Thread: Vent Stack Size

Attached is a pic from the rear of a house I looked at yesterday. In the pic I circled the only plumbing vent pipe I noted at the house and it was about a 2" copper pipe. What is the minimum size vent pipe a house should have for adequate venting of the plumbing system? The house had one bathroom and it was on the upper level adjacent to the kitchen. I did get some gurling from the bathroom sink drain when it was emptying so I wouldn't be surprised if this vent pipe is undersized. The vent pipe exits the house below the bathroom. Shouldn't there be another vent closer to or above the bathroom?

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It's wrong Nick, which you already suspect. Recommend an evaluation by a state licensed plumbing contractor because drainage system venting is an engineered process in which you should not have to guess the number and size of the venting system. The gurgling sound was an excellent piece of evidence that things where not right.
UPC Chapter 9 and IRC Chapter 31.

Plumbing code in MA says this.

"The vent extension through a roof shall not be less than 2 inches in diameter and shall extend not more than 24 inches and not less than 18 inches through the roof.
FROST CLOSURE: Where frost closure is likely to occur, each vent extension through a roof shall be at least three inches in diameter.
PLUMBING VENT STACK EXTENSIONS OUTSIDE THE BUILDING: All soil, waste or vent pipe extensions shall be installed inside the building. For remodeling vents may be installed outside the building with permission of local inspector"

All soil, waste or vent pipe extensions shall be installed inside the building.
That can't be right???

Originally Posted by Jerry McCarthy

All soil, waste or vent pipe extensions shall be installed inside the building.
That can't be right???

Not terminate inside the building.

Originally Posted by Jerry McCarthy

All soil, waste or vent pipe extensions shall be installed inside the building.
That can't be right???

That's 'inside the thermal envelope', or else they need to be insulated or have heating tape, or both, on them.

That freeze line is basically along Jacksonville, Florida, to Tallahassee, to Pensacola, across to Texas and up and around, it's in the Appendix to the plumbing code.

I assume they meant this.

Originally Posted by David Banks

I assume they meant this.

Yep.

Not only does that pipe need to be protected from physical damage, it needs to be protected from freezing (the part under discussion here).

I don't see a support or strap for the offset or the top section of pipe (is there a strap at the top?, looks like it's next to a gutter and has nothing to strap to).

In the pic I circled the only plumbing vent pipe I noted at the house and it was about a 2" copper pipe. What is the minimum size vent pipe a house should have for adequate venting of the plumbing system?
Aside from what has been addressed so far what about the minimum sized vent stack? Table 3005.4.1.

IRC- P3103.2 Frost closure. Where the 97.5-percent value for outside design temperature is 0 degrees F or less, every vent extension through a roof or wall shall be a minimum of 3 inches in diameter. Any increase in the size of the vent shall be made inside the structure a minimum of 1 foot below the roof or inside the wall.

Table P3107.3 Common Vent sizes- Vent sizes 1 1/2-3 inches.

Other tables show up to 4 inches depending on number of fixture units.

Easy for me as I will just use MA Plumbing code above as my guideline

For one full bath and one kitchen, a 2" vent should be plenty. However...

If that vent enters the home between the first and second floor, then that can't be venting the sink. I'm assuming there's no AAV attached to the sink in the bathroom. I'd be more willing to bet that it's gurgling because the sink's not vented at all and on an S-trap more than the fact that it's "only" a 2" vent.

here every fixture requires protection from a trap and vent. the area of all the vents are required to meet or exceed the area of the sewer pipe serving the building. something technical to do with equal pressure etc..

So, if you have a single bath house, and have a 3" line, you need a 3" or greater vent through the roof?

yes. a three inch vent is required