Length of Vent Lines

What is the rule concerning the maximum length of a vent line?

For example: can a vent line combine with other vents in one exit point say 20ft (not linear) from the furtherest fixture? Is there a max length number and or a max horizontal length?

I can't find a straight answer. I read something about 1/3, then another spot said there is no rule.

Thanks!

The code probably varies depending on your location. I don't know about the IPC/IRC. But, the UPC table 7.5 has rules on vent lengths. Vents can be combined. The pipe size depends on the number (and kind) of fixtures served. There are length maximums. If you increase the vent by one pipe size the total length is unlimited. But, and a big but, only one third of a vent's length can be horizontal.

vent

I have never had an inspector enforce that 1/3 rule, which would be somewhat ridiculous. If the house were only 12' high, which is common in this area of flat roofs, giving a 9' vent and a 3' maximum horizontal offset, when the vents to be combined are 6' or more apart.

hj said:

I have never had an inspector enforce that 1/3 rule, which would be somewhat ridiculous. If the house were only 12' high, which is common in this area of flat roofs, giving a 9' vent and a 3' maximum horizontal offset, when the vents to be combined are 6' or more apart.

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Actually, according to the UPC 1/3 of the TOTAL PERMISSABLE LENTH may be horizontal, not 1/3 of the run of the vent. A 11/2 vent can run 60 feet. 1/3 of the total permissable lenth is 20 feet. So you can run a 11/2 vent horizontal 20 feet irregardless of the total lenth of the vent.