[bamatbucket]

I have a 50-year old house that has a 4” cast iron drain and vent that serves one bathroom (with tub, lavatory sink, and toilet) on the main floor. There is a full basement that used to have an improperly plumbed bathroom that I have removed and intend to replace with a new bathroom and kitchen. I have done a fair amount of research on the internet and from several plumbing books and I think I have a good basic understanding of what I need to do to correct one problem in the existing bathroom and put in the new one in the basement, but I have a few questions anyway.

1. There are two problems with the existing toilet. It has an 11” rough-in from the wall to the center of the flange and the toilet drain arm is sloped down from the sanitary T in the stack toward the toilet. I think this worked OK for many years because of the high volume flush of the old toilet. This is the original construction – the house has not settled an appreciable amount and it looks like no alterations have been made to this part of the plumbing. I plan to correct this before I buy a modern low-volume flush toilet by removing the toilet drain arm and slope it ¼” per foot toward the stack from a new flange with a 12” rough-in. I will use 4” pvc to a pvc sanitary T spliced into the old stack. I have purchased some of the materials I need, but have noticed what could be a problem with pipe size. The 4” pvc pipe has an OD of 4 ½”, but the old cast iron pipe has an OD of 4 ¼” and ID of 3 ¾” (measured at the top of the vent through the roof). Is this common? Will a 4” banded coupling from the big box store work to join the pipes with different OD sizes or do I need a coupling with different ID’s on each end? I intend to eventually replace all of the cast iron with pvc, but will do it in stages and will need to connect the pvc to the cast iron while the project is underway. I know about using riser clamps when removing sections of the cast iron and the increase of noise through the pvc.

2. I would also like to change the lavatory and tub drains and vents. The tub is drained and wet-vented through a 1 ½” line to the stack through a side-outlet sanitary T for the toilet and the lavatory is drained and wet-vented to the stack above the toilet T. I would like to change both of these drains to 2” and back-vent them to the stack. The drains would be dropped vertically into the basement and horizontally connect to the stack below the toilet. Is it overkill to use 2” pipe for these drain and vent lines – would it cause any problems to do so? I have seen several comments in the plumbing books that encourage use of larger pipe sizes than the code-required sizes and it intuitively seems like bigger is better in this case.

3. The final phase of my plumbing project is to install a bathtub, toilet, shower, lavatory sink, kitchen sink, and dishwasher in the basement. I was planning to run a 4” pvc horizontal drain under the slab with a 4” branch to the toilet and 2” branches to the other fixtures. Each fixture would be back-vented to the stack above the main floor bathroom through 2” pipe. Is a 4” pipe too large? One of my plumbing books says that 3” is better for the new low-volume toilets in order to maintain enough velocity to carry the solids with the liquid. I suppose the same question would apply to the main floor toilet arm – is 3” better than 4”?

Thanks for any help and suggestions.