I want to install a 1/2 bath on the main floor of our house. I have a 3" drain coming out of the basement floor that serves the washer and will serve the 1/2 bath. My question is venting the 1/2 bath toilet, lavatory, and washer located in the basement. Can a 3" elbow with a 2" side-inlet to drain the sink vent the toilet, lavatory, and washer? The 2" inlet piping would be extended into the attic.

Thanks

Everything sounds fine until that last sentence "into the attic". You don't want to terminate in the attic, go through the roof line. Also, are you saying the basement bathroom is currently unvented?

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My question is venting the 1/2 bath toilet, lavatory, and washer located in the basement. Can a 3" elbow with a 2" side-inlet to drain the sink vent the toilet, lavatory, and washer? The 2" inlet piping would be extended into the attic.

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Let me see if I have this correct. You are going to cut into the main with a wye or combo and pick up the toilet with a 3" closet bend,( not a ell) with a 2" side inlet that will pick up a lavatory and a washer station. Am I correct so far?
I think you have the drains down OK but you must run separate vents for the lavatory and the washer. The washer trap and stand pipe will come off the drain with a sanitary tee and the washer vent will take off from top of the tee.
The lavatory will come off the drain line the same way and pick up the bowl stubbed out from another sanitary tee. This vent will also come out of the top of the tee. You may tie the two vents together with a inverted san tee and run up to the attic where you will revent back into a existing dry roof vent using another inverted sanitary tee. You will slope the vent back to the basement using 1/4 inch to the foot. Good luck, Tom

After reading my question I realized it gives the reader bad information. Let me start again. First off the home was built in 1912. While replacing the basement's concrete floor I tapped into the main drain with a 3" line which sticks about 2' out of the basement floor; which serves the washer also in the basement. An air admittance valve sits on top of the 3" pipe. I want to extend the 3" pipe up to the main floor to serve a new toilet and small sink. I've read numerous books at the library on how to drain and vent but because of conflicting info I still don't know how to properly drain and vent the entire system. Can the bathroom sink (located on the main floor) drain into a 2" side inlet and continue the piping into the attic to wet vent the toilet, sink and washer? As mentioned the 2" would continue into the attic and terminate with an air admittance valve. I can not connect the 2" into the main stack due to location. Our local codes do permit an air admittance valve as long as the home has a main stack. Does this make better sense. If not, I can attach a schematic.

Thanks for your patience and earlier response.

As explained earlier the answer's yes you can. The only difference in my answer is the venting and if a AAV is acceptable in your area thenby all means terminate the vents with a AAV after the lav and washer vents are tied together.
Let me repeat my instructions. I think you have the drains down OK but you must run separate vents for the lavatory and the washer. The washer trap and stand pipe will come off the drain with a sanitary tee and the washer vent will take off from top of the tee.
The lavatory will come off the drain line the same way and pick up the bowl stubbed out from another sanitary tee. This vent will also come out of the top of the tee. You may tie the two vents together with a inverted san tee and run up to a AAV. Good luck, Tom