Adding a wye to a horizontal drain?

[Tom Boissy]

I essentially need to add a wye into an existing 3 inch drain to account for a new bathtub service. the horizontal line i want to tie into runs under the second floor joists and is enclosed in a soffit.

The 1.5 inch tub drain line is about 5 feet from the drain i need to tie into. The tub drain will run inside the floor joist space and feed the horizontal drain from above with a 1.5 inch sweep elbow that ties in to the wye that I intend to install into the existing drain.

i will insert a 1.5 vent line half way between the tub drain and the existing 3 inch drain line. The vent ties into the home's existing vent system behind a knee wall and the horizontal run will be 6 inches higher than the tub's over flow. both the new tub drain and vent will have .25 inches of pitch per foot for drainage/ condensate.

My question is......there is no play in the 3 inch horizontal drain line, when I cut it to add a wye with a 1.5 inch tie in, I will not be able to move either side in order to use glue in couplings.

Whats the best way to add this 3 inch wye fitting with a new 1.5 inch service? two 3 inch rubber flex couplings, one on either side of the wye assembly and almost no play or gaps between the PVC pipes? I will have to glue in some 3 inch pieces into the wye so that I can ensure a tight fit and give the rubber couplings something to connect to.

How much play or gap are you allowed between pvc pipes when using rubber flex couplings in a horizontal condition? 1/8" or 1/4" or greater such as 3/4" or 1"?

I've never had to work on pvc pipes where i couldn't create some movement to add glue in couplings/fittings. sorry if this is a stupid question.

Anything else I am missing?

 

[hj]

1. You cannot use "flex couplings" for that purpose. You use "Pro-Flex" or similar banded couplings designed for plastic piping. You need at lease 1/8" on either end to accomodate the "ridge" 'inside the coupling.
2. Technically, the horizontal vent should be 6" above the overflow RIM of the tub, not the tub's overflow, and even then the second requirement is that it be 42" above the floor if the "overflow' level of the tub is less than 36" above the floor.

 

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[Tom Boissy]

thanks HJ. I was not aware of those couplings. I take it those are specifically for horizontal pvc drain lines? you are recommending two of them, one on either side of the spliced in wye, right?

I was going to use a fernco flec coupling 3 x 3 which home depot carries. it does not have the ridge in the middle. t take it those are for vertical pipes?

I will find the couplers you suggested.

btw, the horizontal leg of the tub vent is actually 43 inches above the floor, i just didn' go into detail. the tub is 22 inches from floor so I do need to be at least 42 inches.

thanks for the very detailed response.