Different types of venting?

My 1949 house seems to have two different venting setups.. is this common or even possible?

The cast iron stack has a vent pipe through the roof. However, there's a couple galvanized pipes that go directly down into my basement floor with a couple things draining through those (laundry sink, sink & dishwasher upstairs). The galvanized doesn't seem to connect to the cast iron at all.

In the attic, i can see the cast iron pipe that goes straight up through the roof. But I can also see the galvanized pipe come up, go across, then go back down.. almost like it loops.

And sure enough, the dishwasher gurgles and the laundry sink drains VERY slow.. both of which are connected to the galvanized pipes... there's absolutely no drainage problems with anything that ends up in the cast iron.

Why wouldn't everything just be cast iron and why would there be more than one main stack?

There were almost no dishwashers in the 1940's and washing machines all drained to the sink. Plumbing methods have changed over the years & there have been changes made to the home's plumbing, right or wrong.

Depending on the layout, it may be possible to connect the unvented plumbing to the C.I. vent stack.

Cast iron was too unwieldy to use for most smaller drain lines, but it did happen. You are assuming that there are missing vents, but without any experience in what a vent looks like you could be completely wrong, and probably are. THe plumbing in the basement would NOT be connected to the cast iron there. If it were it would be vented improperly.

Keep in mind that once a pipe becomes a drain, it cannot be used for a vent on anything below it. You can keep the upper part as a vent and vent other things into it, but those connections must be above the flood rim of the highest drain. A common number is 42" above the floor, or 6" above the flood plane of the fixture. galvanized pipes could be nearly or totally obstructed with rust and may be part of the problem.

Well I guess I'm just trying to figure out a way to fix the slow drain. If I can't do it myself, that's fine, but I don't know how much it's worth is all. I'm all about the DIY but this is beyond me.

Doing a large wash brings the water up to just a couple inches from the rim of the laundry sink .. scary.