Can I tie a basement vent into drain pipe of upstairs sink?

[crazynick]

My stand pipe is galvanized, but the sink drain from upstairs sink is PVC. Can I tie the vent pipe into the existing drain (ie pipe in ceiling of basement) and make it a wet vent?
Is this legitimate, as long as the total horizontal remains under 5' for a 2" pipe?

Note: The pvc goes into the galvanized stand pipe, and that is the only vent pipe. Upstairs bathroom doesn't have separate vent pipes.

Basically, This would keep me from having to crack the galvanized pipe to add in another wye to fit the vent. There aren't walls upstairs that line up to put another vent pipe up.

[jadnashua]

If I understand it correctly, you'd need to connect to the drain above the flood plane of the highest fixture; sounds like that would be the upstairs bathroom.

[crazynick]

Connecting above the flood plane, do you mean above the trap of the sink?

All of the old pipes in this house are using the stand pipe as their vent, and the critical lengths don't match with todays regulations, but I can understand they did when built. So now I'm trying to figure out how to tie into the stand pipe without having to crack the galvanized pipe.
There is a 4-inch cross pipe that connects both upstairs toilets (1 1/2 bath) to the stand pipe. On one side of that cross is a 1 1/2" input that has a wye connection right away. One of those connections goes to the shower (6' run), and the other takes both sinks (5' is the longer of the 2 runs).

Can I break one of those open and put in another wye to tie into the pipe, or do I need to vent clear up separate?

[hj]

You have to tie the vent pipe into the drainage system at a point where the water would overflow the sink before it could enter the vent pipe.

[crazynick]

Just to make sure I understand for future reference.

If I have a 2 story house and wish to use a single stand pipe as a vent, all the separate vent lines must be connected above the overflow height of the highest connection. The connections I am making are downstream of the existing line, and thus need separate vent lines going upward. So I would need to tie the vent lines above the upstairs sink overflow height if they aren't using just the wet vent design.

As I see it then, I must run a separate vent up to the attic at least, in order to make the connection. At that point, it might be easiest to poke a separate hole through the attic. Bummer.

[jadnashua]

Sometimes the inspector will give you some leaway. But, if there is a clog part way down, things could back up and run down the vent and out the other fixtures. It could get nasty quick, so it is best to do things to current code. If you never have a clog, you may never have a problem.