Tying together branch vents in attic question

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Mr_Magoo, Mar 8, 2008.

  1. Mr_Magoo

    Mr_Magoo New Member

    Mar 8, 2008
    I am owner/builder trying to get my rough-in plumbing complete. We are governed by UPC. I finished the master bath venting and wanted to confirm that what I did was OK. I ran a 2-inch main vent line horizontally to tie all the individual vent lines together. To get to the branch vents, I used 2-inch San-Ts laid on their side with a 2 inch branch line to each fixture. I have the San-Ts oriented for drainage. There is no place for rain water to collect. All horizontal lines have 1/4 slope. I have a cheat sheet and not the full code. UPC 905.2 says no flat horizontal dry vents. I assume this means when you are tying vents into drainage lines. My question is once in the attic and all vent lines are dry, is it legal to have a San-T laying on its side to tie branch vents to main vent line? Since the line is always dry it seems as though it is fine, but I have been wrong before.

  2. Winslow

    Winslow Plumber

    Dec 30, 2005
    It is permissable to use a san tee on it's side however

    (1) The maximum permitted fixture unit value of a 2" vent is 24

    (2) The combined cross section of vents through the roof need to equal the cross section of the main sewer. For instance if you have a 3" main sewer line (7.065) the cross section of all vents penetrating the roof must be equal to or greater than 7.065. A 2" vent equals 3.14, a 11/2" vent equals 1.766. Technically speaking you would need at least (1) 2" vent ( the minimum size of a toilet vent) and (3) 11/2" vents.

    A 4" main sewer has a cross section of 12.56, therefore you would need the equivalent of (1) 4" or (4) 2" vent penetrating the roof.

    If you are talking soley of 1 bathroom then all on (1) 2" vent will be OK as long as it and the remaining vents (other baths, kitxhen, laundry, ect) are equal to or greater than the main sewer.

    Hope this didn't totally confuse you but sometimes people demo the existing plumbing then without realizing it delete an additional roof penetration that had the purpose of fulfilling the cross section requirement.
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  4. Mr_Magoo

    Mr_Magoo New Member

    Mar 8, 2008
    Thank you...Yes, I had originally planned on having three 2 inch vents in separate areas of the house (master bath, main bath, and kitchen/laundry). But while reviewing my cheat sheet just tonight I caught that info about the total vent area. I put in a 4 inch main drain for the house because I roughed in under the basement slab for a 4th bath. So, I know exactly what you are telling me. I'll just run my kitchen vent separate to get the total required vent area. Thanks again for the confirmation on the sideways San-Ts.

  5. srdenny

    srdenny Plumbing Contractor

    Sep 11, 2004
    Plumbing Contractor
    SF Peninsula
    Since you roughed in a fourth wc, you'll have to run the sink vent in 2" to achieve the 12.56 sq in cross section required in agregate. See Table 7-5 UPC, which except for chapter rearranging, (it used to be Table 4-3) hasn't changed since at least 1973 [my earliest addition].
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