Thread: Maximum drain slope

I've been told that once the slope on a drain pipe exceeds 1/2" per foot it should be sloped at 45 degrees or greater to ensure correct scouring of solids, however I can't find the code requirement mandating this. Is it a code requirement, or just good practice?

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Previous thread:
Drain pipe slope - Home Inspection & Home Inspector Services For Inspections and Inspectors

Wow... I've been around here long enough to have forgotten that I posted in that thread!

Rereading it, however, I 'm not convinced the question really got answered...

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).

Back up and supporting documents in writing? only what I've posted above.

I was talking about this with my plumber a few weeks ago when we were laying out my waste and vent lines. The fittings are designed to fit tight at 1/4" / foot, which is why if you start getting too crazy on your slopes, you have a much higher change of a leak at the fitting. I didn't ask him about the solids being left behind, but it makes sense at some level. He was quite picky about making sure things were either plumb or sloped properly at the 1/4" / foot designation.

Originally Posted by Jim Robinson

He was quite picky about making sure things were either plumb or sloped properly at the 1/4" / foot designation.

Being "uniformly sloped" is quite important and is also a code requirement.

You do not want a dip down, that's real bad (creates a bit of a trap), and you do not want a belly up, that's real bad too (creates a bit of a dam).

Originally Posted by fritzkelly

IMO: The fittings are designed to be installed with the pipe level with the fitting. Any slope should be done with sweeps, etc, not by bending the pipe down.

Drain fittings are designed to start that 1/4" per foot slope, they are required by the code to be designed that way.

Thus, yes, the 1/4" per foot slope fits straight into the fitting with no angle to it.

Jerry Peck has the answer "nailed". Great reply

Originally Posted by Jerry Peck

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.

Uhhhhhhh, 1 in 12 is 1" per foot. No?

Originally Posted by Jerry Peck

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.

Originally Posted by Dennis Webber

Uhhhhhhh, 1 in 12 is 1" per foot. No?

I posted both sections, one is incorrect (as you pointed out) and one is correct (as you did not point out).

Yes, "one unit vertical in 12 units horizontal" = 1" per foot.

And, yes, "1/2 unit vertical in 12 units horizontal" = 1/2" per foot.

I apparently got the fingers typing and made a typo - oops - thanks for catching it.

Originally Posted by Jerry Peck

I posted both sections, one is incorrect (as you pointed out) and one is correct (as you did not point out).

Yes, "one unit vertical in 12 units horizontal" = 1" per foot.

And, yes, "1/2 unit vertical in 12 units horizontal" = 1/2" per foot.

I apparently got the fingers typing and made a typo - oops - thanks for catching it.

I had assumed the vent pipe, which shouldn't be carrying any solids, could be steeper. But you say it was a mistype?

Originally Posted by Egbert Jager

I had assumed the vent pipe, which shouldn't be carrying any solids, could be steeper. But you say it was a mistype?

The mis-type (typo) is where I said that 1" in 12" was 1/2" per foot, as brought up by Dennis in this post of his:

Originally Posted by Dennis Webber

Uhhhhhhh, 1 in 12 is 1" per foot. No?

I included both sentences from my post to show the typo and where it was - that the typo was in the 1" in 12" was 1/2" per foot (1" in 12" is 1" per foot), not in the 1/2" in 12" was 1/2" per foot.

Originally Posted by Jerry Peck

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.

I don't think you actually did make a typo...there are two allowable maximums depending on the purpose of the pipe. Horizontal branch piping can be up to 1:12 when it is a vent section, but cannot be more than 1/2:12 when its a combination waste and vent pipe.

Not sure what one might gain by going that little bit steeper because, as you pointed out, the fittings aren't really designed for that.

Originally Posted by Egbert Jager

I don't think you actually did make a typo...

I did make the typo, where I typed this:

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.

"one unit vertical in 12 units horizontal" is 1" per foot, not 1/2" per foot as I typed in the quote above.

I think the reasons for the max slope, stated in the 2005 thread (referred to by Michael) are valid but isn't it also because if the slope is too great, the water will flow so fast that it will drain any p-trap along it's path?

