your vent at the roof, which plugs the vent, then when you operate a drain with your now unvented plumbing system you siphon out a plumbing trap, and get sewer odors venting back into your house. By the time a plumber gets there to look the vent is thawed, the trap is refilled, and the plumbing system is normal. The plumbing trap in a fixture nearest to your rec room is probably the one that always gets siphoned out. It makes sense that the same trap will get sucked out every time the roof vent freezes.

Check the rubber flashing boot on the roof, many of them will fit a 3 and 4 inch pipe. If so, cut off your 3 inch vent stack pipe in the attic, push a peice of 4" pipe through it and use a fernco fitting to connect it to the house system. You might have to carefully cut some roof plywood away from the underside. This way you wont have to mess with shingling work. Keep the height above the roof length as short as code allows. This will reduce the length that warm vent air can condense in the vent pipe as the pipe goes from warm interior to freezing exterior