# French Drain: Rock before or After Fabric



## jim_bee (Feb 23, 2021)

I would lay the fabric in the trench, add an inch or so of gravel, add the pipe and then backfill with the additional gravel and the soil.


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## Nealtw (Jun 22, 2017)

Think about fabric as filter cloth, we all know filters need changing when they plug up. For a longer life you want the filter area as large as possible.
You want gravel under the pipe so water travels there first and as it plugs with dirt the water will move up to the pipe and by then there will be very little dirt being moved.
I would just put the fabric above the gravel so the cover dirt does not wash down.


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## arminius (Jul 7, 2014)

Nealtw said:


> Think about fabric as filter cloth, we all know filters need changing when they plug up. For a longer life you want the filter area as large as possible.
> You want gravel under the pipe so water travels there first and as it plugs with dirt the water will move up to the pipe and by then there will be very little dirt being moved.
> I would just put the fabric above the gravel so the cover dirt does not wash down.


For extra piece of mind should i also have the perferated pipe with a sock on it?


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## Nealtw (Jun 22, 2017)

arminius said:


> For extra piece of mind should i also have the perferated pipe with a sock on it?


I think that is a great selling feature for people like you. 
If you have the trench dug, put the water hose down there and give it a low pressure (slow) trickle and see how long it takes to run clear water down the trench.
Water only trickles into this space and does not transport a large amount of dirt.


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## huesmann (Aug 18, 2011)

Fabric _always_ between soil and stone. Otherwise your soil will just migrate into your stone, clogging it. Whether you need a sock on your perf pipe depends on the gradation of your stone and the size of the perfs.


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## jim_bee (Feb 23, 2021)

The water in french drains doesn't _always _flow in a trickle--which can be both a blessing and a curse. Slower moving water in the pipe is will allow more sediment to settle out. Soil type, rainfall, and topography can make a difference in the amount of water flow and the likelihood that sediment will settle out in the pipe. Filter fabric may not always be necessary, but it's cheap insurance. On the other hand, I think fabric around the gravel plus more around the pipe would be overkill. Probably won't hurt and if it gives you confidence, as Neal says, go for it.


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## AllanJ (Nov 24, 2007)

A sock also acts like filter cloth. When placed around the perforated drain pipe it can cling to the pipe and only the circles worth of cloth directly over the perforations will pass water through and these small square inches can clog quickly and disable the entire pipe.


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## huesmann (Aug 18, 2011)

AllanJ said:


> A sock also acts like filter cloth. When placed around the perforated drain pipe it can cling to the pipe and only the circles worth of cloth directly over the perforations will pass water through and these small square inches can clog quickly and disable the entire pipe.


Huh? Sock or no sock, the only way water gets into the pipe is through the perforations.


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## puttster (Apr 30, 2012)

If your fabric / weed barrier is commercial grade it will last for years. Otherwise, don't worry about it plugging anything up because it will be gone in a year or two.






How to Install a French Drain


Installing a French drain will help prevent flooding and pooling in your yard. Learn how to install a French drain to direct runoff water out of your yard.



www.homedepot.com


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## FM3 (Aug 12, 2019)

Short version: Dirt trench. Fabric on dirt. Probably no rock under pipe. No sock.

Some people lay the pipe right on the fabric. The theory being more water is drained out of the trench that way and isn't sitting in there to freeze up or for roots to sniff out. But the water has to rise up at least a bit to get into the pipe perforations anyway, and if it can't, then that water will still be left in the trench if there's not enough slope.

And so the best fix for standing water in the trench is to make sure the bottom of the trench itself always has slope and no bellies. Then the water always moves out to the discharge even if the pipe is installed poorly (or not at all).

It's like building a block wall... take extra time to get the bottom layer the correct level so there are fewer problems above.

With fabric, I bet most people would say with rock under the pipe or without rock under the pipe doesn't make much difference. Laying the pipe directly on the fabric makes the most sense to me, and that's how I do it, but I don't think it matters much either way.

I suppose it might matter more if, as previously said, someone doesn't put proper slope in the trench and creates bellies in the trench, then the rock under the pipe can fix the slope for the pipe. That helps when water rises high enough above those rocks to get into the pipe. But water below that in trench bellies would still sit there, and water higher than the bellies but lower than the pipe would still flow out anyway without getting high enough and into the pipe. So maybe rock under the pipe is better if the trench isn't sloped properly.

Again, I doubt it matters much either way.

