I'm looking to reroute part of my sewer drainage for my home. I have old cast iron pipes in my foundation which are giving me all kinds of trouble. I need to know how much drop do I need to allow and if maybe going with 3" pvc instead of 4" inch is acceptable? I will also have to tap into the main line to complete the connection. I was wondering if using one of those rubber T's would be sufficient since I don't know how else to complete the connection at the main line since it's already buried and will not provide me with any movement to work with?

3" pipe requires 1/4" per foot slope
4" pipe requires 1/8" per foot slope

If you have 4" pipe coming out from your foundations, you have to continue with 4". You cannot reduce to 3". If you have 3" under the slab than you can continue with 3". But why ? Material is cheap and installation is the same. 4" sewer drains better than 4".

Install 4" clean out within 24" of the foundation. You will appreciate this detail later.

Final connection should not be flex fitting. I assume you have 4" existing sewer you want to attach to it. Than do this:

Cut out 18" of existing sewer
2. Remove the 18" piece and discard
3. Buy 2 - 4" Flex Seal couplings
4. Slip both Flex Seal couplings on to old sewer, one on each end. Push them all the way pass the cut
5. Measure length of your new Y
6. Figure out how much pipe you will need on each end to connect with old sewer. ( This installation takes place withing those 18" you just cut out )
7. Glue both pieces onto the Y
8. Insert into the void in your sewer you've created by cutting out 18" of pipe
9. Slip 4" Flex Seal couplings from each end of old pipe onto the new pipe. Stop so each coupling is 1/2 way on old pipe and 1/2 way on new pipe.
10. Tighten s.s. band on Flex Seal couplings.

Now, you've created connection in old sewer with outlet to be connected with your new house run.

Now connect your new pipe ( house run ) to this new Y. You can also plan ahead a bit and glue it into the Y before you insert it into the 18" void in old sewer. In this case, you have to measure twice and glue once. It the connection is too tight, you can always use another Flex Seal coupling to connect those two.

Let us know how you did...

Ok, I have one more question. Do I need to use a transit to meassure my drop as I go? Thanks for your time!

You should always use level or transit when installing sewer line.

Thanks for your help!