Minimum drop for toilet in basement

[wallbobby]

What is the minimum amount of space below that's required for a toilet? I understand the drop needs: 1/4 " per foot. Without going into the really long details:

I have a toilet on a basement slab that is 4 ft from the base of the main cast iron 4" stack.

I measured to the bottom of the stack before it turns toward the street and the bottom of the pipe appears to be 15" from the top of the floor(bare slab). I got this measurement by dropping a tape measure from the roof (it's a straight shot down the stack) and looking into the cleanout at the base of the stack. I also inserted the tape measure into the cleanout and bent it so it was going straight down and got 18", but probably the other measurement is more accurate.

My understanding is the allowance is about 6" for fittings, and then 1 inch for the 4 foot run, so this should be possible. Does that sound right?

Second question: if I'm able to tie in right at the base of the stack, and that arm is 4 ft, am I basically stack vented and no additional toilet vent is required?

The back story here is long - but basically converting basement plumbing that is going to a septic tank over to sewer - the sewer was put in 50 years ago but they didn't bother to attach the basement plumbing to it.

Thanks. Have drawings if needed.

 

[Asktom]

The toilet needs to be vented before it ties into the stack. A drain from the floor above cannot serve as the vent.

 

[Sponsor]

 

[wallbobby]

What about a drain from the same floor? There is a shower 3 ft to the right of the toilet (so, looking at the toilet, stack is 4 ft to the left, shower is 3 ft to the right) that has access to a dry vent next to it. Nothing from the floor above ties into that drain. If I just ran 3" or 4" for both toilet and shower (and tied into the vent for the shower trap, upstream of the toilet) would that work?

 

[Cacher_Chick]

I would not try to install a sanitary tee below grade at the base of a stack. Even if you could fit it in, you would have to support the weight of the stack from above and have enough straight pipe remaining above and below the fitting for heavy woight no-hub clamps. I cannot think of many scenarios where I would do this instead of tying into the horizontal buikding drain.

The vents could be tied together for the fixtures, no less than 44" above the floor, or 6" higher than the flood rim of the highest fixture being served by the vent, whichever is higher.

 

[wallbobby]

Thanks. There is potentially already a fitting at the base - they put a wye in that handles a stand pipe that can easily go away. What I see (I can post a pic) is about 18" to 2' to the right of the main stack (towards the toilet) there is a 2 inch pipe coming out of the concrete at an angle. I wonder if this is a simple wye that I can remove and instead just put in a 3 or 4 inch pipe.

 

[Cacher_Chick]

I am guessing that you are looking at a cast iron stack with lead & oakum joints. If you can properly support the stack, you might have enough room to cut the supposed wye out and replace it with a sanitary tee.

Once you have it a dug out, you will first have to take some good measurements to determine if the inlet of a new sanitee can be placed at the correct depth to provide 1/4 per foot of slope to the proposed new closet bend. Until this is done, there no way to know if if your idea will be possible or not.