1/4 inch drain slope precision
We want to install a first (ground) floor bathroom. We have a crawl space and septic tank. This is getting more complicated every time one question is answered.
The existing vertical distance is 21" from the top of the cinderblock foundation to the top of the ~3" (maybe 3 1/2") diameter waste PVC pipe at the point it enters the cinderblock and then enters the septic system. So using a slope of 1/4" per ft of drainpipe (waste pipe?) the maximum length of drainpipe should be 84ft. So presumably, the maximum bathroom fixture could be placed at approximately 84 linear ft from the 3" diameter entry, minus a few inches for the tie-in. That is, the new fixture's drain pipe has a total horizontal distance of approximately 84 ft to its tie-in to the 3" pipe.
Is it reasonable to expect a plumber to install pipe to this accuracy? If not, what maximum length should I allow for error (tolerance, whatever the correct term is)?
There are several cinderblock piers supporting the floor in parts of the existing house, so there will probably have to be a few horizontal turns included in the 84 ft.
If the reasonable horizontal distance is,say, less than 60 ft, what are alternative measures, other than redesigning the bathroom placement?