Hi again,

Because of the height of the water table, my new septic tank will be very shallow. The top will be at about ground level, and the entry to the tank will be 7 inches below grade. I'm worried about running a drain to it at the right pitch.

The house is on a slab, with the existing septic tank under the slab. I'm going to have a plumber dig a trench in the slab from the 3" soil stack that gathers the 2nd, 3rd and 4th floors, under the wall of the house, and trench out to the tank. He'll run a 4" pipe from the 3" soil stack to the septic tank. At 1/8" per foot drop, I'll be able to go about 40', assuming I start 2" below the slab to replace the concrete. So that takes care of the top floors.

On the 1st floor, the new 4" drain pipe will go right past (within 2 feet of) the existing toilet and shower. Since the existing toilet drain is in the slab and (I'm guessing) enters the existing septic tank too low to use for the new septic tank, I'll try to connect the toilet into the new drain line as it goes by.

Questions:

1 - what is the minimum depth below the slab the toilet will require. IOW, if I put a 3" elbow under the toilet, how much below grade is it when it joins the drain line? I'm worried that the 3" elbow will leave me starting at 6" below grade, and I won't have enough drop to get to the tank.

2 - since the ID of the toilet is 2", can I use a 2" drain under the toilet instead of 3"?

3 - The plumber told me you must drop 1/4" for 3" drain, and 1/8" for 4" drain. Can you use 5" or 6" drain and lay it even flatter?

4 - What's the best way to abandon the existing tank under the house? Fill it with something inert and cap it off? Just cap it off?

5 - Could I keep the existing tank under the house, and just let the 1st floor toilet continue to fall into it?

Glenn

Hey Glenn,

Questions:

1 - what is the minimum depth below the slab the toilet will require. IOW, if I put a 3" elbow under the toilet, how much below grade is it when it joins the drain line? I'm worried that the 3" elbow will leave me starting at 6" below grade, and I won't have enough drop to get to the tank.

(1) Grade being the top of the slab? Assuming a 4" pour on the slab a 3 X 4" PVc closet Bend would give you 6" from the closet flange to the center of the 3" hub. A 3 X 4" Street 90 would give you 4 9/16" to the center of the 3" hub.

2 - since the ID of the toilet is 2", can I use a 2" drain under the toilet instead of 3"?
(2) Only if you don't poop or put toilet paper down it. 3" is the minimum.

3 - The plumber told me you must drop 1/4" for 3" drain, and 1/8" for 4" drain. Can you use 5" or 6" drain and lay it even flatter?
(3) We use 1/8" slope on both 3 and 4" pipe down here. While 1/4" is preferred for 2 and 3", if you're tight our inspectors let you drop 1/8" to the foot.

4 - What's the best way to abandon the existing tank under the house? Fill it with something inert and cap it off? Just cap it off?
(4) Our codes make us fill the tank when we convert to city sewer.

5 - Could I keep the existing tank under the house, and just let the 1st floor toilet continue to fall into it?
(5) Not a good idea. How would you service the tank? Where would the tank drain out to? If the top collapsed you could find yourself on top of a sinkhole.
Good luck. Tom