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rlhed 07-30-2008 10:49 AM

Plumbing Vent size recommendations
I am building a bathroom on concrete slab, size 8' x 10'

1 Toilet
2 Sink
1 Shower

Sinks are 2" coming up through the sill plate for connection for drain and vent to continue up to the attic.

Toilet is 3" with 2" vent coming up through the sill plate for vent connection

Shower is 2" drain with a 2" vent coming up through the sill plate.

Now for the question. (whew!!)

I need to take all of the vent lines into the attic space and connect them together then penetrate the roof with vent pipe.

Q: What size ABS should I use for the vent pipe in the walls going into the attic space

Q: According to local code (WA) the vent has to be the same size or larger as the main drain leaving the structure which is 3". Should I put in a single 3" through the roof and connect all the vent lines to this 3" penetration

Q: any advice on proper fittings to connect the vent lines to the 3" penetration and supporting that assembly would be appreciated.

mstplumber 07-30-2008 12:38 PM

One 3" through the roof will probably be best, unless there is a reason to run any of the others out separately due to the layout of the attic space.

Use regular DWV fittings to connect the vents, just install them "backwards" as compared to the waste system. In other words, the direction of flow of all fittings in the vent system should be towards the roof, not the sewer.

ABS pipe should be supported at least every 4' horizontally and at 10' intervals vertically. Use 2 x 4 blocking and secure the pipe with band iron (metal strapping with holes every inch).

Alan 07-30-2008 08:05 PM

Check on the exact wording there ........

Our code (OR) is worded that the aggregate cross sectional area of vent penetrations must be at LEAST the same cross sectional area as the MINIMUM REQUIRED building drain.

So, here, you could have two 2" vents and one 1-1/2" vent to meet this requirement for a 3" building drain.

This requirement is for the entire house, mind you, so if you're ADDING a bathroom that is tied into the building drain of the original house, you can count the other roof penetrations as well, in which case, you may only need to stub out a single 2" vent, which would save you a considerable amount of money not using any 3" fittings....

Marlin 07-30-2008 09:33 PM

Our code says any pipe penetrating the roof must be at least 3". You need to either go out with 3" to be safe or call your town inspector and ask if it's ok to go out with a smaller line. Codes vary state to state and town to town, even inspector to inspector.


Originally Posted by Alan (Post 144481)
which would save you a considerable amount of money not using any 3" fittings....

You only need 1 3" fitting. We run the whole thing in 2" then use a 3 x 2 coupling to increase to 3" on the vertical just before you go out. That satisfies the code for only having 3" penetrate the roof. It's stupid and asking for a clog but that's how the inspectors want it.

rlhed 07-31-2008 12:01 AM

Follow up information
Thanks for the replies so far. The vent will be 3". One question I have is to continue the venting past the fixtures in 2" or does it make sense to drop the vent lines to 1 1/2" to the attic then connect together for a 3" roof penetration - Rick

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