I have a 4 inch sewer pipe going to a septic system. The pipe exits throgh the floor into the knee wall of the second floor of an unfinished room after renovations. I extended it into the crawl space in the attic by running it along the rafters in a roof pitch. I downsized it to a 3 inch pipe to do this because 4 inches drops below the rafters and would be a problem when I hang sheet rock. I was told that I can't down size a sewer vent pipe because an ice plug would develop, clog the system, blow up the house and I would die etc... Before I cut into the roof to exit it to the outside, do I really have to maintain it at 4 inches or even increase it to 5 inches? Can I simply insulate the 3 inche pipe in the attic? Can I increase the size back up to 4 or even 5 inches in the attic? The house is in Northern Vermont where it gets bitterly cold.

Chapter 14, Section 1403.3( Main Stack) of the Standard Plumbing Code reads, " Every building in which plumbing is installed shall have at least one main vent stack, of not less then 3 inches of diameter, which shall run UNDIMINISHED IN SIZE and as directly as possible from the building drain to the open air above the roof."

I hope this answers your question. Regards, Tom