4" drain going to city sewer. Installed 4" WYE from drain going to new laundry room. Combo just before foundation for cleanout going vertical. 4x2 reducer and 2" from there. Installing floor drain, laundry tub and washer. Can I use 2" cross, sides going to washer and laundry tub, top to floor drain. Put 2" vent before cross, tie vent with vents for washer and laundry tub together above and 1 stub out? Yes trap the washer above floor and trap floor drain. HELP PLEASE!

I wouldn't use a cross underground. Can you draw up sketch of what you are planning? We should be able to point you in the right direction if you can. I would use wyes and 1/8 bends to make all connections underground. So, you would have a floor drain, then a wye rolled above center line (serving as a vent for the floor drain), then another wye going to the laundry sink, and another wye going to the washer. Then you can install you washer trap, and sink drain stub out, and tie all three vents together at least 4' from the floor. 1 1/2 vent will be fine for all three fixtures. You may be able to use the floor drain vent as the sink drain, if wet venting is allowed in your area, but you can't use the floor drain vent (or any vent for that matter) for the drain on the washer. Hopefully Mark will be along to draw this up for you.