Can I Combine Vent Pipes?
Q: "I have five vent pipes going through my roof. Can I connect them together?"
I have five vent pipes going through my roof. Can I connect them together in the attic and reduce the number of holes in the roof?
— C.M., St. Augustine Beach, FL
Richard Trethewey replies: You're right about keeping the holes in a roof to a minimum — every penetration is a leak waiting to happen. Besides, a house with lots of vents starts looking a bit like a chemical plant.
I can't tell you exactly what will work in your situation because I don't have all the details, but I bet you'll be able to combine at least some of those vents. Check your local codes to see what's acceptable. Here in Massachusetts, the code says that a house must have at least one 3-inch-diameter vertical stack, a pipe that starts at the horizontal drain line to the septic or sewer system and exits through the roof. The smaller vents don't need to penetrate the roof, but they have to tie into the stack above the highest fixture in the building.
Another option, if codes in your area allow, is to cut off the secondary vents in the attic at least 6 inches above the ceiling insulation and cap each one with an air-admittance vent. This is a device with a diaphragm that allows air to enter the stack when water goes down the drain but doesn't allow sewer gases to escape into the house. Theoretically, you could eliminate roof penetrations entirely with these vents, but at the moment, the codes say that every plumbing system must have at least one stack that extends outdoors.