Determine size of final roof Vent

[Dohc]

How do you determine the size of the roof vent for a complete DWV system? This is in WI, and a single family residential house. The WI plumbing code can be found online here...
https://docs.legis.wisconsin.gov/co...and_buildings_and_environment/380_387/382.pdf

WI SPS 382.31 Vents and venting systems - I read it that it says if the Diameter of the Drain Stack at Base is 4 inches, then the vent can be 3" as long as the total length of the vent is less than 180 feet. Am I missing anything here or is my understanding correct?

Thank you

 

[Sylvan]

The vent thought the roof in areas where you can have below 32 DEG F should be 4"

Even when I ran a 2" vent from a kitching sink I increased the vent just below the roof to 4" to prevent Hoarfrost

 

[Sponsor]

 

[Sylvan]

16) Vent terminals. All vents and vent systems shall terminate in the open air in accordance with this subsection.
(a) Extension above roofs. Extensions of vents through a roof shall terminate at least 8" above the roof. Where the roof is to be used for any purpose other than weather protection, the vents shall extend at least 7 feet above the roof.

Actually on a flat roof depending on the average snow depth I would not go less the 2 FT above the roof and 7 feet for a Promenad roof /deck

 

[Jeff H Young]

if you only have one vent in the house it must be equal ( or i suppose it could be larger) than sewer as it leaves house so its either 3 or 4 inch. If the house has multiple vents thru roof then the cross section of the total of those vents must be equal or exceed the 3 or 4 inch main.