Basement Bathroom DWV Layout

[OGKris]

Looking for some direction on this basement bathroom remodel with cast iron drains below slab. Here is the existing layout.

Red = Below Slab Drain
Green = Vent
Blue = Wet Vent

The existing layout was cramped and not to code. The previous owner had to remove the shower door because there was no room in front of the toilet. I am rotating the toilet, widening the shower and moving the lav. Note the two 4" branches in close proximity, I'm guessing the bathroom was an add-on.

Currently the two existing vents need to go as they would be inside the room. Here is my proposed layout:

My line of thinking with this layout was to simplify by combining the basement bathroom branch and the stack branch for the bathroom above into a single branch. I am trying to strike a balance between limiting the scope of the project and future proofing. Also, I am trying to minimize the amount of slab I am opening up. But I'm worried about the cast iron and I'm finding it's often suggested to replace them when there's an opportunity. The house was built in 1968. This is not a flip. I am in Fairfax, VA which is IPC I believe.

Questions:

1. Am I crazy for considering ripping out the existing cast iron?
2. To minimize work under slab and to maintain the grade on the wet vent from the lav across the room, I'm planning to go above slab with the wet vent right after picking up the shower drain. Any glaring issues with this?
3. Am I overlooking a simpler solution? Open to suggestions.

 

[OGKris]

If the long wet vent is a bad idea, what about keeping the wet vent at the shower as well and tying the shower and lav vents together?

 

[Sponsor]

 

[wwhitney]

My understanding is that your proposed DWV layout is IPC compliant. To your questions:

1) I think the operative question is "will this existing cast iron perform adequately until the next time it gets exposed anyway?" That requires deciding when that will be (20 years? 50 years?) and judging its condition; the latter would require considerable experience, more than I personally have.

2) If you're talking about running the wet vent in the wall at the top of your drawing, and entering the slab at the point indicated by the arrow, that seems reasonable to me. Since you have a single lav, under the IPC the wet vent from lav to shower drain connection can be 1-1/2"; from shower to WC it has to be 2".

3) I don't know if this is simpler, but since you have an existing vent available in the wall at the bottom of the page, you have the option to dry vent the shower and use that to wet vent the WC, as drawn below (blue = 2", green = vent). You'd have to figure out how to cross your vent takeoff across the existing stack. That still leaves the question of how to run the lav drain; if breaking the slab is harder/costlier than running it through the wall at the bottom of the page, it may be that your original idea is better.

Cheers, Wayne

 

[James Henry]

Maybe this wet vent.

 

[wwhitney]

That would be more conventional and would work fine, but involves breaking more concrete. So the question is whether the more round about path would work as well, or if it presents some issue and should be avoided in favoring of the direct path?

Cheers, Wayne

 

[James Henry]

Your cutting concrete no matter how you route it.

2. To minimize work under slab and to maintain the grade on the wet vent from the lav across the room, I'm planning to go above slab with the wet vent right after picking up the shower drain. Any glaring issues with this?

You cant jump up and down with a wet vent if you plan on being code compliant.

 

[OGKris]

Thanks for quick input! Wayne, good to have a second set of eyes saying it is IPC compliant. I've been doing a lot of research on wet venting but haven't seen this exact scenario so very much appreciate it.

Yes, wondering whether it's worth the extra effort to cut more concrete to pick up the lav more directly vs going around the wall. I guess overall it's not THAT much more work, so if it's definitely going to work better, I'll strongly consider it. But here's what I was originally thinking with actual fittings.

Your cutting concrete no matter how you route it.
You cant jump up and down with a wet vent if you plan on being code compliant.

The entire wet vent would be graded down as it goes around the wall. Is that what you mean by jumping up and down?

 

[James Henry]

That's not a true wet vent system, I'm not sure that would pass, you would have to confirm it with your local inspector. If I was doing the work I would probably do it like my drawing, I don't see much of a difference in the labor between running everything in the wall and digging.

