I am adding a 1/2 bath to the 2nd level of my house. The closet flange will have about a 2' horizontal run to a new DWV vertical stack that runs to the basement. I have been looking all around the internet for opinions on what's better, 3" or 4". It looks like 4" is less likely to block but 3" will flow better. Walls and floor are wide open, space is not an issue. I just want what is best/ less headaches down the road. Also, what is the max drop I can have off the closet flange? Having trouble finding that as well. Any input is appreciated.

For me, 3" would be the only consideration.

3 inch with a long sweep 90 would be fine.
if you are worried about restriction at the flange then use a 4 x 3 ninety below the floor (with the 4 inch looking up). put a short piece of 4 inch into the 90. tile right up to the pipe and cut the pipe when you do the finish. cut it level with the floor and use a 4 x 3 flush-fit flange. no taper at the ring. glue it INTO the 4 inch pipe and secure well with rust proof screws.
i don't know that there is any limit to the vertical drop. as long as the vent take-off is 20 inches or less from the floor.

this is what i mean by flush-fit. no taper at the ring. yours will fit inside the pipe not over it.

I like the 4x3 90 idea. I will probably make the drop about a foot. I didn't plan on venting the toilet because I'm only 2' from the stack. The sink is next to the toilet, maybe 4' from the stack, I am venting that. I was going to bring the drain line from the sink to a sani-t in the stack above the toilet drain sani-t. Any issue with that?

3" for the piping, 4" for the flange itself. The flange will go OUTSIDE the riser or flush ell depending on how you do the riser.



Edited 1 times.

you are stack venting both fixtures as long as there is no other fixtures above. the sink needs no vent as it is stack vented and within the prescribed distance from the stack.

As long as the sink drain goes HORIZONTAL from the sink to the stack, it will work.

Thank you for all of your help.