Moving bathroom in cottage - verify venting plan

[TorontoTim]

I need to relocate a bathroom in a small 800 sq ft cottage. Currently it is between the kitchen sink and main vent stack, and then the drain continues to drain into the septic system.

It will be located off to the side, as per the diagrams attached. I can leave the vent stack in place, and would like to do so to avoid punching another hole in the roof etc.

So - thinking I can run a vent line from the new bathroom, sloping up and then coming up through the floor and joining the main vent stack above all the drains in the cottage - likely about 7 feet up in the closet the vent runs up through now.

The kitchen sink is not currently vented properly. It's too far from the vent stack to not be vented. I will add an AAV to it.

I cannot go up through the walls with vent lines and join the vent stack in the attic because there is no attic space. I do have a 3-4 foot crawlspace under the cabin where all the drains are located. So, I was going to run the vent line from the new bath in this space, sloping upward and then come up through the floor and join the stack as mentioned.

Issues? Concerns? Diagram shows a rough birds-eye and side view of the plan and basic dimensions.

 

[TorontoTim]

And of course after I drew the diagram and posted, I realized I could/should likely run a vent in the crawlspace for the kitchen sink and join the new vent line before it comes up through the floor and join the main stack - instead of an AAV.

For 2 sinks, a toilet and tub/shower, is a 2 inch vent line large enough, then joining the existing 4 inch vent stack?

 

[Sponsor]

 

[Terry]

You can't run a horizontal vent for the bathroom.
The lav is also not vented.
Would it kill you to run a vertical vent for the bathroom in a way that works?

 

[TorontoTim]

Well it's not going to kill me that's for sure I can go straight up and punch through the roof with another vent stack. I just can't go up with a normal vent and carry it over in the attic space to the current stack as there is no access between them. Its a vaulted ceiling with minimal attic cavity and no access. Also the slope of the roof is such that I'd likely be unable to slope upward from the attic space above the bath (directly below the peak) to where the existing stack is which is closer to the low edge of the roofline.

Hard to explain. But the answer is clear that if I can't run an upward sloping vent in the crawlspace over to the current stack then I go straight up through the roof with a 2nd stack. Do I need a full 4 inch stack for this three piece bath?

 

[hj]

Actually, nothing is vented as you have drawn it, and a "vent in the crawl space for the sink", would NEVER work.

 

[TorontoTim]

Well, then I really didn't get this right at all. I'm honestly a little confused however. If the toilet was wet vented through the sink drain, it would be ok I presume? If there was a vent line continuing up from the sink drain up and through the roof?

What's the difference between that and running it across the crawlspace sloping upward and then up and joining the main stack vent above the drains?

I didn't realize having the vent run across the house above the bathroom to the stack would be different than running below it. I wasn't clearly sure of where the vent would need to connect to the drain under the bathroom, but I didn't realize this would result in nothing being vented at all.

I've drawn it from the side, with the vent pipe I thought I could run in green. It sounds like I need to absolutely come up through the sink drain as a 'wet vent' (if I've got my term correct) and up through the roof or somehow across to the existing stack?

 

[TorontoTim]

In doing some more searching, I came across discussions on venting kitchen islands with loop vents. Could I approach this problem with a similar solution? Is it only applicable for a single drain, or can a 3 piece bath be vented like an island?

https://terrylove.com/forums/index.php?threads/island-sink-venting.43049/

1. Properly installed, there is almost NO reason for a loop vent to become plugged in the first place. The lateral vent COULD get plugged it the main drain were consistently getting plugged and backed up, but then that would indicate a deficiency in the plumbing ELSEWHER.
2. I would hope a properly installed drain system would NOT plug up 3 or 4 times a year. AND it would have to happen much more than that to plug the loop vent even ONCE a year, or maybe once ever few years.
3. If you cannot tell when a loop vent is plugged, HOW would you know when to replace an AAV?
4. A "relief" vent stabilizes and eqializes pressures in multistory building.

 

[Terry]

The vents come of the "top" of the trap arm, not below it.
Venting below will not prevent the siphon effect that pulls water out of a trap.

 

[TorontoTim]

Fair enough. I wasn't 100% sure as shower drains don't seem to follow that rule near as I can figure, nor of course toilets.

One of my thoughts originally was to put the sink and shower traps below the floor so they could be after the vent line but I seem to recall the trap has to be close to the drain opening to prevent momentum from emptying the trap.

So if I go up from the sink and punch through the roof with a vent, can I just do a 2 inch line or is the norm to do a full 4 inch? This is an 800 sq ft 2 bedroom cabin and the only 'legal' way to expand on it in terms of either bedrooms or another bathroom would be to put a larger septic system in. I don't anticipate this vent ever doing more than just the 3 piece bath.

 

[Terry]

Fair enough. I wasn't 100% sure as shower drains don't seem to follow that rule near as I can figure, nor of course toilets.

You can vent serveral bathroom groups with 2"
You can wet vent the toilet with a lav, and you can wet vent the tub/shower with a lav, but it has to be done correctly.

In this drawing, the lav trap is vented above the trap arm.
Though come to think about it, you're in Canada, and maybe once you cross the border, everything changes.

 

[hj]

Your "view from the side" is still completely wrong. NOTHING is vented and the toilet flow is going past the other connections before it gets to your "vent".