Jian
New Member
I am trying to add a shower drain to an existing tub drain. The existing tub drain is 1-1/2'' and has a dry vent. I am wondering if I can use a horizontal double wye tee with a slope to merge the new shower drain to the dry vent and tub drain. I know horizontal double sanitary tee is not allowed in this case since there will be issues on backflow. What about horizontal double wye tee? I have attached the a drawing below to illustrate my idea. The top graph A is the existing plumbing and bottom B is my proposed plan.
Plumbing plan:
Double wye tee:

Double sanitary tee:

Another proposal I find is to use a Sanitary Tee with a side inlet. Can I position the side inlet upwards for dry vent and merge the shower and tub drain. See the drawing below for this proposal and pic for the sanitary tee with side inlet.
Sanitary Tee with Side Inlet

Considering suggestions below from Wayne and Reach4, here is the third proposal of the full bathroom if the 1-1/2'' drain pipe size is enlarged to 2''.
Plumbing plan:
Double wye tee:

Double sanitary tee:

Another proposal I find is to use a Sanitary Tee with a side inlet. Can I position the side inlet upwards for dry vent and merge the shower and tub drain. See the drawing below for this proposal and pic for the sanitary tee with side inlet.
Sanitary Tee with Side Inlet

Considering suggestions below from Wayne and Reach4, here is the third proposal of the full bathroom if the 1-1/2'' drain pipe size is enlarged to 2''.
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