Looking for input on plumbing issue.

I saw this abs plumbing vent in the attic. The through roof portion is 4". Note reducer to 2" abs. This vent serves toilet, and other fixtures. What are the specific issues here.


O.K…I finally found something, I think.

According to an old chart I have a 2" vent pipe can handle 4 DFUs, while a 4" vent can handle 8 DFUs

(What a DFU is–I have no idea. Apparently it’s some measurement of drainage venting)

Bathtub = 2 DFU each
Sink = 1 DFU ea
Shower = 2 DFU
Washer = 2 DFU
Diswasher = 2 DFU

So if that is a 2" vent for the house, you can determine it’s capabilties using the chart.

BTW, UPC (United Plumbing Code) says the plumbing vent should protrude through the attic 6".

I just notice the toilet is not listed on my chart, but it should require some consideration in sizing the vent, don’t you think? My guess is at least 2 DFU for the toilet.

Was there another vent, a 4" main vent? If so, this is probably a secondary vent.

Around here, typically, all vents need to be 4" through the roof.

plumbing (Small).jpg

In cold climates the vent goes to 4" where it exits the building to help prevent frost forming that could block the vent pipe. Also the vent should not extend more that 12" above the roof for the same reason.

I believe a water closet is rated at 3 fixture units.

DFU =Drainage Fixture Units???

Issues at hand…

Looks like the 2"? rizer from wall, tee/elbow runs horiz., then elbows to 2"-4" increaser (not reducer) through the roof. Looks like a cost cutting design to me. But check with a plumber to make sure.

Stains on roof decking (plywood or slatboard?). Active or old?

No visible insulation ?

Probably more, but thats a start.


Jae, would it be possible your old chart is so old that toilets as we know where not around yet.???

Marcel :slight_smile: :smiley: :stuck_out_tongue: :wink:

Correct, as oposed to WSFU - water supply fixture units used in sizing supply piping.

If I remember correctly, the 2" vent pipe is good for up to 24 fixture units.

Like someone said, increasing pipe size to 4" where it goes through the roof is to prevent frost from closing up the pipe

Thanks to all who’ve replied. There is one other 2" vent for a total of two penetrations. This is Hawaii. The stains are old. I was wondering about dia. requirements or some quick easy rule of thumb. I think Jae may be on to something.
The vent in the picture is directly above the bathroom plumbing wall. I was wondering if the main soil pipe in the crawlspace (to cess) is 4", shouldn’t the vent be the same regardless of wet or dry?

I thought a toilet was always a 4inch vent. Maybe it was a cheat for a final inspection when the house was built. From outside roof view its a 4inch vent all the way. Or is just because thats what the builder/plumber had handy when they ran out of 2 inch.

I usually see 4" sewer main stacks, but if the vent is only serving minimal fixtures, 4" may not be necessary.

I agree running 2" is a $ savings, but it is also a space saver.

Thanks guys- I think I’ve leaned something here (frost issue, DFU’s).

Guess no more
Look at the plumbing sample page