I am planning to install a bathroom in a small dormer room on the third floor of my old house. The room sits between two plumbing stacks. The original one with cast iron pipe and a newer one with ABS pipe. I wish to locate the toilet and shower stall on the wall with the cast iron stack. I cannot use the waste line because it is essentially inaccessible, as well as being 4 inches of cast iron. The vent on this stack has an existing horizontal PVC splice above the flood level of the toilet that can be easily accessed. Ideally, I would like to use the waste pipe on the ABS stack and the vent on the cast iron stack.

Under this plan, the toilet would empty into a 3 inch PVC pipe that would fall a quarter inch per foot for about six feet and then turn down through the ceiling of a closet where it would enter the ABS stack. I plan to splice a length of PVC containing the fitting into the ABS stack using shielded hubless connectors, the joining of such pipes by cement being frowned upon in Massachusetts. In order to vent this line, I would extend it about 18 inches back beyond where the toilet discharge enters. At this point I could install a vertical line to the vent for the cast iron stack.

I would like to use a similar venting arrangement for the shower drain, with this line entering the PVC line from the toilet before it joins the ABS stack.

On the lavatory side (opposite the toilet), I cannot access the vent on the ABS stack. The house has a slate roof, which makes installing any more venting through the roof undesirable. I would like to use what I think is a “wet vent” arrangement. That is, use an oversized (2 inch) pipe to drop down into the PVC line before it joins the ABS stack.

Does this arrangement meet code?

Thanks for your help.

Dennis

Hi Dennis,

You have the job laid out in your mind. Unfortunately I don't. Telling me you're going to use a "fitting" tells me nothing. This 18" vent behind the toilet would pick up the vent how? A heel inlet closet bend? If you're going to vent the shower,which again you didn't explain, then you shouldn't need a vent on the toilet if you tie in the shower drain downstream from the toilet. That arrangement will vent the toilet. If your thinking that tying the lavatory into the same PVC line that the toilet discharges into constitutes a "wet vent" your mistaken. The lavatory must be vented through the roof or revented back into a existing vent. However if you tie the vented lavatory into the PVC toilet drain and then tie the shower into the lavatory drain you will have wet vented the shower and the entire bathroom group and kept the job in code. Also 2" pipe is not oversized. It's the normal drainage size we use. If I'm missing something here, get back to me with more details. Regards, Tom

First question:

The waste branch line I'd like to add runs like this. It starts with a 3-inch 90 degree elbow. A 2-inch line would runn up to above the flood line of the toilet, where it would join a 2-inch vent in a tee. (I’d use a reducing coupling to change the size of the new pipe from 3-inches at the elbow to 2-inches.)

A 3-inch pipe also runs from the elbow at a slope of one-quarter inch per foot. In about 18 inches from the elbow, the toilet waste enters in a wye. The line continues for about 6 more feet where it enters a 3-inch vented stack through a wye. Is this arrangement up to code?

Second question:

The lavatory will be in a location where it is not practical to directly connect to a vent going through the roof. Can I use an oversized (3-inch) pipe behind the wall that would run about 4 feet to the vented stack?

If not, can I use an air admittance valve to vent the lavatory?

Thanks again.

Dennis

Hey Dennis, I'm still confused.
The waste branch line I'd like to add runs like this. It starts with a 3-inch 90 degree elbow. A 2-inch line would runn up to above the flood line of the toilet, where it would join a 2-inch vent in a tee. (I’d use a reducing coupling to change the size of the new pipe from 3-inches at the elbow to 2-inches.)

It starts from where? What is the 3" line used for? Drainage? some sort of vent? 2 inch line running from where to the vent. Give me fittings, connections and locations.

The lavatory will be in a location where it is not practical to directly connect to a vent going through the roof. Can I use an oversized (3-inch) pipe behind the wall that would run about 4 feet to the vented stack?

What's with all these 3" pipes? Why don't you just vent the lavatory with 2" and revent back into the vent stack?

What I need from you, my friend, is a clearer picture of what you're planing. How about making me a drawing complete with dimensions and send it to
[email protected] Head it up PLUMBING so I'll recognize it.
Until then** Tom