Somewhere along the way, I got it into my head that aside from a minimum slope for a horizontal waste drain line, there is also a maximum. The idea being that if there is too much slope, the liquids will drain away faster than the solids and consequently the solids may hang up in the line. Then when a vertical drop is necessary, it is done abruptly, more or less straight down vertically.
For the life of me, however, I can’t seem to find an IRC reference to a “maximum” slope for a horizontal run. Did I imagine this, or am I just not seeing well today?? If anyone can point me in the right direction, I would appreciate it!
Aside from the general interest, I do have a specific issue that got me thinking about this. A customer called me who had just recently moved into a “newly constructed” townhome. 3-4 days after he moved in, a first floor toilet overflowed. Turned out that the waste line going from the home in the yard had been damaged, dirt accumulated in the line and caused the line to back up.
The plumber fixed the waste line, but the owner said that after the line was fixed, the angle at the clean out, elbow in the basement noticeably changed. Before the fix, the owner says the the vertical drop came more or less straight down and then swept out at a gradual horizontal run out through the foundation wall. Now the vertical drop “slants” somewhat, goes into the clean out tee which seems to have a fall of 3"/ft. Although not much of the vertical run is visible, I can see the portion in the wall and then based on the angle of the exterior clean out in the yard, it would “appear” that the 3"/ft horizontal angle continues for at least 2’ - 3’.
Does anyone think that there is a problem with this or does it seem acceptable?