Without fabric, you'd of course want a layer of rocks on the bottom of the trench before putting the pipe in.

No sock. With sock shouldn't matter since the outer fabric should be keeping all debris out anyway, so the sock theoretically shouldn't get clogged because there are no contaminates allowed in that can clog it. But that's still not a good reason to use a sock. I've never heard of a good reason to have the sock on unless maybe the soil is very sandy and not using fabric.


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## FM3 (Aug 12, 2019)

Nealtw said:


> You want gravel under the pipe so water travels there first and as it plugs with dirt the water will move up to the pipe and by then there will be very little dirt being moved.
> I would just put the fabric above the gravel so the cover dirt does not wash down.


I don't understand that part. If using fabric on all sides, there shouldn't be any dirt getting in to plug anything anyway. Unless the person is using open surface inlets, which somewhat defeats the purpose of using fabric.


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## FM3 (Aug 12, 2019)

huesmann said:


> Huh? Sock or no sock, the only way water gets into the pipe is through the perforations.


Sounds like he means with the sock clinging to the pipe, all it takes to clog the entire pipe is for those little areas of the sock clinging to the perforated areas to get clogged. Whereas, without the sock, practically the entire rock around the pipe would have to be clogged for water to not get into the perforations.


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## Nealtw (Jun 22, 2017)

FM3 said:


> I don't understand that part. If using fabric on all sides, there shouldn't be any dirt getting in to plug anything anyway. Unless the person is using open surface inlets, which somewhat defeats the purpose of using fabric.


I wonder what they did for the thousands of years before we had fabric?


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## rogerwh (Mar 1, 2021)

FM3 said:


> Short version: Dirt trench. Fabric on dirt. Probably no rock under pipe. No sock.
> 
> Some people lay the pipe right on the fabric. The theory being more water is drained out of the trench that way and isn't sitting in there to freeze up or for roots to sniff out. But the water has to rise up at least a bit to get into the pipe perforations anyway, and if it can't, then that water will still be left in the trench if there's not enough slope.
> 
> ...


Sounds like you are all over the place. How exactly are you building French drain?

Roger


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## FM3 (Aug 12, 2019)

Nealtw said:


> I wonder what they did for the thousands of years before we had fabric?


You said, "I would just put the fabric above the gravel so the cover dirt does not wash down." We know they didn't do that.


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## FM3 (Aug 12, 2019)

rogerwh said:


> Sounds like you are all over the place. How exactly are you building French drain?


That's what I get for considering both sides of the rock or no rock under pipe debate and offering arguments for each side to give people something to discuss. 

I said how I'd build it in the first sentence: "Short version: Dirt trench. Fabric on dirt. Probably no rock under pipe. No sock." 

Also, "Laying the pipe directly on the fabric makes the most sense to me, and that's how I do it,"


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## Nealtw (Jun 22, 2017)

This my idea of a french drain.


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## huesmann (Aug 18, 2011)

Well, that one is gonna get clogged pretty quick. The only place you are protecting the stone against clogging is on the top, where there is the least soil. I guess if your purpose for the FD is to drain _surface_ water, it'll work OK.


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## 3onthetree (Dec 7, 2018)

FM3 said:


> rock or no rock under pipe debate


There is no debate on any part of it. French drains have been studied and tested by research engineers, Dept of Transportations, and pipe product engineers.
Trying to find a pretty internet picture, here's a couple for just a representation:

















The gravel provides a sieve better than the surrounding soil. For yard drainage, minimum 2" gravel under the pipe. Minimum 4" gravel above and to the sides of the pipe. With vehicular weight gravel cover starts at 4"-6" and12" corresponding minimums depending on size of pipe. Geotextile "burrito" wrap outside the gravel, 6" overlap on the top crossing flaps. An additional sock (and cutting the geotextile for this would be too laborous, you can buy rolls of "socks'') on the pipe within this system is optional. If you are using solid pipe, have cleanouts, and the gravel sieve is at least 1 size larger than the perforated pipe sieve, then a sock is probably overkill.

There are technicalities on soil makeup where you could say a geotextile is not needed (e.g. native clay content), but yard drainage is not as technical as miles of roadway that are soil tested and material + labor costs add up. So completely wrap the french drain gravel in your yard.


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## FM3 (Aug 12, 2019)

3onthetree said:


> There is no debate on any part of it.
> The gravel provides a sieve better than the surrounding soil.


