toilet drain/vent all wrong

[quayhog]

My downstairs bathroom was improperly designed and poorly constructed. The sink and toilet share a vent which is well below the drains. I think I read that the vent should be at least 6 inches above the highest drain it serves. Also the toilet waste pipe runs very noticeable uphill. Needless to say, performance is not good.

So, I need to re-do both drain and vent. Here’s my layout. If you were sitting on the toilet, the sewer line exits the house directly below and behind you. The toilet drain, though uphill, runs between your legs for about six feet, then 90s down for about 2 /12 feet to meet the main sewage line. So, visualizing a flush, the waste goes between your legs, out in front of you for six feet, 90s down to the sewage line, then exits under you into the street.
I want to run a new toilet drain, just like the old one, except slope it down, rather than up. (By the way, what’s that formula to determine the proper slope?). So, where on this six feet run of new pipe would I put the 3’’ vent line? And, how can I tie in the sink vent, or should they be separate vents?

I would like the vent to do this if possible. 45 (or 90) degree off the new drain line, 90 (or 45) degrees up into the garage (about 11 feet up, turn 90 to run 12 feet horizontally across the garage ceiling to the exterior wall. The other alternative is to exit above the garage door, but that would put the vent right outside the bathroom window, which seems not so good in the warmer months. But, this would eliminate the long horizontal run. Thanks.

 

[Jadnashua]

You want 1/4" per foot of run for the slope. You should have a separate vent for each fixture. It should come off of the back of the 90-degree bend under the toilet, then go up. You can tie it into the one from the sink drain. Basically in the wall, you use a special T, it directs the water down to the drain, and you continue up to the vent from the toilet. You can combine them at around 42" above the floor (6" min above the rim of the sink). They can run horizontal or vertical anywhere you want as long as the horizontal runs still have slope to them, so if rain, or condensation get in there, they will drain and not collect. You should run these up the wall and through the roof. You can run them in the attic and connect with the existing vent, if you wish.