Originally Posted by Doug Campone

I think the reasons for the max slope, stated in the 2005 thread (referred to by Michael) are valid but isn't it also because if the slope is too great, the water will flow so fast that it will drain any p-trap along it's path?

Hopefully once the drain line has started on it's path to the final destination there are no more P-traps. But I suspect you are suggesting other traps that tie into the main along it's route. Sucking a trap dry is not rare but that's why there are venting systems.

Originally Posted by Jerry Peck

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).

Back up and supporting documents in writing? only what I've posted above.

Maybe a few years late, BUT the code section referred to above applies to circuit vents only. I have read about the "liquid out runs the solids" for years but have NEVER seen any real studies that confirm that theory. Nor have I seen a maximum slope listed for a regular drain or sewer line in an adopted code book. I have also discussed this with "very knowledgeable plumber (sic)" (ones that have worked in the field for decades, including me (licensed plumber for over 40 years)) with the same conclusion.

Originally Posted by Jerry Peck

Drainage fittings ...

Drainage piping ...

Table P3005.4.2 ...

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.

Originally Posted by Ralph Holden

Maybe a few years late, BUT the code section referred to above applies to circuit vents only.

Only the last two above are related to vent piping, the others, as stated above, are from chapter 30, drainage piping. I gave the slope for BOTH drainage piping (Chapter 30) and vents (Chapter 31) with the original code sections

From the 2018 IRC P3005.3 Horizontal drainage piping slope - same "1/4 unit vertical in 12 units horizontal (2-percent slope) for 2 1/2 inch (64 mm) diameter and less, and not less than 1/8 unit vertical in 12 units horizontal (1-percent slope) for diameters of 3 inches (76 mm) or more."


Maybe it is different in CA codes, but that is why I said it was from the IRC - so the code reference can be looked up and verified.

ybe it is different in CA, but that is why I said it was from the IRC, the original post had quotes from the 2006 IRC.

Originally Posted by Michael Thomas

I've been told that once the slope on a drain pipe exceeds 1/2" per foot it should be sloped at 45 degrees or greater to ensure correct scouring of solids, however I can't find the code requirement mandating this. Is it a code requirement, or just good practice?

Something else I've not yet seen addressed is in conjunction with the trend towards low flow fixtures, a worthy water conservation measure. This occurs to me as I do some inspection for a green program that awards points for low flow fixtures. I support the overall goal, but as one who inspects under houses a lot, it leads me to wonder about the following.

1. Will this leave solids in drain lines due to lower volume flow?
2. With (1) considered, is it perhaps time to reconsider slope requirements in conjunction with a building specific (fixture flow rates considered/included) design?

Are there any plumbing designers in this group? Or former plumbers with thoughts?

Originally Posted by Jerry Peck

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).

Back up and supporting documents in writing? only what I've posted above.

This is CORRECT

Originally Posted by Jerry Peck

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Only the last two above are related to vent piping, the others, as stated above, are from chapter 30, drainage piping. I gave the slope for BOTH drainage piping (Chapter 30) and vents (Chapter 31) with the original code sections

From the 2018 IRC P3005.3 Horizontal drainage piping slope - same "1/4 unit vertical in 12 units horizontal (2-percent slope) for 2 1/2 inch (64 mm) diameter and less, and not less than 1/8 unit vertical in 12 units horizontal (1-percent slope) for diameters of 3 inches (76 mm) or more."


Maybe it is different in CA codes, but that is why I said it was from the IRC - so the code reference can be looked up and verified.

ybe it is different in CA, but that is why I said it was from the IRC, the original post had quotes from the 2006 IRC.

The only reference I see in your original post quoting IRC Chapter 30 refers to Table P3005.4.2 which doesn't say anything about maximum slope. P3005.3 also contains no mention of a maximum slope. I must be missing something here?

Originally Posted by Ralph Holden

The only reference I see in your original post quoting IRC Chapter 30 refers to Table P3005.4.2 which doesn't say anything about maximum slope. P3005.3 also contains no mention of a maximum slope. I must be missing something here?

Look back at your original post - Post #17 - in which uou invluded a quote of my post ... and you will find it.