I have observed that when we receive a lot of snow here in Ontario, at least one of the drain vents on my roof is covered by snow. While after one winter in the house I have not yet observed any ill-effects from this, I know it can cause some problems that I'd like to proactively avoid. I also have a belly in my main sewer that has caused a number of backups (see this thread) so I'm trying to prevent anything that might make this worse.

The colour of the vent is black so I am assuming it is ABS.

Can I use a coupling and some ABS pipe to extend it a couple of inches, or is there a more preferred/correct method of doing this?

  • Too localized. Building code questions are generally too locale specific. Commented Jan 16, 2012 at 18:13
  • I will remove the code part of it, still curious as to the correct way to extend this.
    – Steven
    Commented Jan 16, 2012 at 18:15
  • @TheEvilGreebo remove the building code aspect of the question, and it's a good question. It boils down to two questions. "How can I lengthen my vent stack?" and "Are there any negative effects to changing the length of a vent stack?"
    – Tester101
    Commented Jan 16, 2012 at 18:22

1 Answer 1


IANAP (I Am Not A Plumber) but I have recently replaced my old cast iron stack.

The relevant part of the International Plumbing Code looks like section 904. Specifically the following sections may apply to you.

904.1 Roof extension. All open vent pipes that extend through a roof shall be terminated at least [NUMBER] inches (mm) above the roof, except that where a roof is to be used for any purpose other than weather protection, the vent extensions shall be run at least 7 feet (2134 mm) above the roof.

Note the [number] is usually specified by your local codes as it is very specific to each region. Some web searching turns up a few forum posts from plumbers in Ontario who say it needs to be 6 inches above the roof. That does not sound like much to me and if you are already at 6" and having issues it seems reasonable to go higher.

904.2 Frost closure. Where the 97.5-percent value for outside design temperature is 0°F (-18°C) or less, every vent extension through a roof or wall shall be a minimum of 3 inches (76 mm) in diameter. Any increase in the size of the vent shall be made inside the structure a minimum of 1 foot (305 mm) below the roof or inside the wall.

904.3 Flashings. The juncture of each vent pipe with the roof line shall be made water tight by an approved flashing.

904.7 Extension outside a structure. In climates where the 97.5-percent value for outside design temperature is less than 0°F (-18°C), vent pipes installed on the exterior of the structure shall be protected against freezing by insulation, heat or both.

I don't know the preferred method for doing this but if you plan is to add a coupling from inside the house and "push" the pipe up, I bet you will compromise the water tight seal around the pipe at the roof. The simplest solution would be to add this extension at the very top so you don't have to mess with the flashing at all.

If this is the main waste/vent stack for the home, I don't think you are allowed to reduce the diameter of the pipe (even up at the "vent" portion). I don't know if that is still true for smaller vents that service just a few fixtures.

If it is black plastic, it is ABS and should be easy to cut and glue.

  • I believe the Ontario code specifes 6" above the "estimated max amount of snow" or something like that - obviously whoever installed this one did not estimate it well. I was planning on just installing the coupling at the roof so I do not need to modify any other part of the vent. I will keep it the same size.
    – Steven
    Commented Jan 16, 2012 at 23:30
  • I've seen anywhere between 6"-18" depending on location, for minimum stack height above a roof. Keep in mind, however, the minimum height is the length of the shortest side of the pipe. So if the roof is slanted, the minimum height is measured on the high side of the pipe (closest to the peak).
    – Tester101
    Commented Jan 17, 2012 at 16:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.