Heh, that first picture makes the same argument I did for rock under pipe: "Use gravel under pipe to fine-tune the pitch of the ditch." Though that isn't a concern if the trench itself is sloped properly.

So let's say the trench is sloped properly. Gravel provides a sieve better than the surrounding soil, but with fabric the fabric is the sieve. And with fabric and a sealed system, there's no contaminates getting in to sieve. Also, if contaminates do get in, getting them into the pipe at the bottom gives a better chance of washing them away and keeping the drain clean than if they are stuck between the rocks on the bottom. 

If you have links discussing that particular aspect, I'd love to read them. Pipe on the bottom still makes the most sense to me, but I've already made an argument against that (an improperly sloped trench), so I'm open to other reasons.


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## jim_bee (Feb 23, 2021)

I can see Neal's point. Before filter fabric, french drains were installed with only some barrier on the top of the gravel to prevent the loose soil filtering into the gravel when backfilling the trench. I've specced a number like that myself. OTOH, I've also seen agricultural drains clog from siltation. Now that we do have filter fabric, I think it's a reasonably cheap way to add another layer of protection.


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## 3onthetree (Dec 7, 2018)

> Think about fabric as filter cloth, we all know filters need changing when they plug up.


Geotextile that "clogs" does not fully block water going through it. It does "slow" the transfer as the sieve is now much smaller, which is why you use the proper geotextile, not landscaping fabric. _Needlepunched_ geotextile has been tested to maintain performance where fines have "clogged" the remainder of the very same sheet of textile. Socks have not been tested as far as I have found, and usually they are not of the same quality as needlepunched geotextile anyway. But if a textile were completely "clogged" somehow, it should perform just as well as certain strata of the surrounding soil that has the same content of fines.



> I would just put the fabric above the gravel so the cover dirt does not wash down.
> If you have the trench dug, put the water hose down there and give it a low pressure (slow) trickle and see how long it takes to run clear water down the trench.
> Water only trickles into this space and does not transport a large amount of dirt.


It's correct that deep undisturbed soil does not move as much fines, and a legitimate argument a textile cap just guards against the looser backfill above. But surface drainage is just too shallow and the soil composition is looser than say, at the footing depth. So the water will move laterally under the textile into the pipe (via typical seeks level and capillary action, not just during rain events). But if you have a drainage situation with good slope, non-corrugated pipe, and a strong velocity of water, you could argue just this textile cap (instead of full wrap) is sufficient because the fines would just wash out. I think that was done in the early days of textile use, now we've learned a full wrap is better.

Note Neal I used your quotes just as subject topics, not arguing them as you really didn't say anything wrong.


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## Nealtw (Jun 22, 2017)

jim_bee said:


> I can see Neal's point. Before filter fabric, french drains were installed with only some barrier on the top of the gravel to prevent the loose soil filtering into the gravel when backfilling the trench. I've specced a number like that myself. OTOH, I've also seen agricultural drains clog from siltation. Now that we do have filter fabric, I think it's a reasonably cheap way to add another layer of protection.


Farm field drainage can be a different animal as it often goes in with out gravel, 








Clay tiles were put in with a rectangle of tar paper over each join and worked somewhat for 40 or 50 years before we find them plugged








But when they were surrounded by gravel you still may find them working after 40 or 50 years.


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## 3onthetree (Dec 7, 2018)

FM3 said:


> Pipe on the bottom still makes the most sense to me


A gravel bed:

allows additional controllable sloping of the pipe
provides the largest sieve where the perforations are (in solid pipe)
allows the "drawing" of water into the trench below the pipe
allows any fines inside the wrap to settle below the pipe
differing materials (gravel vs soil) and larger sieve space allow more movement if shifting occurs
most importantly, provides a foundation for pipe support. Gravel ("gravel" is defined as crushed/chipped rock) is self-compacting to an extent, which gives a controllable, consistent compaction rate around the entire pipe cross-section. In comparison, for agricultural installations not utilizing a gravel bed, pipe support is recommended by a 'V' groove for 3"-6" pipe and a shaped trench for 8"+ pipe, like this:


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## jim_bee (Feb 23, 2021)

I can't disagree with Neal's comment about agricultural tile, and I wouldn't claim that a french drain won't work without filter fabric. How long it will work depends on a number of factors including soil type, slope, etc. I see the use of proper fabric (as 3onthetree points out) to be cheap insurance and a "best practice". 

We could have similar discussions about many, many construction issues.


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