Cutting out a wye under my slab

Hi there - was going to make my first cuts into drains actually being used and I figured I would write out my plan here, ensuring a) I have a plan, and b) if I'm about to do something dumb, some nice person here might save me from myself. Sorry if this is a bit long-winded.

House is 1959 and all the drainage below slab is cast iron. A previous owner added a bathroom in the basement, but the drainage wasn't done correctly, so the shower could overflow and the vanity sink also wouldn't drain well. The bathroom had a poor layout and no one would even use it, so I decided to make redoing the bathroom a covid DIY project, and demolished it. I discovered the vanity sink drain in the wall had a 1 1/2" trap arm that went over about 3', then down 1', then went into a CI sanitary tee on a 1 1/4" drain used by the sink in the bathroom on the floor above.

We eventually decided that the only way to fix the layout we'd like would involve moving drainage under the slab, and we decided if we're gonna do that, we'd actually prefer to have the bathroom 15' away in a different part of the basement.

I made a long hole in the slab where the old shower trap was, and discovered an assembly I'd seen on several "top 10 mistakes DIY plumbers make" lists - the shower trap arm ended with a 45 and a wye instead of a sanitary tee, and also was sloped toward the p-trap instead of away from it. This ABS drain continued down and turned to horizontal and eventually connected straight into a 2" CI pipe with a fernco. Since it's not used, I cut out the whole thing near the main house drain, and put on temporary caps there and on the vent pipe.

To get my 3" pipe to the new bathroom started, I plan to hook it into the main house drain where the old 2" shower drain did - it ran into a CI 4" double wye with 2" pipes coming out of both sides. The 2" pipe on the other side of the wye used to be the drain for the bathroom sink, the upstairs bathroom sink, _and_ the kitchen sink, but in previous drainage adventures I redirected both the kitchen and upstairs bathroom sink so they drained elsewhere, so today, this double wye drains everything in the house through it's straight part, and nothing drains into either side.

My plan, hopefully for tomorrow, is to cut out this 4" CI double wye, and replace it with a 4" ABS double wye, but one with larger 3" pipes coming out of the sides. One side would serve the new bathroom 15' away, and the other side will eventually become a new toilet stack for the upstairs bathroom once we get to renovating it. For tomorrow though, I just want to do the minimum so when the family gets home from their outing, everything works like it did before.

Here's what I've prepared:

• 4" ABS double wye with 3" on both sides. I've added and glued two short 3" pipes on each side, and capped them with 3" fernco caps I've tightened to 60 inch-pounds with torque wrench.
• Few feet of 4" ABS pipe to measure and cut once the CI wye is cut out
• New 9" reciprocating saw blade for cutting the 4" cast iron pipe out, and the saw
• Two 4" Mission Rubber Adjustable Repair Couplings (see picture) to connect the ABS and CI
• Torque wrench so I can tighten the couplings to 50 inch-pounds
• Sanding sponge for the ABS
• Iron file for sanding and smoothing cast iron
• ABS cement

The plan:

1. Cut the CI wye out by cutting the pipes to it, making a gap large enough to fit the new wye and the two couplings. This should be the length of the new wye plus 4" and change. The new wye is about 2" shorter then the CI one.
2. Sand and smooth and round off the outside edge of the cuts so they don't damage the couplings.
3. Clean the outside of the two 4" CI pipes I'll be connecting up again so they both have 4" of clean pipe to slide the couplings all the way onto.
4. Measure and cut two pieces of 4" ABS pipe, carefully deburring and rounding one end of each so they don't damage the couplings.
5. Dry fit the cut pipes into the new wye, and dry fit that in place to make sure it all fits.
6. Put all the ABS pieces together with cement.
7. Put the assembly in the gap, and slide the couplings into place, and gradually tighten them with torque wrench until it pops on each clamp.
8. Do some tests to make sure it's not leaking.

