Wet vent with 4" stack

This question is mostly about _why_ plumbing code requires certain things.

I am working on the 3rd floor of a 4 floor, 200 year old building. I recently had a plumber in to repair a leak in the 4" cast iron at the third floor. In the process he completely re-did the sink and washing machine hook up on the 3rd floor. The two fixtures tie into the 4" drain using sweep T fittings, the sink above the washer. This puts the two about 8" apart. On the fourth floor there is only a kitchen.

The washer hasn't been connected yet, and my question is if it needs its own vent. There is a vent pipe that the plumber used for the sink, and I could branch off that and also vent the washer.

Reading the code (Massachusetts) it seems that wet venting is explicitly prohibited if the fixtures involved are a sink or clothes washer! Go figure! But I can't understand why it would really help anything in this case. With a 4" main drain fed by only 2 1.5" or 2" drains, that 4 drain could never have enough water in it to siphon out the washer trap. Is there some other potential problem I am not thinking of? Or are they trying to prevent someone using a 1.5" diameter pipe as drain & wet-vent?

Another assertion in the code is that a washer trap shall not below floor level. Why is this?

You can't plumb without a license in Mass. Especially in Boston.

There is a vent pipe that the plumber used for the sink, and I could branch off that and also vent the washer.

Click to expand...

You can use the same vent for the sink and washer, re-vented at 42" or higher off the floor.
For years we used to wet vent a laundry sink over a washer as long as the sink was run with 2" on the vertical. It's been years since I've been allowed to do that though.