Plumbing ?--Waste line under basement slab, or route around the house?

[Erin]

We're debating the pros and cons here. 
All of our plumbing is on the north side of the house.  The septic, on the other hand, is on the south.  Do we want to route it around the corner of the house and out?  Or would the straight run, but under the slab, be the better way to go?

[glenn kangiser]

Is your septic tank lower than the basement, Erin? 

If not, Suspended to the basement ceiling then out to the south?

Not exactly sure of the situation here.

[diyfrank]

Theres nothing wrong with putting it under the slab if that was your question.
Myself, I would take the path of least Resistance.

[Redoverfarm]

If this is to have a basement with a drain line originating in the basement the choice is obvious.  If there is a story above the basement then as Glenn stated to the basement wall then drop down to go under the footer.  I personally ran mine under the basement floor from one side of the house(plumbing side) to the other side (septic).  The only down side if any to this fashion is to make as little turns as possible so that in the future a snake can be used to clear the a clog. 

Another word of caution is that any septic line laid in the basement floor should be solid against the undisturbed earth.  Otherwise it will settle and a crack could develope in the floor over the location that the line runs.

[JRR]

On a recent TV home show, they showed an (under concrete floor) sewage line having to be dug up.  And I'm also facing the same task in my current home ... old cast iron pipe that has served many good years ... but not long enough to suit me.  I would never chose to run a sewage line under a concrete floor if there is another resonable option.

[diyfrank]

 Use quality material and do a quality job.  Cast iron like a lot of thing used in the past have proved to fail in time.
Sch 40 abs or sch 40 PVC will give you no problems if laid correctly.
Dig a clean trench lay your pipe on a bed of sand. Fill with sand to the top, Compact. Using the native soil to backfill is usually the cause of cracked / settled slabs over trench's.
If for some reason you need to access it later, a concrete saw / rotohammer is all thats needed.
If you don't need to run it under go over like Glenn suggested.

Going around the house will work but its not the best choice.  you may run out of grade going the long way around. You should put cleanouts at the corners of the house if your going a very long distance but, if its to be inspected they will say how far apart.

[Erin]

On a recent TV home show, they showed an (under concrete floor) sewage line having to be dug up.  And I'm also facing the same task in my current home ... old cast iron pipe that has served many good years ... but not long enough to suit me.  I would never chose to run a sewage line under a concrete floor if there is another resonable option.

This was actually our concern with putting it under the slab, but as Frank just mentioned, we also worried about running out of grade (and having to go around a corner) to route it around. 
And yes glenn, the septic is lower than the basement, as well as having a bathroom in the basement that needs to be plumbed.

[JRR]

Obviously, not having enough slope in drain lines would not be a "reasonable" (perhaps I've spelled it correctly this time) option.  In my current case, I do have enough elevation to reroute the sewage line around the concrete floor and still have adequate drain line slope.  Yes, I will install clean-outs at critical and useful locations.  The old sewage under-floor line will just be abandoned in place, avoiding break-out of the floor.  Part of my Spring 2009 list of things to do.

[Native_NM]

I am not an expert, but am in the preparation process for our cabin.  Each time I see a question here I take it as an opportunity to research and learn.  In this case I have determined:

1.  There are two threats to drains run under any slab:  downward pressure and upward pressure.

2. Downward pressure includes the weight of the slab, as well as any weight transferred downward to the foundation. 

3. Upward pressure includes soil and hydrostatic pressure.  In combination with downward pressure, upward pressure can destroy a drain in a matter of months.

4.  Isolation and compaction of drain trench is critical. 

5. Proper selction of materials is critical.  Cast iron is no longer recommended.

6.  Depending on soil conditions and temperature conditions, a "sleeved" system may be desirable.

7. Adequate drainage of the foundation is critical to ensure pressure does not build up under the slab.

8. Adequate bracing is required where drain exits the slab.  In retrofit installations, ensure that the stem or footing is not disturbed.

9. When properly installed, using modern materials, an underslab drain has a life in excess of 100 years.

Again, I am use these questions a chance to learn.  I am a great research scientist, but have no building experience.

[Bishopknight]

I did the plumbing under my slab and I don't anticipate any problems. I put a 2 ft sand pad under my slab to protect from frost heave and cushion the DWV.

[ScottA]

I run lines under slabs all the time. Sleeve all floor penetrations. Put 6" sand under and over the PVC pipe.