Previous owner's old shower drain that would overflow before I cut it out:

The cast iron double wye, using the cap I was using above to block sewer gasses:

The new ABS wye:

Some Canadian Fernco knock-off? It's 4 1/8" long and has 4 fasteners and no inside ridge in the middle.


Any thoughts/advice/warnings? Did I need to buy those extra beefy couplings? What kinds of things could go wrong that I could be better prepared for? Is it ok to have a 15' trench in my slab, or should I pour part of it so I'm dealing with two smaller holes? House is 40' by 25', and the wye is about a foot from the retaining wall-side of the walkout basement. Any tips on making recipricating saw cuts straight? Thanks so much!

 

First: Do NOT use a double wye in the horizontal plane. They are only for vertical runs.
Use, instead, two wyes. Use two 4 x 3 or whatever appropriate, NO HUB wyes. That way, each can have 1/4" per foot (1/4 heavy bubble on a torpedo level) and easily be adjusted.
You did overkill on those nice couplings but they will help, especially because the O.D. of old C.I. is different than No Hub.
All you need are plain old no hub couplings for the connection between the two new 4 x 3 wyes. You also will need: No Hub torque wrench (sounds like you have one). Check specs, but the heavy duty couplings you have may take a different inch/pound torque vs the regular no hub couplings.
Don't mess with standard sawzall blade. They do make a special sawzall blade for cutting C.I.
Best to start with a skill saw with a carborundum blade as much as will cut first. Sawzall cutting c.i. is a pain.
A plumber would use a snapper or a mini grinder with cutoff blade.
When replacing concrete, you must rotohammer drill out and then pin rebar on each side of the slab cuts you have made. This will insure that your inadequate uncompacted backfill doesn't lead to the new concrete sinking on you over time.
Best to backfill with drain rock with overlay of heavy duty visqueen.

the old shower drain: should have gone into a tee, not a wye. Wyes are not allowed for trap arms. Tees go there.

 

So - one thing I just noticed is that the main 4" CI pipe is actually a little steeper than I expected. Across the 14" span I'm planning to cut, it drops 5/8" or maybe even 11/16", meaning the drop is slightly more than double the standard 1/4" per foot. I think that means that if the double wye is level side to side, the sides would still incline over 3/8" per foot. Since I can't change the slope of the cast iron pipes without basically digging them out anyway, does that mean I can probably get away with using the double wye? From what I can see in other threads, the rational is that it's hard to maintain slope in a horizontal double wye. Seems like I actually have a lot of control here since I can even adjust them at my leisure since its the couplings holding them in and not ABS glue.

 

Breplum can correct me if I'm mistaken, but as far as I can tell, the sole issue with a horizontal double wye is ensuring that both branch inlets have adequate slope.

If the straight path of the double wye is at 1/4" per foot slope, and the fitting is dead level on the perpendicular axis, both side paths will have sqrt(2)/2 the slope of the straight path, which is too little. If you increase the straight path slope to 3/8" per foot, then you still have the problem of getting the fixture dead level on the perpendicular axis; if you don't achieve that, one side will be below 1/4" per foot slope, the other will be above.

Given those challenges, it's probably best as a general rule to avoid horizontal double wyes. But your situation sounds like the exception to the rule, in that you should be able to handle those issues without difficulty.

Cheers, Wayne

 

Double WYEs installed in Seattle/King CO. Basically just add a St.45 into each inlet and slightly angle them to provide the needed slope.

https://www.kingcounty.gov/depts/he...cuments/plumbing/double-wye-double-combo.ashx

However, city of Redmond may not interpret that way. Being that a repair is being made and likely the double wye was installed to code in '59, it should be legal to make a repair and replace like for like since the double wye doesn't create a dangerous situation.

However, there may be a way simpler way to plumb the future bathroom using horizontal wet venting depending on the layout.

 

Just got everything tightened up and it all seems to be working so far - thanks for the help. I definitely plan to wet vent the new bathroom, and I'll make a new thread with the details there once I'm a little closer.