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Any length max on horiz. vent branch (to get to main stack)?

Posted by la_koala (My Page) on
Thu, Jul 29, 10 at 20:16


First, is there an exact name for the horizontal piece that multiple secondary vent stacks terminate in, that then runs over to the main vent stack?

As depicted in the bottom, rightmost picture titled "Terminating into header" on this page:

Is it called a "vent header"?
(Asking because when I Google search on "vent header", I get pages that don't match in their descriptions. :-)

And my main question: Does the code put a restriction on how long that horizontal piece can be?

If I have a secondary stack vent running up to the attic, can I terminate it in a horizontal piece that might run 25 feet over to the main stack?
Or would I have to split up that horizontal distance with another stack vent in some way?

If it makes a difference, I'm in MA and I think we're under the UPC. I'm trying to learn enough that I have the right terminology to even see what might be feasible to do in my old house.

Thanks in advance!

Follow-Up Postings:

RE: Any length max on horiz. vent branch (to get to main stack)?

See the link below for Massachusetts CMR Plumbing Code.

Pay particular attention to 'headers' and Table 2 for vent pipe sizing.

Here is a link that might be useful: Mass UPC Vents and Venting

RE: Any length max on horiz. vent branch (to get to main stack)?

Thanks manhattan42! Thanks both for posting the link and for confirming that "header" is the right term to pay attention to.

I always have difficulty reading these ordinance-type of things. For example, it seems that a word or punctuation is missing from this one:

10.16 (5) (d) 2 and 3 reads:
2. The vent header shall connect to a vent extension through the roof.

3. When more than two four-inch soil or waste stacks are connected the vent header extension through the roof shall be five inches in diameter.

Does #3 really mean in essence:
"When more than two four-inch soil or waste stacks are connected to the vent header, the vent extension through the roof shall be five inches in diameter."

(Notice the addition of the word "to" and a comma added after "vent header" above).

I'm asking because #2 uses both the phrases "vent header" and "vent extension", and states that the vent extension goes through the roof. I can picture that.

But in #3 as written in the Mass UPC, it introduces what looks like a new phrase, "vent header extension" going through the roof.

And hence my confusion. :-)
It makes more logical sense (to me anyway) that if you've got more than 2 4-inch stacks basically connected together, that you'd want a higher diameter pipe as the single guy going out through the roof. But that #3 isn't communicating that well.


RE: Any length max on horiz. vent branch (to get to main stack)?

See the link below using the image from your first post.

The 'header' is the horizontal pipe that connects the vent stacks.

The 'vent header extension' is the vertical vent pipe that extends up from the vent header through the roof.

Here is a link that might be useful: Terminating To Header

RE: Any length max on horiz. vent branch (to get to main stack)?

OK, thanks for clarifying that about the "vent header extension" piece.

You've been extremely helpful. If you don't mind, a follow-on question:
Can the 'header' horizontal pipe connect to an existing main stack (soil stack) which already terminates through the roof into open air, connecting above the point where any fixtures are?

Or is the code saying that the header configuration always require the vent header extension and terminating itself through the roof, requiring a second pipe through the roof?

My situation is that the main stack is at the front of the house (second story full bath, soil and vent for that bath, no other fixtures above the second story). The venting for the kitchen in the rear is not up to today's code (outside vent up the side of the house). I'd like to be able to run a new stack vent for the kitchen in that rear part of the house, and then connect that vent to a header running across the attic space above the second story to connect to the main stack vent in the front.

I'd really like to avoid making another hole in the roof and stay within code--thus my question of whether it is allowable to connect the header to the existing main stack vent, in the attic space that exists above any and all of the fixtures on any vent.

I hope my question makes sense. It seems that the last time plumbing was done in this house was over 50 years ago and the external stack is no longer allowed in my area.


RE: Any length max on horiz. vent branch (to get to main stack)?

Talk to your local plumbing code inspector.

I am not that familiar with the Mass. UPC. We use the IPC in my jurisdiction and the codes are different.

It appears that the UPC requires the 2nd stack penetrating the roof if the existing vent is not large enough to serve all fixtures and their DFUs.

Regradless, you would actually produce a better more efficient drainage system if you installed the 2nd vent extension through the roof imho.

RE: Any length max on horiz. vent branch (to get to main stack)?

Hi manhattan42, thank you so much for replying.

And thanks for your last thought about the efficient draining. I hadn't thought of looking at it that way, and that's a good "pro" on the side of going straight through the roof at that spot. (I'm going to need something to tell my spouse--who's queasy about leaking roofs--why putting another hole through the roof is better than the alternative.)

While I did have a brief meeting with our plumbing inspector a week ago (and just to see if the existing rear external vent was viable if we're redoing the kitchen) he seemed reluctant to talk to me, the homeowner directly. He kept saying I should hire the plumber and then he'd be happy to meet with the plumber.

(Which made me a bit dismayed that he only wanted to speak with plumbers, because a few years ago we hired a licensed, neighbor-recommended, well-known local plumber to put in a washing machine hook-up in the basement. We thought he had pulled the permit and was going to do the work to code. That guy put in an air admittance vent -- and now I come to find out from my chat with the inspector that the air admittance vents are not allowed! So all that work has to be re-done. If an inspector is truly going to only speak with the plumber, then I figure I'd better get educated on what's allowed and what's not so that I ask the plumber the key questions before I hire him/her and can feel confident that the right things will be done. Once bitten, twice shy.)

manhattan42, I appreciate all the info you've given--it's helped immensely!

Thanks again,

RE: Any length max on horiz. vent branch (to get to main stack)?

I believe Mass. is one of those states where plumbing must be done by a state licensed plumber.

That is why your inspector doesn't want to talk to you, the owner.

As far as your story about the air admittance valves, they are permitted under the UPC but only with the permission of the plumbing inspector and generally only when no other viable venting alternative exists.

Sounds like your plumber on the washing machine did not ask about the AAV nor call for any inspections...and why the inspector was PO'd...with every right to be.

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