Half-bath DWV Rework

The current DWV layout in my tiny upstairs half-bath is a mess. There are leaking fittings, horizontal vents, and drain pipes sloped in the opposite direction! The "stack" for this bathroom comes up from the basement through a closet in the first floor. I plan to cut out the existing cast-iron stack and replace it with the one shown in the diagram below. I've already removed the existing toilet and sink, so re-installing and moving fixtures is not a problem. Would the layout in the following diagram be correct? The toilet drain passes behind the horizontal drain for the sink in this vertical view. The toilet and sink are very near one another.

I do have a problem with the venting, however. The "stack" is not contained in a wall and lies directly below a window in the bathroom above. Can I attach an air admittance valve at the vent location in the figure, or can I attach a single vent at the rear of the sink p-trap? How else might I get the vent to exit the building? Any advice would be greatly appreciated.

half-bath dwv

First of all does your local plumbing code authority recognize the use of AAV?IPC allows AAVees,while UPC does not,anyway whether the sink+toilet are side by side or facing each other it does not matter.Establish where your cloest bend will be,it can be 3",run it horizontialy and the install a 3x1.5" wye before you connect the 3" to the 4" stack use a 4x3 no-hub or mission coupling to make that connection.Now out of the 1.5" outlet of the wye run it horizontially and the up thru the floor and the stud wall that the sink will be located,if your allowed to use an AAV you would turn a 1.5"90 into your vanity or cabinet lay a 1.5"tee on its back,the riser would be where the AAV is installed,the horiz.would be where the p-trap of the sink would be,Now if you can not use an AAV you would run the 1.5" pipe for the lav install a 1.5" san.tee about 18" off your finish floor run hoizontial length of pipe to be the waste arm for the trap of the sink,keep it less than 5' vent to trap distance.The vertical out-let of the 1.5" san. tee becomes your vent pipe,run it vertically above the flood level of the sink then you can use 90's+45's for change of direction and tie into your existing venting system or run it thru the roof but you must increase to 3" 12"above + below the roof line.What I have just described is wet venting the half-bath.

Ahhhhh yes

Thanks tbplumbloco!

I understand now. This is a branch drain line and the sink is the furthest "downstream". I've attached a drawing with my interpretation of your message. The state plumbing code (NC) allows AAV's, but I'm not sure about the locals. Is the flood level of the sink simply the top of the rim? Also, the "code" also states I don't need to make the vent 3" wide through the roof (or wall) if the 97.5% temperature for outside design is above 0F. How do I find out if that's the case here? The current vents on the house are not 3" in diameter. Of course, a previous owner also installed the kitchen with no vents at all as well as the monstrosity I'm trying to replace.

I've attached a sample of this handiwork. From the cast-iron stack, some sort of black tubing is attached using pipe clamps and barbed fittings. Every one of these connections leaks. The vent goes horizontally out the back wall of the house and the sink drain proceeds up to the bottom of the floor above. There, it DROPS about three inches before going upwards into the bathroom.

Thanks

half-bath dwv

Yes your diagram is correct,I can only speak of my locale and the 1.5" vent would have to be increased to 3" thru the roof as described ealier.I can also see by your second photo the need for re-work your typical radiator hose + clamp job.

How about this?

Thanks for the help tbplumbloco.

As you might be able to see from the photograph I posted earlier, I have a problem fitting a wye into the horizontal run before the toilet. There's not enough room! Can I offset the vertical drain pipe by using two 1/8 bends and have it lead vertically to a wye. The side of the wye is then connected to a 1 1/2" pipe for the sink. This connection comes out of the page in the attached drawing before bending to the left. The top of the wye is connected to the closet flange. Would this work? Also, where should I use long sweep 90's instead of plain ol' 90's in the 3" pipe?

Thanks again,

Al

If you are going to wet vent the lav over the toilet, I would run the vent out 2" all the way and use a 2x1.5" tee for the lav.


Where the waste goes from vertical to horizontal, it should be a long turn 90

Thanks

Thanks Terry and tbplumbloco,

I think I have the layout down now. I'll use 2" for the horizontal run and on up for the vent. Unfortunately, the closest snap cutter for rent is about 50 miles away, but its still cheaper than buying. I'll post my progress in a few weeks.

Al