French drain - which contractor is right?

StitchinsonFebruary 11, 2013

Hi there,

Can anyone help me with a question on French drains?

I have a very soggy back garden (mainly turfed on top of clay) and am looking for a solution to ensure my lawn is useable this summer and that I can plant nice things in my borders and give them half a chance of not drowning!

I have had 3 contractors in to give me quotes (as well as the French drain I am getting my existing borders dug out, sleepers put in to raise the soil level and filled with top soil - hopefully this will make a difference) and replaced with top soil. Anyway, the question is:

One of the contractors has quoted for laying a layer of stone/gravel across the whole area including between the trenches before laying the turf so that there is effectively a soak away between the turf and the existing clay. This sounds sensible to me, but I mentioned it to another of the contractors whose opinion was that it was not necessary, pointless and perhaps just a way of increasing his fees. However, the first contractor's price is not significantly who should I listen to?

The other thing is that the second guy didn't seem interested in quoting for the sleepers and borders at this time as he said we need to get the drainage sorted out and working effectively then think about other stuff...whereas the first guy is quite happy to do it all in one job...

I just want the job doing right and if that means paying a bit more to the person who will do the best job then fine, but not having any knowledge of French drains and such work, I don't know who that is...can anyone help?

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Around here, 9 out of 10 french drains should have never been installed. They are usually the wrong solution for the problem, and sometimes cause additional problems, especially for houses with a basement. Some have to be removed costing the homeowner even more.

Nothing in your post indicates a need for a french drain. If you go forward with it, I suggest that all gravel be at least 2 feet below finished grade.

    Bookmark   February 11, 2013 at 3:47PM
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Thanks for your reply. If not a French drain, what other solutions are there?

    Bookmark   February 11, 2013 at 4:08PM
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Manage the water with surface drainage and fix the soil.

    Bookmark   February 11, 2013 at 4:48PM
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I second what pls8xx has said about fixing surface drainage and soil. I would not want a layer of gravel under my topsoil as an alternative (or a bunch of probably near worthless french drains!)

    Bookmark   February 11, 2013 at 9:34PM
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So, I'm not a soil engineer so hopefully pls8xx can speak to this better than I can, but... I was told that putting gravel at the bottom of a potted plant actually SLOWS drainage because the disparity in particle size between aggregate and soil prevents water from passing easily between them. By that logic, wouldn't a layer of gravel under topsoil cause the yard to drain less well?

    Bookmark   February 12, 2013 at 8:33AM
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Stitchinson, I'm sorry I was so cryptic in my reply. This forum has a reputation of being rude. Some of us, rather than answer a simple question tend to tell a homeowner what we think they need to know.

Why do contractors sell a french drain where one is inappropriate? For the same reason stores sell bedding plants at the wrong time for planting: that's when people will buy them. Many homeowners equate french drains with a drainage fix. For the contractor it's an easy sale of a profitable job. Keep in mind the contractor is selling construction, not results. If you don't get the results you anticipated, it's your problem, not his.

Then there is the placebo effect. Homeowners who have just spent a large sum on a french drain tend to see an improvement, real or not.

Marcide raises the issue of a perched water table which occurs when a freely draining material underlies a less permeable soil. And yes it works in the ground just as it does in a pot. Water in the upper layer drains out fairly quickly, but only to a certain point, leaving a saturated zone at the bottom that doesn't drain at all. The depth of this saturated zone is a function of the particle size of the upper layer. In the case of many silt top soils this is often 4 to 5 inches. The effect can be either good or bad for plants. If the upper layer is 10 inches and the lower 5 inches is saturated, summer heat cooks roots in the top 2 inches, and you are left with an active root zone with a depth of three inches. On the other hand if the top soil is 24 inches deep then you have a root zone of 17 inches (24-5-2=17) with a water reservoir below, which can be very good for plants. The cost would be huge to dig out three feet of material to do it. It don't happen.

Clay does not give up its water to a french drain. If clay is saturated it will remain unchanged by a french drain.

    Bookmark   February 12, 2013 at 10:01AM
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Thanks pls8xx. Good to know I was on the right track.

Ok, here's the big question to the OP: where is the water going to go? If you can regrade and push the water somewhere (legally), you have a solution. If not, a french drain in your yard will alleviate the problem as effectively as a french drain in the bottom of a plugged bathtub will empty the tub.

    Bookmark   February 12, 2013 at 12:14PM
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Thanks for the replies, was just checking in but wasn't really expecting to see any more! Really appreciate the time taken to post.

Ok, taking your points on board, assuming a french drain isn't appropriate for the various reasons outlined...what does manage the water with surface drainage and fix the soil mean in practical terms (sorry I realise I am probably being thick)? I just want the issue fixed, I'm not hard over on the method.

To answer the last poster, my garden has a slight decline to the rear and handily located behind the back fence is a stream into which each of the contractors proposed to direct the flow from the drains. Apparently, being rain water only, this is legal. Does this make the French drain solution any more feasible?

    Bookmark   February 12, 2013 at 4:03PM
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You could have one of the few properties where a french drain is appropriate, it's just not likely.

It's not that we don't want to help, but every situation is different. Most of these problems occur on very flat properties and to make suggestions, one needs detailed data to work from. Few of the people who come here are willing to do the work of providing adequate photos and measurements. There are exceptions, here is one �.

    Bookmark   February 12, 2013 at 4:56PM
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deviant-deziner(Oh zone)

Which contractor is correct : Consult an engineering contractor.

    Bookmark   February 12, 2013 at 8:55PM
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