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Which one is preferable if you have a straight line 2 feet deep trench in mostly clay soil?

I'll put either one in a sock with gravel.

There are some roots around the trench so I thought the solid pipe would be sturdier.

Any comments on this would be appreciated.

Edit: I need more comments please.

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Tim Carter at AskTheBuilder.com has a lot of articles on French/Trench drains. He likes solid pipe and I agree. You are correct in that the holes point DOWN. Here's why - the water won't magicially find the holes if they are pointed up, but if they are down, the water can fill the trench and then flow into the pipe. Put a cleanout on the upper end if you can, if you don't want it sticking out of the ground, then come up with a 45 elbow and bury the cap right below the ground. I would NOT put a sock on the pipe. That will just clog. If dirt gets into the pipe, most will flow out because its rigid, and hopefully you can put a cleanout on it, or you can run a snake in from the outlet. I would put some straw or landscape fabric right under the top soil. Use 3/4 gravel as filler, not smaller. Read Tim's articles if you get a chance.

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  • I was going to put not only a sock but also garden fabric around the gravel (all 4 sides: bottom, sides and overlap it on the top). My thinking was that this way the sock has lower chance of clogging.
    – Peter Q
    Apr 21 '11 at 11:46
  • I had not thought about the clean out on top; thanks for the idea.
    – Peter Q
    Apr 21 '11 at 11:51
  • Use the garden fabric to line the ditch, that should be sufficient. We had to redig a ditch that hadn't been lined, basically to wash the silt out of the gravel. We put back rigid pipe with ground cloth lining the ditch between the gravel and the soil. This was the first French drain we'd installed on the property. The two others all were ground cloth lined and seem to be holding up quite well. The first ditch took about 15 years to clog up, the second has been there about 18 and is flowing as well as when it was first put in.
    – Fiasco Labs
    Jan 28 '13 at 22:19
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Use the solid pipe -- two feet of clay soil will be very heavy, especially when wet, and I can see the flexible pipe deforming as you fill in the trench, potentially causing a blockage in your nice new French drain.

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  • I'll have gravel around it; all the way to the top possibly.
    – Peter Q
    Apr 21 '11 at 2:05
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    @PeterQ: that's not going to be as bad, but in the meantime I found this link that gives more reasons not to use corrugated: rigid is easier to install and easier to clean out afterwards if you need to.
    – Niall C. ♦
    Apr 21 '11 at 2:10
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    The solid pipe has round holes that are of large size. The flexible corrugated pipe has slits that can easily get clogged.
    – Fiasco Labs
    Jan 28 '13 at 22:06
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The holes point down, below the pipe is a 2-3 in layer of gravel that the pipe sits on. The pipe needs to drop 1/8 inch per foot of distance, if I'm not mistaken. 80 feet distance equals 10 inch drop from the start of drain to finish (where it drains to). I'm unsure which pipe is better to use overall but I would assume solid PVC (with holes pointed down) would work better if a lot of material is piled on. (Deep trench, heavy soil). Picture the trench without the pipe, just a thin layer of gravel in a trench sloping down gradually. The water would flow just fine through the gravel and down the trench until it filled past gravel and eroded the dirt into the trench. If there is very little water the pipe doesn't fill and just flows through gravel under pipe to the low end. When there is a lot of water it fills the thin gravel layer and continues upward into the holes of the pipe which gives the water an easy path to follow in large amounts without eroding dirt on side of trench. The sides of the trench wall and all sides of pipe have gravel around them to let water go through the gravel from the grass, dirt, sidewalk etc. to enter the pipe. If holes are up or on sides of pipe then the water has to fill the trench until it hits a hole to enter the pipe and drain away to low end. Holes down keeps the water from having to fill so high before the pipe can do its job. Socks on pipe and fabric in trench containing the gravel keep dirt and other small particles from mixing with gravel or entering pipe decreasing the likelihood of clogging or otherwise obstructing water flow. It will make all your hard work last longer without issues and cleaning out.

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Has ANYONE actually set-up various french drain configurations behind clear plexiglass (cut away-like an ant farm display) to see (for real) how it performs?

In my 'minds eye', the possibility of collapse (despite the corrugations (I've seen crunched perforated pipe at the store!)) and the idea of standing water within the corrugations makes the flex pipe less of a candidate behind a retaining wall 4ft or higher in a clay environment.

Solid perforated PVC: Hmmm...Holes up lets water in directly above the pipe and that's it. Holes down hopes water will find a path of least resistance and voluntarily enter the pipe on a journey to a desired outlet away from the retaining wall to alleviate hydrostatic pressure.

It seems best to combine the best attributes of both systems to use solid perforated pipe with holes drilled/punched all around to ENSURE water finds its way inside to travel down stream.

Regarding the water seeping out...yes, water will seep out under low or zero precipitation conditions where water behind a retaining wall will not be an issue. However, during moderate to heavy precipitation, water will be taking the path of least resistance, once it finds its way into the pipe, and since water is draining through the backfill stone everywhere, flow into and through the pipe downstream until precipitation subsides. Keep in mind, appropriate airflow needs to exist for water to flow. Keep the outlet above ground/water level taking into consideration for water pooling at the outlet.

None of this is proven...just my minds eye perspective.

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Sorry, I have to disagree to solid pipe. Regardless of what type of soils surround the trench, the back fill for the drain must be pea stone or gravel. If you use solid pipe, how is the water going to enter the pipe to be drained? Solid pipe would be fine for a floor drain etc, but by definition, a French drain is a continuous porous downhill drain. Use the flex pipe, point the holes up and cover it with landscape cloth, and backfill with a good draining material.

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    But, shrilock, the solid pipe does have holes: 2 holes on either side of the bottom line. That way the water is allowed to get into the pipe while still allowing the water to flow at the bottom of the pipe.
    – Peter Q
    Apr 21 '11 at 2:04
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    @PeterQ: confusion over terms -- solid vs. rigid. :)
    – Niall C. ♦
    Apr 21 '11 at 2:08
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    yep Niall, I made the assumption solid pipe was like conduit pipe. Any perforated pipe will work, rigid or flex. The corrugation on flexible pipe actually make it quite strong however.
    – shirlock homes
    Apr 21 '11 at 6:13
  • Apologies for the confusion over my misuse of terms. @shrilock, what about cleaning the pipe?
    – Peter Q
    Apr 21 '11 at 11:42
  • Once you have buried your drain pipe, you can't really clean it unless you have intentionally made a clean out port with a street Y or something like that. If you wrap the pipe in a good silt filtering landscape cloth or pipe sock then be sure it is covered with stone before backfilling with gravel and final topsoil, you shouldn't have any cleaning requirements for many many years. you can also add a layer or cloth over the stone for double protection from silt infiltration.
    – shirlock homes
    Apr 21 '11 at 12:10

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