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# sewer slope translation

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• Author
Posts
• #279263
MasterPlumbers
Keymaster

It’s been a long time since my algebra years. What equivalent would 1/16″,1/8″ and 1/4″ per foot slope be in decimal numbers of slope ft/ft? (ie…S= 0.0104 ft/ft)

• #300390
John Aldrich1
Participant

[email protected] it must be a longer time since your arithmetic days.

1/4 in. /ft. = .25 inches / ft.

1/8 in./ft. = .125 inches / ft.

1/16 in./ft. = .0625 inches / ft.

Which is one precise sewer pipeline slope. :>)

» This message has been edited by John Aldrich on 08 July 2003

• #300391
JoeH
Participant

Maybe I wasn’t clear enough………Fraction to decimal conversion in inches/ft. easily done but what about fraction to decimal FT/FT?
(ie…1/16″ slope = ___________ft/ft.)

» This message has been edited by JoeH on 16 July 2003

• #300392
nicktheplumber
Participant

I’m not sure why you want to make the calculation, but it seems simple enough:

1/4″ per 12″ = 0.0208

1/8″ per 12″ = 0.0104

1/16″ per 12″ = 0/0052
.
.
.
1/n” per 12″ = 1/12n

NtP

• #300393
John Aldrich1
Participant

JoeH, upon rereading your original inquiry I find that you were clear enough, but I just thought that you had made an error by requesting the decimal equivalent in terms of ft/ft. I have never heard anyone define the slope in those terms, and I have never found a need to make that conversion. In the USA, the slope is typically described in terms of inches of fall per foot of run. So now I am curious, why do you wish to define the slope in decimal terms of ft/ft?

• #300394
Phil_H
Participant

John,
Ft/ft is the same as percent. 1/4″ per foot equals 2% grade.

• #300395
John Aldrich1
Participant

Phil H, OH!!! Man, am I embarrassed. I have certainly heard of, and used myself, the description of slope in terms of percent grade. Must have been experiencing a senior moment. Thanks for clarifying this issue.

• #300396
nicktheplumber
Participant

Yes, the figures you were looking for are called “percent grade.” My post above shows you the general formula for calculating it. Just move the decimal point two places to the right and you have the % grade: e.g. 1/4″ per 12″ = 2.08%. 1/8″ per 12″ = 1.04%, and in general, 1/n” per 12″ = 8.33/n % grade.

The question is still why you would want to calculate grades this way for plumbing work? % slopes are not used in routine residential plumbing specs. They may be used in large scale civil and commercial engineering projects.

NtP

• #300397
Phil_H
Participant

Gosh Nick, I guess you are one of those “curiousity killed the cat” kinda guys. LOL. For me, anything that has to do with pipe, piping or fluids is cool stuff. I am also kind-of curious why Joe wants to know the slope. I am guessing that he is playing around with the Manning Equation for calculating the velocity of open channel flow. In mathematics slope usually is denoted with the letter m, whereas formulas like Mannings use S.

Now some may ask what does this have to do with plumbing. Well, a properly designed sanitary sewer will have a minimum velocity of 2 ft/sec under full flow conditions while a properly designed storm sewer will have a 3 ft/sec minimum. These velocity insure proper scouring of the pipe. The Manning Equation provides an easy way to calculate this. And, slope is needed for this formula. Joe may have found one of the online calculators for this because if he does not remember enough mathematics to remember slope, he is likely to have problems with the rest of the equation.

Stuff like this can even be used around the track home. Perhaps junior has a science project. Maybe someone is constructing a artificial brook in there backyard and needs to know what size pump is needed to fill the steam. Who knows why Joe asked, who cares. It has to do with pipes, piping and fluids so it is cool stuff!

Phil

• #300398
Retired plbg1
Participant

Easy way for 10′ pipe grade
1/4″ fall per. ft. 2-1/2″ fall
1/8 ” per. ft. 1-1/4″ “
1/16 per. ft. 5/8″ “
Now you have it.

• #300399
nicktheplumber
Participant

In reply to message posted by Phil H:
Gosh Nick, I guess you are one of those “curiousity killed the cat” kinda guys. LOL. For me, anything that has to do with pipe, piping or fluids is cool stuff…

Stuff like this can even be used around the track home. Perhaps junior has a science project. Maybe someone is constructing a artificial brook in there backyard and needs to know what size pump is needed to fill the steam. Who knows why Joe asked, who cares. It has to do with pipes, piping and fluids so it is cool stuff!

Phil

Yeah, I guess you’re right, Phil. The % grade way of describing slope (or even the mathematical definition of dy/dx, where and y are in the same linear units) lends itself to plugging into the standard hydraulic engineering equations and calculating all sorts of nifty things that are of interest to engineers or physicists.

However, as a practical plumber, I’ve never had any cause to calculate drain runs this way. Of course I understand the basic math and physics and I even took these courses when I went to college, but I never really needed them in my work as a plumber. The guy I apprenticed with never went to college and was a great plumber (much better than me) and the only time my physics knowledge ever helped me was when we were installing a one-pipe hot water heating system in a new residential addition. The radiators had to be connected to a single hot water pipe that ran around the room that was added to the house. I figured out that we could connect the four radiators that were required to the hot water pipe by using the “Bernoulli” effect, by connecting the radiators to a small and large diametr tee that would force the hot water to circulate in the radiators without a pump. Otto, my mentor, was a bit skeptical, but I assured him that the system would work, based as it was on on sound hydraulic principles. He gave the go ahead, with the understanding that if it didn’t work I’d be doing a lot of unpaid work to make it work. Well, it worked just fine.

NtP