# Venting question

### Users who are viewing this thread

#### Tomski

##### New Member
I've attached a layout of my proposed plumbing for an addition. I'm in California so UPC.

I've read that a 2" vent can take care of 24 DFU's. I have a total of 8 DFU's so I thought I could combine the two 2" vents in the attic to one 2" vent through the roof.

However, I've also read that the area of all the vents must be greater than the area of the main drain. My main drain is 3" so by this standard I need 7.07 sq. inches. Even if I use a 2" vent through the roof for the kitchen sink and another 2" vent through the roof for my bathroom group I am still short of the 7.07 sq. inches. Am I missing something or do I have to add another vent pipe somewhere? Thanks for your input.

#### Attachments

• latest plumbing layout.jpg
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#### Terry

##### The Plumbing Wizard
Staff member
Through the roof, two 2" vents and a 1.5" vent.
The kitchen sink is getting the 1.5" vent, and the bathroom group is the 2".
You have other plumbing in the home, right?

#### Tomski

##### New Member
Yes there is other plumbing and vents in the main part of the house.....forgot about those so that would take care of the total vent area.

Just to clarify, you said 2 2" vents and one 1.5" vent. I assume you meant one 2" and one 1.5"?

#### wwhitney

##### In the Trades
Under the UPC, if you have 3 WCs or less, the minimum size building drain is 3". With 4 WCs, the minimum size building drain is 4". The total vent area through the roof has to be at least the minimum size building drain.

For 3" minimum area, you could obviously do a 3" vent. Or you could try (2) 2" vents, but 2^2 + 2^2 = 8 < 9 = 3^2. So that's not quite enough, you'd need to use (2) 2" vents and (1) 1.5" vent, which was Terry's point.

For 4" minimum area, you could do a 4" vent, (2) 3" vents, (1) 3" vent plus (2) 2" vents, (4) 2" vents, (3) 2" vents and (2) 1.5" vents, etc.

Cheers, Wayne

#### Tomski

##### New Member
Ahhh. I'm so focused on the new plumbing, I keep having tunnel vision. I wasn't realizing Terry was talking about the house in total, not just the new addition. Sorry, I'm usually not so dense. Thanks again to both of you for the help.

#### Nick_A

##### New Member
Hello, this thread is helpful to me, thank you. On the vent size requirements, is this requirement for the size at the interface with the roof, or the size of the vent stack in the attic space? Thinking in terms of the upsizing one inch at the roof due to freezing temps in the winter.

Thanks
Nick

#### Tuttles Revenge

##### In the Trades
Hello, this thread is helpful to me, thank you. On the vent size requirements, is this requirement for the size at the interface with the roof, or the size of the vent stack in the attic space? Thinking in terms of the upsizing one inch at the roof due to freezing temps in the winter.

Thanks
Nick

Code does call for upsizing a pipe size for the potential freezing of condensation in a vent at the roof. The upsizing should occur at least 1ft below the roof but also within an insulated space.. so potentially the entirety of the attic space starting at the insulated portion. I had to look that up since we don't deal with climate that cold.

The discussion was originally about the proper sizing of venting at a group of new fixtures in an existing house. The OP's drawing was perfect for horizontal wet venting the bathroom and separately venting the sink in wich they could be combined and vented through the roof as 1 penetrations.

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