First of all, let my thank all of you pros out there for the daily challenge you face keeping up with the rules that code ninjas throw at you. I also thank you for taking your time to help out us DIYer's. Now, to my question:

What are the Pros and Cons of wet venting bathrooms?

From my perspective, it would seem that dry venting toilets is just a setup for a clogged vent in the future. I would think that always wet-venting a toilet would be a good idea. Dry venting bathroom fixtures other than toilets seems ok as less solid waste passes through their drains. Another aspect is that despite newer code trying to cover it with dedicated vents for every fixture, practically speaking nobody drains every single fixture at the same time (obviously more likely in a public restroom). Therefore, dedicated vents for every fixture just seem to lead to an excessive amount of DWV plumbing that is complicated to lay out properly.

On the same note, what has driven code to require the toilet (water closet) to be the most downstream fixture in a wet-vented system? Ref UPC 2009, 908.2.1. ... The water closet fixture drain or trap arm connection to the wet vent shall be downstream of any fixture drain or trap arm connections ...

Just a curious DIYer here trying to understand some of the "why" behind the codes that must be complied with.

A properly installed dry vent will not clog any quicker than a wet vent, key word being properly, now when you start running into horizontal dry vents below the flood level then yes problems arise. Wet venting saves a lot of time and lots of piping. I use the lavatory as a wet vent for the tub or shower about 90% of the time, only when conditions deem it unrealistic to wet vent will I dry vent a shower.

Thanks North Carolina Plumber.

Two scenarios come to mind based on your response.

1. If you have a dedicated vertical dry vent coming out the top of the horizontal portion of a toilet drain, and fairly close to the fixture (but greater than 2 pipe diameters away), it would seem that solid waste might collect around the edges of the intersection. What would keep that area clean since there's no regular drain through the vent? Purely a hypothetical from my perspective since I don't have the practical experience you have.

2. If you have a tub drain that has a portion of the vent running horizontal before it then goes vertical to at least 6" above flood level of the highest fixture before connecting to the main vent, I can't see any issues with the venting of the tub in a brand new system. I guess I can see solid waste potentially getting trapped in that horizontal run because water may not regularly be draining through the area. I totally get why the vent needs to tie into the primary vent above flood level, just not fully grasping the issue with horizontal runs below this. Again, probably because I lack practical experience dealing with these.

I see both of the above scenarios creating physical challenges with layout, but then that's why we need professional plumbers.



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one important point about wet venting.. if a dry vent clogs you will not know it until your house is getting filled with bad odors and sewer gas. if a wet vent clogs or even partially clogs you will know as the wet venting fixture will not drain or drain very slowly.
i'm not sure i understand your question about a toilet being downstream of a vent ??

Thanks packy.

I totally agree with/understand your logic on wet-venting. It seems like code changes are making it more and more difficult to do wet-venting and I just don't understand why.

Regarding the toilet downstream thing, it's quite possible I misunderstand the intent of the following code snippet:

UPC 2009, 908.2.1. (under Horizontal Wet Venting for Bathroom Groups)
... Each individual fixture drain or trap arm shall connect horizontally to the wet-vented horizontal branch drain or shall be provided with a dry vent. The trap to vent distance shall be in accordance with Table 10-I. Only the fixtures within the bathroom groups shall connect to the wet-vented horizontal branch drain. The water closet fixture drain or trap arm connection to the wet vent shall be downstream of any fixture drain or trap arm connections. ...

As I understand this code,

... the following wouldn't be legal:

- Toilet drain into horizontal branch (to use wet-vent), then wet-vent vanity sink into horizontal branch, then dry-vented tub into horizontal branch, then main stack drain

illustration: [drive.google.com]

- Wet-vent vanity sink into horizontal branch, then toilet drain into horizontal branch (using wet vent), then dry-vented tub into horizontal branch, then main stack drain

illustration: [drive.google.com]

... only the following would be legal:

- Wet-vent vanity sink into horizontal branch, then dry-vented tub into horizontal branch, then toilet drain into horizontal branch (using wet vent), then main stack drain

illustration: [drive.google.com]

Perhaps I'm just not understanding the code correctly.

