I recently visited a friends condo that he recently purchased at the beach. It has a huge partially underground (approx 7 feet below grade) garage. I noticed that the drain piping was almost flat for about 40 feet. It appeared to have about 1" of fall over that length. There were a number of these flat areas for different drops as the Unit has approximately 200 condos. I commented that normally the code requires 1/4" per foot fall and that it should have 10" of fall from the fartest drop to where it enters the Verticle mainline on each section. But I then thought that the inspector may have overlooked this installation due to the 4 storys of drop that would cause a flushing of the flatter area. It aparently is working OK as he states that there is no mention of abnormal plumbing drain problems in the Disclosure Statement that was required to be signed off in Escrow prior to the Close. Do you guys see any problems with what I have described? This system is Cast Iron No Hub and there are numerous No-Hub caps on the end of the runs on combinations for cleanout purposes.

4" and larger can be run at 1/8" per foot.

I am not a plumber. Just speculating...

I would guess if the drain pipe has a large enough diameter, than the water would flow even if the pipe doesn't slope ideally. Points to ponder...

1. Flushing effects of water from high above as you mentioned.
2. If the slope is too little or even backwards, small solids may drop out at the low spots and eventually, naturally, create the slope needed for water to flow. Doesn't this happen in very long city sewer lines or in ditches on the side of the road.
3. Maybe access to air to balance any air pressure differences (vacuums) is more critical than slope. That is, if the pipe diameter is big enough, it can breath along the air pocket at the top of the pipe and water will flow.
4. However, you would think that solids dropping out at low spots would trap other solids coming along and cause a traffic jam. In a ditch, that probably happens...dam forms...water rises and breaches over dam and destroys dam...cycle continues.

Yes I understand that grade can be 1/8th inch on 4" or larger pipe with approval of the Administrative Authority or Inspector. But in this case that would be 5" amd this Condo only has 1" of grade. I think they plumbed it this way too retain a maximum head room but I am only speculating. I don't know for sure but I believe it all drains to a sump pump that pushes the waste up to the City Street Sewer level.