I am installing new clothes washer drain in a 8" stud wall. There is a 4" vertical cast iron vent stack about 1 foot away. What is the best method to attach the 1.5" drain to the 4" vent stack?
The best method is to not do it. Whether or not a "wet vent" is legal in this situation depends on local code interpretations and several details about your specific situation.
Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed. --Herman Melville
If there are any fixtures connected to the 4in below without idividual vents you can't connect a drain to it, what you have is a STACK VENT not a vent stack, you have to find a different location for the drain and then you can run a vent into the stack vent.
Keep in mind that a wash machine hook-up has to be 2in.
"If all else fails, read the directions"
installation is necessary to determine if a connection to this vertical pipe is legal or advised.
Is this a continuos pipe with no connections beween the main drain pipe and the vent through the roof? Is this on the second floor or the first floor?
Wet vents are acceptable in many instances, but not all. It depends on what else is going on. IF you were able to connect the washer drain to this pipe, 1) it should be 2", 2) it should include a separate 2" vent and 3) cutting and adapting to a 4" cast iron pipe would require properly supporting it top and bottom, cutting out a section and adapting in either a 4 x 2 soil tee in PVC with rubber boots or doing so in cast iron. Not an easy task for the uninitiated.
Legal or not, the most practical solution is to look elsewhere for a place to drain the washer OR hire a competent professional and let them worry about it.
"Is this a continuos pipe with no connections beween the main drain pipe and the vent through the roof? Is this on the second floor or the first floor?"
The stack vent goes from the slab to the roof in a one story house. There was a toilet connected to it, but the toilet we removed and a washing machine will take its place. The toilet connect is plugged. There are no other connections to the stack vent.