NOTE: UPC 2006, 908.4.1. (same subject) doesn't have the downstream water closet restriction.



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I think #2 and #3 would be legal. Code states bathroom fixtures "shall be permitted" to be horizontally wet vented, not that they have to be wet vented(as in all or none). Code states other fixtures shall not enter the drain in the wet vented section and I feel code is referring to non-bathroom fixtures, which are not allowed to be horizontally wet vented. Drawing #2 shows dry vented tub, downstream of toilet. It shouldn't matter what type of fixture it is, it's dry vented and has to connect downstream of the wet vent(if it wasn't a bathroom fixture). #3 has a dry vented tub draining into the horizontal wet vented section. If the tub drain connects horizontally to the horizontal wet vented drain pipe, it doesn't need the dry vent and I feel that's just extra venting available.

Wet venting or "dry venting" ANY fixture does NOT mean the vent will plug up, or not, any easier. After all, even a "wet vent" has MUCH LONGER "dry" component. As for the toilet, iF it flows past an "unvented" connection, (which is meant by "any other" connection), the venturi effect can cause it to siphon the trap.

Thanks Steve, good explanation. #1 is definitely questionable, but still wonder if #1, since it's draining vertically without a horizontal arm, would be considered "upstream" of the toilet drain; at least by some inspectors/plumbers. If properly sized, it seems highly unlikely the sink trap could get siphoned by anything.

Thanks hj, totally get how unvented branches can result in the siphoning of the trap.



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Not a plumber just a DIYer.

My understanding of having a horizontal run in a dry vent below the highest flood level is to prevent potential clogging of that section due to a backup. Any pipe above the highest flood level will stay clear of any back up because water will just spill out of that highest fixture which in most cases is the sink bowl and flood the floor. So the section of pipe above that level would only be clogged by birds and critters coming from up top. If there is a drain backup the water (and waste) rise up the vertical dry venting pipe and that undesirable horizontal run, eventually the drain is cleared downstream by a plumber and water leaves that vent pipe...but solid waste that floated up that horizontal pipe section would stay behind, you won't know about it, and there is no way to clear it either since most people make these dry vent connections in such a way to avoid a snake accidentally navigate into one.

As to the toilet having to tie in at the most downstream point, I also think the illustrations you had are legal. I am wondering if that may be referring to a trap arm connection scenerio. Where your main stack is receiving flows from a lav sink, tub/shower and toilet via say three sanitary tee connections, and you have the vertical clearance and the fixtures are all within the maximum trap arm distances for their respective pipe sizes, to dictate which one is connected up top, in the middle and at the bottom? In that case I would tie the toilet in at the bottom because it is a major fixture. Just like I would tie a washing machine standpipe at a lower connection point than the adjacent utility sink.

IF the "dry vent" portion above the wet vent clogs, it creates the same situation as if there were no wet vent. A wet vent is NOT some "magical" way to keeps drains from clogging.

so hj, tell me the last time you ripped out a vertical "DRY" portion of a wet vent (galvy, cast, copper or plastic) that was directly above the san tee behind a sink and found it clogged. say the portion of vertical pipe 18 to 34 inches off the floor.
i have seen hundreds of galvy dry flat vents that were clogged solid but never seen what i mentioned above.

hj, something just occurred to me, wouldn't a "chicago loop" venting a kitchen island drain contain a horizontal portion of pipe which is always going to be below the flood elevation of the sink?

yes it does. this is why MA requires a cleanout in the vent pipe.
also the top of the vent must be brought up as high as possible to the countertop. so a sink with 4 -5 inches of water in will not be enough to spill into the vent.
how many times do people fill their kitchen sink to the brim??

