Adding a sink to a laundry room

I'm in the process of adding a sink to the laundry room in my condo and would like a double-check on my plan.

Here's what the plumbing looks like now:


Here's my plan to add the supply/waste lines for the sink:


Basically I would cut out the existing sanitary tee on the 4" common waste line, and replace with a double sanitary tee like this:
http://www.pexsupply.com/Spears-P429-420-4-x-2-PVC-DWV-Double-Sanitary-Tee

I think I'll have to use a no-hub coupling as there is no wiggle room on the 4" line. Was thinking something like this:
http://www.homedepot.com/p/LDR-Indu...-x-FPT-No-Hub-Coupling-808-cNHC-400/202310691

Then I'll just make a mirror-image copy of the existing washer waste line on the left side, going up to a washer box. On the right I'll plumb the new sink waste line over to a 90 where it comes out of the wall, with a standard p trap under the sink.

For supply lines I'll cut them off at where they 90 up to the current washer box, add tees for the sink supplies, and continue the lines over to the washer box.

Does all of this look correct? Is there a better solution than the no-hub coupling for the 4" line? I've never used one, not sure how reliable they are.

Thanks for your help!

You may have a problem, unless you are on the top floor. If that 4" is a waste line for upper level fixtures you cannot use it the way you show, and it should not even have been used the way it was for the washer drain. This is one situation where a sanitary cross could be used, because they do not make the proper fitting in the size you need, and you cannot use a double "Y" or combination Y-1/8 bend. But, if you are not on the top floor and the line is a drain line, then both the washer and sink drains will have to be vented.

Thanks hj - there are 3 floors above me and that 4" stack is a common for all laundry rooms in upper/lower units. The existing plumbing setup is how it was done when the building was built in ~1979. I've never had an issue with backup on the washer drain, but I know that doesn't mean it's right. What's the correct way to vent both washer/sink waste lines?

They must have used the "Philadelphia single stack" method, which is cheap to install, but not approved for most areas. Venting them would be a problem, because you apparently do NOT have a vent stack parallel to the waste stack, and an "air admittance valve" would not protect against the positive pressures the upper floors would create.

However, if it's just four washers on the stack, and it's run in 4", that may "be" the venting.
Like hj mentions, not doable everywhere.

IPC SECTION 910
WASTE STACK VENT
910.1 Waste stack vent permitted. A waste stack shall be considered a vent for all of the fixtures discharging to the stack where installed in accordance with the requirements of this section.
910.2 Stack installation. The waste stack shall be vertical, and both horizontal and vertical offsets shall be prohibited between the lowest fixture drain connection and the highest fixture drain connection. Every fixture drain shall connect separately to the waste stack. The stack shall not receive the discharge of water closets or urinals.
910.3 Stack vent. A stack vent shall be provided for the waste stack. The size of the stack vent shall be not less than the size of the waste stack. Offsets shall be permitted in the stack vent, shall be located at least 6 inches (152 mm) above the flood level of the highest fixture and shall be in accordance with Section 905.2. The stack vent shall be permitted to connect with other stack vents and vent stacks in accordance with Section 903.5.
910.4 Waste stack size. The waste stack shall be sized based on the total discharge to the stack and the discharge within a branch interval in accordance with Table 910.4. The waste stack shall be the same size throughout its length.

I assume the 4" line is being used as the vent, there is no other plumbing in that wall. It's a 5 story building so I have 3 above and 1 below. Layouts of all units are identical so I assume the 4" line is only used for washer discharge (unless someone has added a sink like I plan to). If I install as shown in my design should I expect backups or flow-through from the washer side to the sink side? Also would a double wye be better than a double sanitary tee on the 4" line? Thanks for the help.