the 3.5'' is because of the ibco/ iron body clean out.
my expanding cutters will easily go through a 3.5'' opening and expand out to 6''.
now pulling out a hugh wad of baby wipes from a toilet with my k-50 5/8'' cable was fun:D
Years ago I was reprimanded by my plumbing inspector for using 4". I have 6 commodes on the system, and a swimming pool waste. He said I should have used 3" anyway (but he passed me).
His explanation: Though code requires 4", in a single family residence more commodes does not represent increased usage, only increased convenience. Pipes are designed to be self-cleaning. If you oversize a pipe (based on actual usage) it will not properly clean and the likelihood of plugs increases.
That makes sense to me. Pipes are sized based on the amount of flow from the fixtures. When we cut commode flush quantity to less than half, why did the fixture units not decrease. Logically we could flush twice as many at the same time without an overflow condition.
the low flow 1.6 went down to 3 fu on private and 4 on public.
the greater than 1.6 are still at 4 fu. for private and 6 on public.
i have a few large scale buildings with powder rooms that plug up due to the distance prior to tying into a common waste main. the 4 powder room toilets run anywhere from 40-80' without any help from the rest of the system.
Hmmmm, so if I do have a 3" drain all the way to the street, and I change out the three existing higher than 1.6Gal toilets to new 1.6Gal they would let it go and add the 4th toilet in the shop?
So here is another angle, the drawing corrections request a FU Count if it is 3". So I can provide the count which is below the maximum number and slide by the toilet note about having 3 on a lateral. It is worth a try...