How bad is an UPHILL waste run?

[CFO]

I'm remodeling my basement bathroom (on slab). I cut away about 3' of the slab to relocate the toilet and tub drain (to make a shower) and discovered that the exposed 3" pipe carrying away the waste from the toilet, sink and tub has a very minor UPHILL slope to it -- maybe 1/8" per 2 feet (see pic). We never had any problem with things not draining properly so I guess the momentum of the water was enough to push it through the slightly uphill pipe until it came to the main waste.

The existing pipe is ABS and I'll be using a Fernco coupling to switch from 3" ABS to 3" PVC near the end of the cutout. All the ABS in the pic will be replaced with PVC.

I really don't want to cut out more concrete -- I'm not sure where this pipe finds the main waste stack or how far I would have to cut before I came to a downhill slope -- I don't want to cut the slab beyond this bathroom.

Can I ' tweak' the Fernco coupling just a little to raise the PVC pipe up a half inch or so to at least give it a good downhill run before it hits the ABS. Any suggestions?

 

[Jadnashua]

FWIW, that installation should never have passed a rough-in inspection. Now, it could be that things settled a bit.

Waste does not flow uphill. It will have to get deeper in the pipe before it can flow, assuming it can. The depth of the water in the tub and the elevation probably isn't more than a pound or so of water pressure (0.43#/foot elevation). So, since you've not had a backup, somewhere before the pipe would cause a backup, it must turn down but it will have been flowing slower than it could. Having a negative slope means that the line won't get scoured clean as much. Luckily, the trap will prevent that rotting hair, etc., from smelling up the room.

If you can raise your p-trap a little, yes, you can improve that initial slope. Water flows out of a tub fairly slowly, so it's not the momentum that kept it going, somewhere it must eventually turn down to let gravity do its thing. There will be standing water in the pipe until that happens, though...generally, not a good idea.