Horizontal Vent Length in Illinois Misunderstanding

[MavMan]

I'm a little unclear on how far a vent can go horizontally. According to Illinois code it says.

e) Size of Vent Piping. The size of vent piping shall be determined based upon its length and the total number of drainage fixture units connected to the vent pipe as provided in Appendix A.Table K. No more than 20 percent of the maximum developed length as determined from Table K may be installed in the horizontal position. Vent piping serving floor drains shall be installed so as to minimize horizontal vent distances.
(ftp://www.ilga.gov/JCAR/AdminCode/077/077008900K15800R.html)

At first I was thrown and thought it could only be 20% of length of vent. But that almost seems impossible as you would have a vent poking out of the roof at drain location in the house and I'm pretty sure that is not how professional plumbers do it. Then I did some googling and I saw where someone said the 20% is based on the max length in Appendix A Table K, not on actual length of vent pipe. That seemed to make more sense.
(ftp://www.ilga.gov/JCAR/AdminCode/077/07700890ZZ9996akR.html)

Everything seemed fine until just now my brother said that a plumber at a large university where he works told him it is the actual vent pipe length as I had at first read the code. This plumber has been around for quite a while and works in a commercial setting (so I would think he would know). Can someone confirm what is actually right? In a 1 story house your vent pipes would approximately 7 ft to attic and then another 7-9 ft out the roof being a total of about 15 ft in developed length. This would allow for maximum 3 ft horizontal vent. Is this right?

Where we live, there is actually no plumbing inspection but I want to do things right. Thanks!

 

[Terry]

I read that as:
1.5" pipe with up to 10 fixture units, Max length is 100 feet, of which 20% of that would be 20 feet of horizontal allowed.

 

[Sponsor]

 

[MavMan]

Terry, thanks for the confirmation. I'm not sure where the disconnect is with the other plumber but it seems to be very clear as to how you mentioned when I go back and reread the code. If it was actual vent length there would be no reason to mention "Maximum" developed length. Thanks.

 

[Terry]

My dad was a judge, a mayor, on the Puget Sound Metro area planning board and a lawyer.
So maybe I got that right. I plumb using the UPC code in Washington State, which is a bit different.
I grew up listening to my mom and dad discussing law around the dinner table. he was a politcal science major.
I still read articles in the paper discussing laws my father had written fifty years ago. I remember my parents talking over how the laws would be written.

 

[hj]

as a practical matter, over 65 years, and I have NEVER worried about the lengths of my horizontal vents and have never had to revise them.