Any advice on adding p-trap, for Standing tub, to existing shower drain?

[Mranderson1984]

Any advice on adding p-trap, for Standing tub, to existing shower drain? My concern is that the downward ~45 degree angle(connecting to the main drain) will create a siphon, and pull the water for the p-trap.

 

[wwhitney]

Yes, you need to vent the tub trap arm before it falls more than one pipe inside diameter.

If the wall above is open and you have access to the shower vent, you could vent the tub the same way the shower is vented: point the tub trap to get underneath the wall, use a san-tee on its back for a vertical dry vent takeoff, and then turn down to hit a wye in the stack. In the wall above, at a height at least 6" above the tub flood rim, the vent can turn horizontal and join the shower vent.

If you don't mind having the trap take up space in your basement area, one possibility would be to drop the trap so that your trap arm can go straight under the joists and can connect to a san-tee in the stack just below the quarter bend where the shower drain turns downward. There may be some controversy as to whether that is an allowable wet vent.

Cheers, Wayne

 

[Sponsor]

 

[Jeff H Young]

I dont know where the upstairs wall is but you might be able to wye off right where youve drawn put a street 45 creating an upright wye. continue up add a 2x2x1 1/2 santee vent straight up with 1 1/2 and 2 inch goes to shower trap.
yea gotta have that vent.