Maximum slope for waste drain pipe

[techinstructor]

I've read that the ideal slope for the waste drain pipe is around 1/4" per foot. The waste pipe in my new home will have to run through a poured concrete wall. The pass-throughs that were put in place when the walls were poured were located per instructions from my general contractor. However, I'm concerned about the amount of slope that will be required to go from the pass-through in the interior wall to the pass-through at the exterior wall where the waste line will exit to to to the septic tank.

Here's a picture to show the slope that will have to be used. (I haven't actually measured it.)
http://lh6.ggpht.com/_JWULuIxmfCo/SXYB5xuIr-I/AAAAAAAAAw4/Z4GHq1q7iHY/s640/P1190047%20copy.jpg

The toilet will be on the main floor -- to the right of the wall on the right of the above picture - about 19' from the exit location. You can see that the pass-through for the waste pipe is about 6" from the top of that interior wall. (The red line was added in Photoshop.) The pass-through for the waste pipe to exit the house is at least 4'6" down from the top of the wall. Obviously this pipe will be falling a lot more than 1/4" per foot to go from one pass through to the other. The exterior pass-through was determined by the level of the grade and to achieve proper slope to the location of the septic tank (about 15' from the house). I plan to call my contractor to ask him about this but I would appreciate knowing what is considered an acceptable MAXIMUM slope for the waste drain pipe. I've read that too much slope can cause the liquids to flow through and leave the waste behind.

[Don_P]

I don't think there is a code maximum. I've checked my code (table 710.1(1));
https://www2.iccsafe.org/states/Virginia/Plumbing/PDFs/Chapter%207_Sanitary%20Drainage.pdf
A 20 year old plumbing code I have read the same, minimums but no maximum. and did a search onm the inspectors website where the same question had been asked with the same answer.

There is the scouring issue but the inspectors also pointed out that when a washer or dishwasher dumps there is alot of flushing action. !/4" to a vertical drop would be another way but I don't think its a problem.

[Richard A Hetzel]

Heck, it can be, and almost always is, vertical (in places) with no problem, and that's about as maximum a slope as you can get. What, me worry?

[techinstructor]

I talked with my general contractor about the drain pipe slope. (I've got a GREAT general contractor - should have known he had it right!) When my contractor explained his plan for the location of the drain pipe it all seemed too obvious. I was visualizing it all wrong. The horizontal pipe was put as high as possible so that it would be out of the way and it will run at the proper angle to a vertical pipe which will lead to the exit. Sort of like this:
http://lh4.ggpht.com/_JWULuIxmfCo/SXeAzEHMXsI/AAAAAAAAAys/Zap6P6dcqAI/s512/drain%20line.jpg

From what I've read, vertical pipes are NO problem; it's the horizontal ones that have to have the correct amount of slope, so it seems to me that this plan will work just fine.

It's interesting that the house I currently live in does have a drain pipe that exits on a slope of a 45 degree angle or greater. We've never had a problem, but I didn't want to take any chances. Nobody wants to deal with clogged up drain pipe.

[Richard A Hetzel]

A pipe on a 45-degree slope is never going to clog. It's the pipes that have to run nearly flat that we have to worry about. As I said, the pipes can be vertical, and you don't get any more sloped than that! =) However, your contractor's plan is the right one. Just make sure he installs a cleanout at the exit, and at all other 90-degree bends. Clogs are much more likely at elbows than in a straight pipe.

[Don_P]

Actually you can have issues with too much slope, hence the comment that 1/4" to a vertical drop would be another way, which is what the contractor is doing. Water can run away from solids leaving them stranded. The slope is to provide proper "scouring" action.

[gcan]

Don P is correct the first option you show will have the water out running the solids, not good!

the second option is best

[homebild]

All due respect to Don P but he is NOT correct to suggest that there can be 'too much slope' with any waste pipe.

'Slope' is not and never is the issue...

Pipe 'diameter' is the issue.

And scouring action only applies to 'horizontal' pipes....(Pipes greater than 45 degrees from vertical)

[mjpliv]

I found this in another forum for building inspectors http://www.inspectionnews.net/

Drainage fittings are required to be designed to maintain 1/4" per foot slope.

Drainage piping is required to have a minimum slope of 1/4" per foot for sizes 2-1/2" and smaller, and, a minimum slope of 1/8" per foot for sizes 3" and larger.

Table P3005.4.2 shows three slopes: 1/8" per foot, 1/4" per foot, and 1/2" per foot for calculating number of fixture units allowed.

The maximum slope of horizontal branch piping is 1/2" per foot.
- From the 2006 IRC.
- - P3110.3 Slope and size of horizontal branch.The maximum slope of the vent section of the horizontal branch drain shall be one unit vertical in 12 units horizontal (8-percent slope). The entire length of the vent section of the horizontal branch drain shall be sized for the total drainage discharge to the branch in accordance with Table P3005.4.1.



The maximum slope of a horizontal combination waster and vent pipe is 1/2" per foot.

- P3111.2.1 Slope.The horizontal combination waste and vent pipe shall have a maximum slope of 1/2 unit vertical in 12 units horizontal (4-percent slope). The minimum slope shall be in accordance with Section P3005.3.



Yes, it has to do with the water speed. Too much slope and the water speed will outrun the waste it is carrying with it, leaving the waste behind to clog the line up.


If you need to exceed 1/2" per foot slope, then either use a 45 degree slope or a tie into a vertical drop.


The above information is from what I have learned over the years and from very knowledgeable plumber (the ones who teach the courses for apprenticeship programs).

[WV-Mike]

There is the scouring issue but the inspectors also pointed out that when a washer or dishwasher dumps there is alot of flushing action. !/4" to a vertical drop would be another way but I don't think its a problem.


I also think of any flushing of non-solids as scouring.

-Mike