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French Drain


Onoff
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I need to build a French drain. Any pointers? This to take some rain water run off where the land slopes down to the house. It's going to be temporary (my temporary :) ) until I landscape.

 

My thinking so far is a 9" wide trench (bucket width) with a 110mm brown soil pipe laid in it to falls.  (I've a load of scrap, brown soil fittings with perished rubbers that'll do for joins). Pipe wrapped in landscape fabric. For the "fill" I'm thinking to use small 'ish local flints and believe me I've got them in spades!

 

To give a "side" to the drain I was thinking maybe 300mm gravel boards?

 

I've read that the pipe needs holes in it but only along the bottom. Are they in a line? What diameter holes & spacing?

 

Cheers

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23 minutes ago, recoveringacademic said:

That's not far off the mark, Clive.

The holes do not need to be drilled by a German; you are allowed to be a couple of mm left or right of bottom dead center (but no more OK!) ^_^

 

(My great grandfather was in fact German! :) )

 

It's the ideal hole dia and spacing I'm after at the mo.

 

 

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22 minutes ago, Stones said:

You could also cut slots in the pipe if you happen to have a grinder or stihl saw to hand.

 

I could even route them! :)

 

More thinking Starrett holes. I have from 14mm dia up to I think 152mm dia.

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Not sure if it has something to do with our soil ( clay) but our local land drain expert told me not to put pipes in the bottom as they will block up, he digs a trench, lines it with membrane and fills it with 50mm stone and a little covering with soil ( and membrane ) if you dont want to see the stone. On my many many trips to sight this appears to be the way drains are laid at the side of the A30 going to Devon. ThIs is the way I have done the French drains around my garage and house and they work very well.

Edited by joe90
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1 minute ago, joe90 said:

Not sure if it has something to do with our soil ( clay) but our local land drain expert told me not to put pipes in the bottom as they will block up, he digs a trench, lines it with membrane and fills it with 50mm stone and a little covering with soil ( and membrane ) if you dont want to see the stone. On my many many trips to sight this appears to be the way drains are laid at the side of the A30 going to Devon.

 

It would be really no problem to dig say a 9" wide 1' deep trench. Line with fabric, fill with flints up to say 3" from the top then lay some more fabric and another 3" of flint. It'd be easy 'ish then for maintenance / sprucing up. AND no pipe to clog. Think I'll still put gravel boards in though.

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1 hour ago, Russell griffiths said:

The stone idea works very well as opposed to the pipe option, I have done one also were we concreted the bottom,( lean mix not very thick ) and laid to a fall then refill wth the stone, this creates an impervious layer that the water runs along until the drain. 

 

So do you have to concrete the bottom of the trench?

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4 minutes ago, PeterW said:

Membrane and gravel - you don't need boards in them as they will just rot ..!! Don't try and improve on roman engineering ..!!

 

How do you "maintain" their level without adjacent dirt etc encroaching if nothing to stop it?

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18 minutes ago, PeterW said:

The membrane ... pack the gravel in well and it goes nowhere. 

 

This wont be gravel it'd be assorted flints. Thinking whether it'll work, I mean it's hardly pea shingle!

 

Did similar alongside the garage. Doesn't look too bad though SWMBO though the addition of pine cones would enhance things! This isn't used as a drain btw just to "hide" the ugly footings:

 

20170809_200034

 

20170809_200021

 

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1 hour ago, Onoff said:

 

How do you "maintain" their level without adjacent dirt etc encroaching if nothing to stop it?

 

Fold the membrane over the top and overlap, then put whatever you want on top .. I usually do 2-3 inches of 20mm gravel on top or decorative, then if I want to I loose lay pavers.

 

If the pipe is inside the membrane at the bottom of the gravel fill it stays clean. The pipe increases capacity should it need it since gravel makes most of the space to be stone not void.

 

You want your holes to be smaller than the general size of your gravel.

 

F

Edited by Ferdinand
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With gravel I can see all the detrius like leaves sitting on top and being fairly easy to keep tidy. With my proposed flint topping being a more "open" fill i.e. with gaps guessing that's more prone to filling with rubbish.

 

 

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  • 3 years later...

I'm a bit confused by the discussion here about not needing a pipe in a French drain. thepavingexpert.com website that gets lots of rave reviews here, just says the following:

 

"A big part of the problem with French Drains was that the vast majority of them were poorly constructed. As already stated they often comprise nothing more than a trench backfilled with any old gravel that happened to be lying around, but a true French Drain should include an unjointed pipe and the better ones would be surrounded by a graded filter of selected gravels and sands."

 

There is then a somewhat helpful picture:

 

drains.jpg.39e0af3540c6e13871b4b9444f89ece6.jpg

But apart from describing the third option as "deluxe" it doesn't really tell you why. Why would it be helpful to have a graded filter? As long as the perforations in the pipe are smaller than the size of the pebbles/gravel/pea shingle it presumably won't impact performance, or does the filter make sure that the closer the water gets to the pipe the cleaner it is and therefore the less likely the pipe is to block up?

 

The other point, which I'm sure is obvious to everyone else, but not to me, is whether the pipe needs to be installed at a fall or whether it is sufficient for the bottom of the trench to have the fall?

 

And finally, what are the pros and cons of a fully perforated pipe (top and bottom) versus one that is only perforated on one side (presumably the top side)?

 

For context, I'm trying to finalise the spec of my french drain for the perimeter of my house, which includes two damp walls where a significant cause of damp was insufficient drainage of water away from the wall.

 

It would also be incredibly helpful to have people's views on what diameter pipe I should specify. Architect has specified 60mm and two types of gravel coarse and then a finer decorative type on top. I've also found an 80mm pipe for similar money. Which is better?

Edited by Adsibob
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17 minutes ago, Adsibob said:

Why would it be helpful to have a graded filter?

Depends on the soil type.  You don't want the pipe clogging up and I guess the graded thing should stop this.  Depends on soil type.  In heavy clay, most land drains fail over time because they clog up.  I have seen lots of examples of old pipes completely blocked with clay.

 

Regarding fall, as long as the pipe does not get silted up it is fine to lay level if it is sited low enough and has a gravity outfall.

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7 minutes ago, Mr Punter said:

In heavy clay, most land drains fail over time because they clog up.  I have seen lots of examples of old pipes completely blocked with clay.

 

Does a wrap of permeable geotextile around the pipe or say under the top layer of aggregate help prevent blockage?

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38 minutes ago, Onoff said:

 

Does a wrap of permeable geotextile around the pipe or say under the top layer of aggregate help prevent blockage?

I believe that for a clay soil a geotextile membrane won’t help much since the membrane itself is prone to clogging up. Sandy soil is much more suited to a membrane.

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