Drain and Vent Layout Help

Hi everybody,

First time posting, this forum has been a huge help to me as I work on remodeling an old house (1940s). I'm decent with most trades but haven't had a lot of plumbing experience and need some advice before attempting to add a bathroom on a concrete slab. I will be hiring a licensed plumber to do the fittings and make the connection to the sewer line, but I'm very price conscious at the moment and want to do as much as I can to knock the price down (i.e. get the trenches cut and dug etc...) so I won't take up much of his time.

I'm in the Houston area and we follow the UPC 2006, but also allow AAVs.

Attached is a layout of the proposed bathroom. This was an addition to the house and was poured on slab (the brown wood floor area). The green area is the exterior of the house, and the light brown area is the old porch that was taken out and is now just interior dirt. I've trenched through the dirt and was hoping to make as many connections in the light brown area so I could minimize cutting the concrete as much as possible. The red lines are the drain lines that I was going to run, with the main line from the toilet being 4" and the rest being 2", except for the 1st sink, which was 1 1/2" (2x4 exterior wall). I was also going to run 2" vents everywhere, but I assume the main vent had to be the same size as the sewer.

The double sinks on the exterior wall concern me because of the studs that the drain pipe will have to pass through to get to the 2nd sink. Was considering using AAVs here to minimize further cuts to the top plate and working in a low attic. Also considered tying the sinks in directly but the old sewer pipe is concrete and I'm concerned about connecting to it at the sink and again at the main drain because I don't want to create a section of concrete pipe between 2 flexible connections that could create a belly.

My other concern is that I want to minimize roof penetrations. There's already a 2" vent in the attic from an old drain that I was curious about tapping into. I guess this isn't possible if I have to make the main vent 4" though.

The last thing is that the washing machine drains goes through the standpipe and P trap and drains into the tub's trap arm. I'm not sure if this is a permissible wet vent, but I can always run it into the main stack I guess. I wanted to have the connection on the side so I could access it easier since there'll be a dryer on top of it.

Just wanted to get thoughts on whether this would generally work for a drain layout and if anyone has any other ideas of things I can do to help knock the cost down a little. Please pardon my lack of knowledge - I really appreciate the help. Feel free to ask any questions and I'll respond ASAP.

I guess it's ok providing your trap to vent distances are right. It's hard to make out on the drawings though. You also can reduce your vertical vents to 1-1/2"

IF you really have "concrete pipe" under the floor inside the house, you have a disaster waiting to happen.

It transitions from cast iron to concrete as it leaves the house (certain it's not clay). Why is it a disaster? Would I be better off having everything run the opposite direction and tying into the sewer line outside the bathroom (where the X is on the sewer line) instead of under the slab?

From your description, it sounds like your connections to the concrete would be inside the house, and if so, the concrete should be changed to a more durable material. Concrete pipe does not conform to ANY other material's dimensions do any "coupling' would have to be improvised, and if it is corrugated concrete, forget about making any connections to it.

Thanks for the replies!

In the drawing above the connection would be under the house. I could replace some of the concrete with PVC but thr concrete pipe continues on for about 20' under the slab (not shown). Not sure I can afford to replace that much and tear up all my floors.

If I reversed the direction of all the drains and had them exit the house right below the toilet and connect to the concrete pipe outside the house (at the X in the yard) instead of behind the tub then it could be done without making a connection to the concrete pipe under the house. It's not corrugated FWIW but I'm sure its brittle. This would make the drain plan a little more complicated but I guess it's worth it because it wouldn't affect the pipe under the slab?

If I did that, could I have the sink drains run along the outside of the exterior wall and meet the 4" line from the toilet?

You are NOT listening. Concrete pipe ANYWHERE that is not accessible is a disaster. It was never meant to be a "long term" material. If you were to get a permit and the city found out about it, they SHOULD make you replace it, regardless of the cost, since it will NEVER be cheaper than it is now.

Ok I wasn't sure if you were saying it couldn't connected to under the house only. So it's impossible to connect to concrete pipe in this case? So my options are either replace the entire 100' sewer line (which I've had scoped and isn't leaking) or just not add a bathroom? Seems like there should be some middle ground - that's why I was thinking it could be better to tap the concrete pipe outside - at least it would be somewhat accessible.

You should at least replace the concrete pipe INSIDE the building, and I doubt that any "scope" could verify the integrity of the pipe.

Does the shower need that separate vent there or could it vent thru the next vent down the line by the washer instead? Assuming that the shower drain is 2 inch and the distance to vent is less than 5'?

If so, what is size would the vent need to be since the shower, washer, tub, and toilet are using it (I'm not sure if the tub would be considered to be using the lavatory vent or not)?

Anybody know if something like this would work better?