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Question DetailsAsked on 5/8/2016

I'm putting a new shower and tub in can I connect the drain lines to the 2" line that the washer drains into?

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In almost all plumbing codes, no - for a couple of reasons:

1) shower requires 2" drain line, and when two lines run together the general rule is you upsize the combined flow pipe to the next size above the larger of the ones coming together - so 2" from tub going into a line with another source requires 3" for the collecting DWV pipe - whether the washer is using 1-1/2" drain pipe (old standard) or 2" (new standard to accomodate higher flow capacity washers, which sometimes overflow going into even legal 1-1/2" pipes).

2) assuming you are talking located on same story, doing that is inviting backflow from the washer to the tub - because during high discharge, particularly if aerated and sudsy during the wash cycle first pumpout (which greatly inhibits flow capacity in the pipe) the water in the washer discharge pipe will very commonly back up quite a bit - one of the main reasons the washer discharge hose come way up off the floor to the discharge point.


So - your washer discharge line and the tub discharge line should both discharge into the main stack - presumably 3" or 4" depending on your house.


Don't forget proper venting connection to the vent stack for the tub line too - and if there is a toilet "upflow" of the tub and the tub ties into the 3" or 4" line coming from the toilet and heading to the main stack, the tub needs a separate vent connection, not just an "end-of-line" vent upflow of the toilet, because the toilet flow going past can draw the water out of the tub trap and release sewer gases through it.


Most, but not all, code areas allow "end-of-line" venting if the only upstream items on the line the shower is going into are normal (not washer) basins or sinks with 1-1/2 or 2" drain lines, but best practice is to be sure each fixture draining into a DWV line has its own connection to the vent line - and the "mouth" or bottom end of all vent connecting lines has to be "washed" by flow going past - it generally cannot vent into a section of pipe that is "dry" or above fluid level.


This is a point where "end of line" venting comes into conflict in a lot of code areas, and why it is a bad practive in general. [End of line venting is where the drain line or main stack which the fixtures are connected into continues "upflow" and is connected to the vent pipe - which does serve the purpose of providing venting to the stack, but does not prevent flow in the main drain branches from pulling the fluid out of traps in fixtures it passes by as it flows. This can get particularly bad when a drain is brought in on a vertical run of the stack as opposed to on a relatively flat-lying run.


I would recommend a sketch of the drain lines be made and run by the building department, or a plumber at the place you are buying your pipe and fittings if not buying at a box store - because you do NOT want to have to go back in there to redo it after a failed inspection - either building inspection now or home inspector report at sale time.

Answered 2 years ago by LCD




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