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Old 06-05-2006, 07:22 PM  
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Default New Plumbing under slab

Im getting ready to put some plumbing (drain) down for the house before more backfill and then slab is poured. I guess Im suppose to have around .25" drop per foot. Well I have almost 60' from start to septic and the current grade is roughly .40" to .45" per foot. Why would this not be acceptable ? Im asking because the ground below in the house is very hard chert. We currently have a septic system we have been living on now for about 9 years and the drop in it is somewhere between 5 to 10 feet difference for about the same 60 foot length and we have never had any problems. So if I didnt measure right and my fall for the new house septic drain was close to 1/2" drop per foot then would it still work just fine ?

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Old 06-06-2006, 02:01 AM  
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I would make sure this won't be a problem because it will be a whole Costco sized jar of Ibuprophin if you have trouble with it. chert is hard - concrete is harder. try the UPC

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Old 06-06-2006, 07:17 AM  
Daryl in Nanoose
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My calculations show .25" x 60 ' = 15 feet. If I were you I would double check your measurements and make sure you follow the .25" per foot drop rule because you can't change it later.
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Old 06-06-2006, 09:07 AM  
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OK its time for a lesson from the old master:
First the fall on plumbing drains should be at least 1/8" per foot and a maximum of 1/4" per foot. You say the drain is 60' long; 60'X1/4"=15" total fall for the drain. The reason for the min/max fall per foot is to keep the solids moving without the water running off and leaving solids behind. This was determined by extensive testing by old timers. In hard ground you can put your drain rather shallow under the slab keeping a proper fall; then, when you get outside the building line, put in a test tee and drop on a 45 degree angle so you will have 18" of cover over the sewer and then carry your proper fall on down, using the backhoe (save your back).
For the shallow run in the building line, you may use wyes to join fixture drains to the building drain; otherwise, if you use tees they must be laid on their backs or no more than a 45 degree angle, never lay a tee on its side. That gets you to digging too deep in the hard ground by hand.
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Old 06-07-2006, 06:38 AM  
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Plumbing codes set only minimum pipe slopes with pipe diameters of 2" or less requiring slopes of at least 1/4" per foot of run and pipes of diamters greater than 2" requiring slopes of at least 1/8" per foot of run.

There is no maximum slope of drain pipe and never has been and the myth of water separating from the solids is completely untrue.

Otherwise vertical waste stacks would not work.

Anything less than the 1/4" or 1/8" slopes mentioned does not produce enough of a "scouring velocity" to move solids. It is when pipe slopes are too shallow that liquids separate from solids, not when they are too 'steep'.

1/8" per foot is acceptable slope for any plumbing code for building sewers.


Backfilling of sewer lines is regulated by plumbing codes, most require a 6-12" layer of sand to be laid in the trench first, then the pipe covered to a depth of at least 12" with clean fill free from rock and debris.

Successive backfilling of the trench usually must be done in 6-12" intervals and tamped before additonal layers of fill can be added in the same sequence of filling and tamping.

Call your local plumbing code office for details on what will be required and inspected in your code jurisdiction, minimum depths for sewer line placement, and everything else related to this process.

It is the only accurate information you will receive.

Last edited by Square Eye; 07-15-2006 at 05:36 AM.
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Old 06-07-2006, 06:46 PM  
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Thank you all for your help and information.
It is very much appreciated
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Old 06-07-2006, 07:44 PM  
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Default Plumbing is all a bunch of &$$*

Interesting ain't it?
One more thing to include is before you poor your slab of concrete, make sure your plumbing which pokes up through the concrete is wrapped so it does not later move with the concrete.
As water goes through the pipe and things settle the pipe if connected to the 'crete can break the plastic if it does not have a sleeve type of separation, your plumbing supply house will have this material,not the big box.
Good luck ....wear your muckin boots.

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