Thanks hj. I agree wet-venting isn't a magical solution to keep a drain from clogging. I believe the philosophy, and most likely reality, is that if the drain experiences restriction or clogging the liquid and solid waste will back up into the vent system. When the restriction or clogging is cleared the vent system will evacuate. In a pure dry-vent, any residue left over really has very little chance of being washed away. But in a wet-vent, particularly one done off a vanity-sink, is likely to receive regular scrubbing with liquid waste ... with flooding unlikely to occur in the dry portion above the sink. Additionally, as packy has pointed out, you should see evidence of the dry-vent clogging with poor drainage or stoppage at the sink (or whatever fixture serves as the wet-vent) which can possibly go unnoticed in a purely dry-vented fixture due to pulls from nearby vents. Again, not a magical solution, just seems to offer some real advantages under some conditions.

By the way, can a toilet dry-vent exit at 45 degrees (above centerline) and be considered vertical or must it come directly off the top?

Sum, thanks for the inputs. Section 909.0 of the UPC covers "special case" venting of island fixtures.

Thanks to all for the discussion; it's very beneficial to know what you pros see from a practical experience perspective as most DIYers don't see enough plumbing examples to know the reality.

I honestly can't remember the last time i dry vented a toilet..
on a top floor installation (if possible) i like to use a side inlet sanitary tee so the toilet and tub or shower are stack vented and the lav drains into the stack.
to answer your question.. i don't know for a fact but i would think 'yes'..

So our former house in NY (built in 1972) was plumbed exactly like #1. Master bath toilet at the end of the run, tubs/sinks joining in as shown, hall bath toilet joining in, before the main turned to go downstairs and out. It seems to me that fixture traps are protected from siphoning...would this not pass code today? Why not?

number 1 is my preference. i would have tied the sink drain into the tub vent so...
the toilet is wet vented thru the tub and lav vent.
the tub is wet vented thru the lav..

The discussion above is saying that #1 doesn't meet code, but Packy you're saying it's OK?

Here's #1 again for reference:

Don411, I believe it's may a matter of code interpretation. Based on UPC 2009, 908.2.1., is the water closet (toilet) in #1 "downstream" of any trap-arm connection? It appears "upstream" in the illustration, but then again the lav/sink is on a different level than the water closet (toilet). Does the intent of the code include "downstream" of the trap arm that's part of the wet-vent? To me #1 would seem to meet the intent of the code but is a confusing situation based on the letter of the code. #3 seems to most closely resemble the letter of the code as the water clost (toilet) is the most downstream fixture.

As long as fixture to vent distances and vent size meet code, agree the tub vent is not necessary in any of the illustrations. If I understand the code correctly, the entire wet-vent portion up to the point of the dry-vent would have to be 3" or larger if it's the only vent since a 2" wet-vent is only good for 4 fixture units (3.0 toilet + 2.0 tub + 1.0 lav = 6 FUs).

Ok thanks that makes sense. I struggle to understand the code requirements when it comes to DWV setups, so when I see a discussion like this on the forum I like to jump in and try to follow the logic. I am getting ready to add a new master bath upstairs that will be back-to-back with the existing hall bath, once I have the walls opened up I'll be posting some pics here for guidance.

it is 100% OK..
why would the tub trap being vented the way it is be OK but a toilet vented the same way not be OK ?

Don411, I'm a DIYer going through the same thing right now ... remodelling an upstairs master bath that's back-to-back with a hall bath. The existing new construction plumbing has some very questionable vents in it, so I'm working to improve my knowledge both of code requirements and the practical issues that the pros see day to day.

Packy, to me it wouldn't seem to matter. To those who see #1 as a code violation, the toilet is "upstream" of the lav and tub drains, either of which could act as a wet-vent for the toilet; this, in their estimation violates the code in UPC 2009. Since the tub has a dedicated dry vent in all illustrations it really doesn't matter where you bring it in, but the toilet must rely on something for it's vent, which in the eyes of some must be "upstream" of the toilet. At least that's how I understand their point of view.

Anyone who see's #1 as a code violation care to express your reasoning why?

By the way, thank you very much for your inputs.



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How about NEVER, nor the dry vent portion of ANY system.

i have seen many galvy vents put in 50 years ago that were installed in what they thought was proper but they were full of rusty flaakes..