Slope of waste drain line.

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?








You are correct in saying a drain line can have “too much” slope, as far as what the specifics are, call one of your local plumbers and ask.

But what are you using as a reference or basis for saying that “a drain line can have too much slope”?

This might help.

Normally 1/2" per ft is accepted (without a fitting - 45 deg or other). Most code inspecters pay little if any attention to this in plastic. It used to be a larger issue in Cast Iron when you actually had to use long turn sweeps at the bottom of a vertical stack. Crap sticks to cast iron.

Thanks for the help guys!

Instead to say " to much", say “improper” , make you room to explain general requirements. We are not design advisers. let plumbers do their jobs.

I recently took a class on CA Code changes which go into effect on Jan 1, 2017. Thi sis something that is being researched. One draw backs to the studies so far is they have been aimed at commercial buildings with long drain runs so they are not stating results are accurate for residential as well. However CA is looking at the PERC reports with interest.

One of the findings was the use of toilets below 1.28gpf showed increase in blockages. So for now CA is using 1.28 as of Jan 2017. The other impacts are tinsel strength of toilet paper used and slope. Too much trailing water appears to block airflow contributing to potential blockages. Too little trailing leaves waste behind again leading to potential blockages. “Trailing Water” too much or too little causes increase potential for blockage. They also made the recommendation that any older structure being retrofitted to newer lower flow toilets should have their drainage systems inspected prior. One of the short falls with the studies so far is they focused on commercial buildings with little additional inflow. The only comment made in the reports is that additional inflow in residential from washing machines, dishwashers, etc would increase inflow but it was not tested. I would think that the location of this additional inflow would be a consideration as to its impact. More studies are being done by the PERC group.

Though this does not specifically answer your question it does show there are many factors to the blockage question.