 

[wwhitney]

A typical lav wet vent is a sequence (going downstream) that is vertical, then horizontal (and basically 2% slope). No other changes of elevation angle. Both James's suggestion and the last diagram (with both dry vents) satisfy that criterion. The last diagram is unusual in that the dry vent serving the wet vent for the shower and WC is not actually the primary lav vent, but I don't see how that makes any difference.

The original Proposed DWV Layout (with only one dry vent) doesn't satisfy that criterion. It is vertical-horizontal-vertical-horizontal, which is not conventional. So the question is whether that will still work OK, and whether is it allowed under the IPC (or UPC).

For the former, I don't see how it makes any difference. I guess the concern would be that the drainage in the second vertical portion would somehow be more likely to block the entire pipe cross section than the drainage in the first vertical portion? That doesn't make sense to me physics-wise. And I don't see anything in the IPC (or UPC) that explicitly prohibits it.

If the second vertical second is to be avoided, then it would not be hard to change the last diagram (with the second dry vent deleted) to avoid the second vertical segment. All you'd have to do is use a 22.5 degree elbow instead of a 90 (san-tee in the diagram, but the second dry vent has been deleted) to more gradually enter the slab. Then everything would be horizontal except for the initial vertical segment.

[Edit: This would still be non-conventional, as the horizontal portion is usually all at 2% slope, but again I don't see any reason it would cause problems, or any prohibition in the IPC (or UPC).]

Cheers, Wayne

 

[OGKris]

Wayne, the technical explanation is very much appreciated. I was thinking along the same lines that it wouldn't matter physics wise if the wet vent went horizontal and vertical more than once, for the same reasons that a wet vent is allowed to have unlimited length. But if there's even a question of whether or not it is allowed under IPC, I'm going to err on the side of caution.

James, thanks for the suggestion. I think I'll bite the bullet and do it like your recommendation.

Here's my latest version. I also made the change to go 3" after the stack to the upper level. See any other issues?

 

[wwhitney]

I don't see how your latest rendering has any fewer supposed issues than your original plan--you still have vert-horiz-vert-horiz. The conventional approach would be to have the two horizontal wet vent runs at the same elevation (well, 2% slope, or a bit more if it helps), so you'd eliminate the short vertical near the shower and rotate the combo where the lav and shower drain join from vertical to horizontal.

Cheers,
Wayne

 

[OGKris]

I don't see how your latest rendering has any fewer supposed issues than your original plan--you still have vert-horiz-vert-horiz. The conventional approach would be to have the two horizontal wet vent runs at the same elevation (well, 2% slope, or a bit more if it helps), so you'd eliminate the short vertical near the shower and rotate the combo where the lav and shower drain join from vertical to horizontal.

Cheers,
Wayne

Hmm, I thought that's what I did. Might just be a perspective issue with the render. Here it is from the top view. It's all on the same elevation (with 2% slope) except for the vertical at the lav.

 

[wwhitney]

Hmm, I thought that's what I did.

Ah, yes, the isometric rendering is ambiguous for the configuration shown. [Although perhaps the lack of any curvature at the lav/shower drain joint should have been a clue.] Looks good.

Cheers, Wayne

 

[OGKris]

I did one more variation. I'm trying to go around the existing wall instead of under the wall since I already opened it up just to the inside of the wall. I am offsetting around the wall and have a wye to the stack for the upper level group. Also going down to a 3" for the stack. Does this look good as well? If it's definitely more optimal to not go around the wall, I'll just bite the bullet. But if it will work just as well and saves a little work, why not.

 

[wwhitney]

Looks good to me, as long as the existing upper floor stack you're picking up is 3".

Cheers, Wayne

 

[OGKris]

Looks good to me, as long as the existing upper floor stack you're picking up is 3".

Cheers, Wayne

Yes, taking the opportunity to replace all of the upper level group as well so it will be 3".