French Drain, What to do?

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French Drain, What to do?

Postby The_Laddie » Wed Aug 01, 2007 8:33 am

Hi folks,

I am in the process of digging a trench/French drain around my victorian house to try and rid some of the walls of damp as the grass level is at the same height as the DPC. See picture
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The couple of problems that I have and would like some advice on are, My main sewer/drain pipe whatever it's called is running along the bottom of the trench so I can't really make it a dry area as if there was ever a problem with it I would have to break out any concrete etc. on top of the pipe and probably damage the pipe in the process as it's clay
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and secondly, I have what I can only describe as Gulley drains??? which are also at the same height as the DPC and therfore can't really tap into them to rid the trench of it's water. Can these drains be lowered at all as the water level inside them is higher than the trench I'm digging and if not what other solutions does anyone have????
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Thanks for your advice and sorry for the long post
Cheers
Chris
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French Drain

Postby tim smedley » Sat Aug 04, 2007 7:14 pm

Chris,
I had a similar problem with a property in Bournemouth some years ago. We got around all the issues buy filling in the trench with large pebbles/cobbles from the beach - in those days you were allowed to hep yourself!
They allowed the trench to be as deep as we wanted (about 18") and provided support to the trench wall. Being large and irregular in shape there was always a good air gap between each one ensuring that the wall dried out.
With regards to the drains, if the pipes themselves are sound and the water is being retained within them, then this method also means you do not have to change anything as they cannot be seen.
It might be worth doing this first and monitoring the damp levels in the walls before considering a re-plumb, which may in itself cause a host of other issues that need resolving.
Tim
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Postby The_Laddie » Mon Aug 06, 2007 5:22 pm

Thanks for the reply Tim. What you've said was exactly what I set out to do but I then thought that it might just turn into a big pebbley moat instead of a drain which was when I decided I might be better to drain off the water into the soil pipe. The problem with that being that I couldn't just run a pipe into it because of the smell and secondly the level of the gulley drain is about the same level as the top of the ditch I dug so there was no simple way of just draining into it.

Cheers
Chris.
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Postby Lydia » Fri Aug 17, 2007 8:54 am

We had the same issue in our house - apart from someone had built the concrete drive to over the DPC level, so that it happily drained though our airbricks.

Ideally, you'd try and get whatever surface to come 150mm under the DPC, so as to avoid splahback - and pea shingle will help absorb the rain droplets. We could only get just over one brick-course down, but it seems to have done the trick.

We dug nearly 300mm down from the ground level as it was, and filled with 150mm pea shingle, retaining the border with a wood edging. Any pipework we encountered was carefully excavated around (but not underneath!). I think it's a question of finding the best solution possible and working with what you have. Any intervention will help.

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edited to say that our "trench" is about 200mm wide.

Also, you probably won't get much water in it at all, especially if your RWPs still drain away properly. It'll function in much the same as normal ground, but just at the right level for the house.

http://www.photobox.co.uk/album/album_f ... 1367892654
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Postby The_Laddie » Mon Aug 20, 2007 1:27 pm

Thanks for that Lydia. Like you say, anything is probably an improvement on the ground level being on or in places above the DPC. The only problem I am still pondering is that the ground has rubbish drainage properties, The soil is like clay and the water just sits on top.
At the present time the front garden is looking something more like a swimming pool after all the weekend rain :?

Cheers
Chris.
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