3" drain pipe pitching

jeremiah18702 · Jun 29, 2006

I am currently installing a 3" inch drain pipe in my basement under the first floor to serve a back bathroom. I am running a pitch of 1/2" per foot with a distance of roughly 10 feet. Will this be too much pitch for that size pipe which requires a minimum of 1/8" per foot?

Toolaholic · Jun 29, 2006

1/2" is max pitch

1/4" is target

Sponsor · 2022-01-29T12:20:58-0800

hj · Jun 29, 2006

pitch

In most cases 1/4" is the "target" slope, but there is no problem with 1/2".

Mikey · Jun 30, 2006

General slope question

1/4" per foot is OK, and vertical is OK, but in-between isn't totally OK. Does anyone have any data or authority defining the not-OK in-between range?

hj · Jun 30, 2006

pitch

There is none. 1/4" is OK, vertical is OK, 45 degrees is OK, and so is anything in between them. Why would horizontal and 45 be Ok, but 1/2" or 30 degrees not be? The same with anything betwee 45 and 90 degrees.

Mikey · Jun 30, 2006

Don't know, but I've been told by various people in the trade, and one inspector, that "too much slope" is cause for flunking an inspection. Never made sense to me, but one story I've heard is that if there's too much slope, the water outruns the poop (if I may use a technical term), and the poop stays behind, eventually causing a blockage.

Prashster · Jun 30, 2006

The 'theory' is that the water in the drain pipe needs to act like a 'flume' and carry the solids with it. If the pitch is too steep, the water runs off, leaving the solids to get caught up in the friction of the pipe.

I have been advised that this is more of a problem in large diameter, high friction concrete pipes carrying large volumes of waste. Most plumbers (I've asked) say it is not a problem in practice with smaller, pvc pipes.

On threads I've read, 1/2" seems to be a non issue. It's when you get steeper to 30 and 45 degrees that you'd even consider adhering to this "Old Wives Tale".

I CAN say that in the plumbing books I've read, some empirical testing has been done on this and 1/4" has been deemed to be 'optimal'.

hj · Jun 30, 2006

pitch

Many laboratories have tested the theory and all have debunked it. 1/4" is the "optimal" because that is the least that works properly. I have never had a system fail because of too much pitch, and it it did, I would contact his superior before changing it.

Winslow · Jun 30, 2006

hj said:
Many laboratories have tested the theory and all have debunked it. 1/4" is the "optimal" because that is the least that works properly. I have never had a system fail because of too much pitch, and it it did, I would contact his superior before changing it.

if it is a straight run then too much slope makes no difference. If you have a lot of turns in a long run the eventually the water will outrun the solids. Old time plumbers used to "dive" the pipe all the time (at least in hawaii) with no problems.

hj · Jul 1, 2006

pipe

If you have a lot of turns in a long run, you have a poorly installed system, and it doesn't make any difference how much, or little, slope you have it will create a problem.

Winslow · Jul 2, 2006

hj said:
If you have a lot of turns in a long run, you have a poorly installed system, and it doesn't make any difference how much, or little, slope you have it will create a problem.

1/4" per slope provides the most efficient scouring capabilities of the waste water running through the pipes (2 ft per sec on pipes 3" and larger). Less or more slope degrades the scouring capabilities of the water running through the pipe. Properly installed you can have many turns without problems. Ideally you use the least amount of fittings/turns as as possible, but it depends on the particular situation. To say that you will have problems or a poorly designed system simply based on numerous turns ie erroneous.

hj · Jul 3, 2006

ells

winslow said:
1/4" per slope provides the most efficient scouring capabilities of the waste water running through the pipes (2 ft per sec on pipes 3" and larger). Less or more slope degrades the scouring capabilities of the water running through the pipe. Properly installed you can have many turns without problems. Ideally you use the least amount of fittings/turns as as possible, but it depends on the particular situation. To say that you will have problems or a poorly designed system simply based on numerous turns ie erroneous.

Okay, then I will say that a design with numerous turns COULD BE an poor design, or installation, by someone without the necessary imagination to CLEAN up the design. If it were not a potential problem, cleanouts would not be required after every 90 degrees of turning.

3" drain pipe pitching

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