DWV remodel issue...??

I have a 3" horizontal main running under my slab floor with the toilet at the end of the line. I am now moving my toilet downstream 8 feet. The existing lav ties into the main down stream from where the old toilet location was located. With the new location of the toilet, the existing lav will now tie into the main upstream from the toilet. I plan on leaving the old main active upstream from the toilet to keep the existing lav tie in. The question is, does the possibility exist of some toilet waste back flowing up the main to where I have the existing lav tied into? The pitch on the system is correct and I would expect water from the lav to help wash down any waste that back flowed up the main to where the lav ties in. Any thoughts??? Thanks

A lav cannot scour a 3 or 4" line by itself; The under slab line for the lav should be 2". What you are doing will also require a vent for the toilet.

Who's code are you under? IPC will allow that. The 2" requirement for all underslab drainage has been changed and you can go with an 1 1/2" line there. If the lav is vented it will wet vent the toilet but.... under the IPC the toilet to vent distance is "unlimited" so as long as there is a vent in the house you are good.

Tom, are you saying that the IPC allows a wet vent to be only 1-1/2" ?
Does not sound right to me.

Thanks to all who responded, but both the toilet and the lav will be properly vented. The real question I have is if it is OK to have a lav empty into a main upstream from the toilet connected to the same main. Someone thought solid toilet waste might back flow a bit towards the lave and was concerned of a build up. The original toilet location was upstream from the lav and is now relocated downstream or past the lav. So now the lav still empties into the same main but with the toilet downstream. I hope this explains my situation.

Why yes indeed, as stupid as it may sound, you can now run 1 1/2 in slab Dooooh..

Thanks to all. Just to play it safe, I using 2" tie in for the main coming from the lav, and another 2" from the shower. Both should scour the line enough for my original toilet question.