905.3 vent exceptions

I have a tub and sink that are plumbed using S traps, on a 1.5 inch line that is 6 feet to the stack.
I would like to change these S traps out for P traps and replace part of that drain line this weekend or next. I can not run a vent in the typical routing as the tub drain comes back under the tub and 1.5 inches of cement and tile and is butted up to a 6"x12" supporting beam on the front side.

If I extend the horizontal line another 4 feet and then go vertically about 16-18 feet and tie in to a 2" vent that goes to the roof, will I gain enough to make the effort worth while? Would this be consider a valid except under 905.3?





905.3 Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than six (6) inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and whenever two (2) or more vent pipes converge, each such vent pipe shall rise to a point not less than six (6) inches (152 mm) in height above the flood-level rim of the plumbing fixture it serves before being connected to any other vent. Vents less than six (6) inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain.

quote; If I extend the horizontal line another 4 feet and then go vertically about 16-18 feet and tie in to a 2" vent that goes to the roof, will I gain enough to make the effort worth while?

No way to tell, because your description, while it makes sense to you, does not really tell us HOW you intend to run the pipe. WHAT 'horizontal line"?

The tub drains out via an S Trap to a horizontal drain that connects to the stack.
There are no vents on this line, other than the stack itself. Most of the drain lines are 68 years old and original install.
The tub is the furthest upstream item, followed by a sink and lastly, the WC a few inches before the stack.
I understand a vent should rise from a San Tee, but that can not be done in this case.
Hardwood beams box it in on the three sides and the cast iron tub drains back under neath itself and 1.5 inches of cement and tiles.

If I cut out the old drain and remove the s trap it,
install a p trap, that connects to the horizontal line that connects to the stack with a wye and a 45 street to the P tray,
but extend that line upstream to another wye, add a clean up and head vertically to the existing vent for the kitchen sinks and out through the roof.

I can't see another way to remedy the S traps/venting until the bathroom gets a full tear down and reno.

question. If the horizontal pipe you are connecting to is the same one the tub is now connected to, how is there room for an "S" trap? Are you misinterpreting what an "S" trap is?

The sink S trap is newer ABS and then attaches to the old steel line with a Fernco just below the floor, the former owner painted the Fernco white.

The Tub is original steel to ABS and then back to original steel with Ferncos, the rubber on both Ferncos on this drain have also been painted white.

The plan is to cut the entire bath drain out, remove the Ferncos, and all older ABS, run new drain, use one shielded mission clamp at the steel to ABS or PVC joint, then further the upstream section of the line over one more joist length (which would be to the right of the bath drain) to allow a full clean out and go straight up to tie in to an existing 2 inch vent.

While there, I will remove the 40 amp electric wire for the stove that runs under the bath and replace it and re-route it away from the bath tub base.

That assemblage of ABS fittings, the horizontal section running to the wye. It's flat and it can't be.

Tom Sawyer said:

.... It's flat and it can't be.

Click to expand...

That is why I asked about exceptions to 905.3, what is the best Plan B approach?

905.3 Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than six (6) inches

I can't cut in to a 6x12 hardwood main supporting beam to install a picture perfect vent installation to go vertically because of the original plumbing installation, how best to remove the S trap and add some degree of venting is the question.

There has to be a better setup that what the last guy installed on top of a less than great original install.

If I understand what your are trying to do with the tub, then lower the trap to the correct level and take off the vent vertically "from" the trap arm.
If needed, the trap arm can run "the wrong way" until it meets the vent, then it becomes a drain and can turn around and go any distance to the stack. Getting the proper slope for the entire run may mean tying in lower on the stack...

bluebinky said:

If I understand what your are trying to do with the tub, then lower the trap to the correct level and take off the vent vertically "from" the trap arm.
If needed, the trap arm can run "the wrong way" until it meets the vent, then it becomes a drain and can turn around and go any distance to the stack. Getting the proper slope for the entire run may mean tying in lower on the stack...

Click to expand...

Thank you so much! Simple enough but I never thought about going backwards to go forward!


I made a replica this morning to rough fit it, it gives me a 14 inch trap arm to hit the next joist, a straight up vertical vent, a clean out and up to 1 1/4 inch play room for slope over the distance of 14 inches out and back.

Thank you sir!

I have a question on what they mean by "vertically." Does the vent have to go vertically straight up(90 degrees), or could the vent pipe go up on an angle until it gets to the wall and then go straight up from there. If it can go on an angle, is there a minimum angle it could be?

905.3 Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than six (6) inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and whenever two (2) or more vent pipes converge, each such vent pipe shall rise to a point not less than six (6) inches (152 mm) in height above the flood-level rim of the plumbing fixture it serves before being connected to any other vent. Vents less than six (6) inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain.

I think the intent is straight up until 6 inches higher than the flood line.

r_ventura_23 said:

I have a question on what they mean by "vertically." Does the vent have to go vertically straight up(90 degrees), or could the vent pipe go up on an angle until it gets to the wall and then go straight up from there. If it can go on an angle, is there a minimum angle it could be?

905.3 Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than six (6) inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and whenever two (2) or more vent pipes converge, each such vent pipe shall rise to a point not less than six (6) inches (152 mm) in height above the flood-level rim of the plumbing fixture it serves before being connected to any other vent. Vents less than six (6) inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain.

Click to expand...

Going up at a 45 or steeper is considered vertical.

I think I may have fixed one problem only to move another one further down the line.

I installed a new Kohler pedestal sink, new drain line and new water lines.
A few weeks later I pulled the the S trap and drain line from the bath tub and installed all new drain line joining up with the new sink line, which ties in to the stack via a 4x4x1.5 wye and 3 foot section of original 68 year old steel 1.5 inch pipe. This section of pipe has some build up on the wall of the pipe that a snake could not remove, so it has an actual size of maybe 1.25 inches. I know this is next up for replacement.



The trap arm on the bath is 16 inch, the vent is 1.5 inch that runs vertical for about 20 feet and ties in to a 2 inch vent that goes through the roof.

The bath trap no longer (appears to) suck itself dry, nor does the sink.

However, it seems like the volume of water rushing from the bath drain now hits the old steel section and tries to back up into the sink drain (you can see the wye of the sink drain entry just next to the mission clamp).

When you listen to the audio, does this make sense or have I come to the wrong conclusion?
There is an AAV on the sink drain that kicks in as the water level falls to the drain line level as the bath tub drains.
The AAV is there only until I gut the bathroom and can install a proper vent during a full bathroom reno.



Audio of sink is here: