# 1/4 inch drain slope precision

canishelJanuary 7, 2008

We want to install a first (ground) floor bathroom. We have a crawl space and septic tank. This is getting more complicated every time one question is answered.

The existing vertical distance is 21" from the top of the cinderblock foundation to the top of the ~3" (maybe 3 1/2") diameter waste PVC pipe at the point it enters the cinderblock and then enters the septic system. So using a slope of 1/4" per ft of drainpipe (waste pipe?) the maximum length of drainpipe should be 84ft. So presumably, the maximum bathroom fixture could be placed at approximately 84 linear ft from the 3" diameter entry, minus a few inches for the tie-in. That is, the new fixture's drain pipe has a total horizontal distance of approximately 84 ft to its tie-in to the 3" pipe.

Is it reasonable to expect a plumber to install pipe to this accuracy? If not, what maximum length should I allow for error (tolerance, whatever the correct term is)?

There are several cinderblock piers supporting the floor in parts of the existing house, so there will probably have to be a few horizontal turns included in the 84 ft.

If the reasonable horizontal distance is,say, less than 60 ft, what are alternative measures, other than redesigning the bathroom placement?

Thanks.

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homebound

A 1/4" slope is done all the time and can be expected. There's even a mark on levels for it (one mark off the center marks).

It might be able to be tied on the side of the waste line, giving more length. But what that max length is, I don't know. Stay tuned here and you should soon get a professional opinion.

January 7, 2008 at 8:34PM
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lazypup

1/4" per foot is the minimum and pitch and it is not the distance from point A to point B, but rather it is measured on the actual developed length of the pipe.

In fact, there is so much emphasis on pitch that most experienced plumbers or inspectors can eyeball a pipe clear across a room and tell if there is not enough pitch.

January 8, 2008 at 1:20AM
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bus_driver

Perhaps the original post is concerned that the distance involved will result in a pitch that exceeds 1/4" per foot, 9/32" for example. As lazypup states, the 1/4" is the minimum. Greater (than 1/4" per foot) pitches are permissible. There is an old tale that greater pitches result in the liquids running away faster than the solids and leaving the solids resting in the pipe. Absolutely false. To accept that, one would have to believe that solids in motion would stop moving sooner in a pipe with steeper pitch than they would in a pipe with shallow pitch.

January 8, 2008 at 7:35AM
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canishel

Thanks to all of you for your answers. I didn't phrase the question clearly.

Please correct me if I'm wrong. As long as the total horizontal distance from point A to point B is less than 84 ft, then there shouldn't be a problem with drainage to the septic tank. By total horizontal distance I mean the point where the fixture's exit pipe (term?) begins horizontal travel under the floor joists, includes any bends (due to obstructions such as piers,ducts, whatever) and ends at the juncture with the 3" pipe.

Whew. I'm just trying to figure out how far away from the septic entry I can put the bathroom and the laundry room.

Please keep the info coming. Thanks.

January 8, 2008 at 9:38AM
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lazypup

You need not worry about the pitch from the house to the septic tank. That line was pitched when it was installed and it doesn't change.

January 8, 2008 at 4:10PM
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marknmt

>>Please correct me if I'm wrong. As long as the total horizontal distance from point A to point B is less than 84 ft, then there shouldn't be a problem with drainage to the septic tank. By total horizontal distance I mean the point where the fixture's exit pipe (term?) begins horizontal travel under the floor joists, includes any bends (due to obstructions such as piers,ducts, whatever) and ends at the juncture with the 3" pipe.If I understand your question correctly you are asking whether you understand the 1/4" rule correctly and I believe you do.

You have 21" of drop between your (planned) toilet connection and the point where the run will join with the septic's pipe as it enters the house. As long as the run does not exceed ~84 feet you'll have the required drainage. And of course you could be closer than 84' and simply lower the toilet connection.

Right?

Good luck,

M

January 9, 2008 at 9:38AM
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