My general layman's explanation (so I'm sure to be corrected), but here goes: The fixture "upstream" (shower or tub, for example) is vented via the "wet vent" that is "downstream", so to speak. The wet vent serves as both a drain (for the lav waste coming down), and a vent (for the air being displaced upward into the dry vent of the lav). The horizontal distance limits to the wet vent connection are affected by the diameter of pipe you are using. Bigger diameter pipe = longer horizontal to reach the wet vent, according to the rules/tables.

One laymen's description only.

Lazypup,

Thank you for taking the time to post that detailed explanation. The diagram is very helpful.

Using your example the TDL for the tub fixture arm would be the pipe length + fitting allowance to the point where it meets the 3" vented branch? The TDL for the shwr fixture arm from the tub would be the length of the 2" fixture arm & fitting allowances + the length of the 3" branch & fitting allowances to the point where it meets the wet vent?

In the case of the Shwr can the distance from the vent be increased because a portion is 3" in diameter?

Before I proceed, I do need to correct a mistake in my previous post. If a 1-1/2" pipe is run with the code minimum 1/4" per foot the maximum length would be 6', but if it was run with a 1/2" per foot pitch the maximum would only be 3', not 4' as I stated above.

Now to answer your question about the length of the shower fixture arm, which is upstream of the "waste & vent" from the lav.

Technically speaking, if the lines are properly sized by the DFU method a line will never be more than 1/2 full under full load, which leaves the upper half of the pipe open for the free transfer of vent air, thus when connecting a fixture arm to a "Horizontal Vented Branch" the "Vent Opening" is the point were the "Fixture arm" attaches to the horizontal vented branch and that is defined as a "Wet Vent" because both liquids and vent air are present in the line, as opposed to a "Dry vent" which only transfers air.

Now your next question really falls in a rather gray area of the code that it generally defined by your local code, or code enforcement officer.

In most jurisdictions they allow that an uptream portion of the vented branch equal to the maximum length of a fixture arm of the same size is still vented. Thus if it is a 3" line they would permit us to consider up to 12' upstream of the actual vent opening (line from lav) as an extension of the vented branch. In that case you could attach the full length of a fixture arm to that portion of the vented branch, but, in some jurisdictions they state that only the portion of the Branch line that is downstream of the vent is vented. In that case, when connecting on the upstream side of the vent connection, the maximum length of the fixture arm would be measured from the trap weir to the vent opening in the manner that I illustrated above and they generally do not make any allowance for the increased pipe size from the vent opening to where the fixture arm is attached as shown above.

If you happen to be in one of the areas that does not permit defining a portion of the line on the upstream side as a vented branch, there is a simple solution. A shower is required to have a 2" drain line, but the code will allow us to increase the size of the line by one nominal trade size, so you could increase the line to the shower to 3" and it could run up to 12' from the vent opening to the trap weir.

Allow me to share some sage advice that an old timer gave me many years ago. Your inspector can really be friend or foe, and that will totally be determined by your attitude towards him/her. Whenever one of these gray areas of the code arise take the time to politely discuss it hypothetically with your inspector. That will have a twofold effect. 1st you are conveying to the inspector that you are trying to do the job correctly and 2nd it establishes the ground rule that you respect his/her judgements & authority. You will find that if you do that many times the inspector will simply point out a minor infraction and caution you not to repeat the mistake in future. On the other hand, if you give the inspector a bunch of attitude and argue code definitions you can be sure that he/she will go over your job with a fine tooth comb, and I can assure you, there is no plumbing job that they cannot find a reason to red tag it and make life misearable for you.

Speaking of making life miserable, I looked at a bathroom/laundry remodel job last week that has many red flags. The original contractor obviously did not know anything about drainwork. The job was partially completed, drains were moved, and all walls and most of the floor was tiled before the contractor skipped out. Now it's become a tough life lesson for the owner.

This previously functional basement bathroom/laundry now has, among other things, a toilet flange with a 3" tee visible underneath (for the upstream shower drain to share!). There is 3/4" of standing water in it and some drywall bits that won't even wash away.

Proceeding over to the shower drain stub, I don't think there's a trap since, using a flashlight, I can't see water in there after I pour some down. And the icing on the cake is that they neglected to install a shower pan before they tiled right down to the concrete floor. Peeking under the wall tile I can even see the wall plate.

And then over to the washer drain, which is now stuffed with rags around the drain hose because the owner says it now smells.

Moral of the story, glad you are taking the time to do your project right.

Without going into all the codes, I will give you another idea. Run the 3 or 4 inch pipe to your toilet, brach off with 2 inch to your tub, then brach off to your lavatory. Vent the bathroom from the lavatory.

The lavatory is in constant use and will help keep the pipes cleaner.

Some plumbers will call that crazy, but thats the way this anal 70 year old plumber taught me.