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Max DWV slope

e hilton

e hilton

Bronze Member
Joined
Jul 2, 2014
Messages
3,156
Location
Virginia
Is there a maximum slope for toilet drain lines? I have heard that too much slope can cause the liquid to flow too quickly and leave the solids behind.
And this questions sounds familiar. I searched but did not find an earlier discussion, sorry if there is.
 
$ $ = $ $

From the 2018 IPC, Section 704

704.1 Slope of Horizontal Drainage Piping

Horizontal drainage piping shall be installed in uniform alignment
at uniform slopes.........The slope of a horizontal drainage pipe shall
be not less than that indicated in Table 704.1, except that where the
drainage piping is upstream of a grease interceptor, the slope of the
piping shall be not less than 1/4 inch per foot (2-percent slope).

TABLE 704.1 - SLOPE OF HORIZONTAL DRAINAGE PIPE
SIZE
(inches)		MINIMUM SLOPE
(inch per foot)
2-1/2 or less1/4a
3 to 61/8a
8 or larger1/16a

For SI: For SI 1 inch = 25.4 mm, 1 inch per foot = 83.33 mm/m.
a. Slopes for piping draining to a grease interceptor shall comply
with Section 704.1.

= = + = =
 
No there is not....because it can be vertical....But I think I saw a 10% suggested max. once when I was researching, but it will also depend on the length of the steep run....
 
e hilton said:
Is there a maximum slope for toilet drain lines? I have heard that too much slope can cause the liquid to flow too quickly and leave the solids behind.
And this questions sounds familiar. I searched but did not find an earlier discussion, sorry if there is.
Click to expand...
I do not believe you will find a max. A toilet is a self siphoning fixture water and solids should disperse under suction.
 
I don't think it is in the code what a max is. A Master Plumber acquaintance of mine says that it does reduce scouring, but I don't recall the number.
 
In Pennsylvania, if you have a septic tank, there is a maximum slope to the septic tank which is no greater than 1/4" per foot for the last 10' of the pipe before it enters the septic tank. This keeps high flow sewage from slamming against the intake baffle.

Therefore, if you are in PA, you should be inspected this. This does not apply to pump tanks, just gravity.
 
e hilton said:
Is there a maximum slope for toilet drain lines? I have heard that too much slope can cause the liquid to flow too quickly and leave the solids behind.
And this questions sounds familiar. I searched but did not find an earlier discussion, sorry if there is.
Click to expand...
Yep, with low flow, the liquid outruns the solids, but the real answer is for the IPC to do the calcs and reduce the pipe sizes IMHO
the original calcs were done in the 1920's with 5 then 3.5 gals per flush.

every one knows it but codes need the data to change, from what I have been told
 
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