French drain

[Woody3]

We're currently digging a french drain around the perimeter of our Cottage.

The internal FFL was approx. 100mm below the external FFL, so we dug a trench, by hand, about 200-250mm deep. By digging by hand you get a better "feel" with regards to ground conditions and as mentioned above, you are less likely to damage anything.

We've already noticed an improvement in the moisture content of the external walls too.

 

[Feltwell]

Zebra, how about using a soakaway crate system to form your trench? This sort of thing:-

http://www.drainageonline.co.uk/Soakaway-&-Geotextiles/Polystorm-Soakaway-Crate.htm

http://www.polypipe.com/polypipe/controller?action=PP-Sustain27

If you wrapped it in an appropriate geotextile, you wouldn't need the upright paving slabs on the graveyard side. These I believe are a closed unit and are designed to support pedestrian weights so you could possibly just leave them open at the top. I don't know that much about them, it may be worth you getting a brochure from Polypipe.

As an aside - my house & driveway are around 3' higher than my neighbour's ground floor level, the side of his house being next to my driveway. When we moved in there were (very manky) bushes and trees along this edge of the driveway, and loose lumps of rock sort of holding back the earth and leaving a bit of a gap to next door's house wall. When we re-did the driveway we took all the vegetation out, put a proper retaining wall in and took all the muck out of the bottom of the trench - leaving a nice clear trench about 18" wide, taking the ground level below the neighbour's floor level for the first time in many years. 2 years on, the neighbour commented recently how much his wall had now dried out as a result - and that's despite it being smothered externally in poor quality cement render.

 

[AMc]

Zebra - I can't answer your question myself but I'd be concerned that you'll seriously undermine the performance of the insulation above.

You may get additional help over at http://www.greenbuildingforum.co.uk/forum114/?CategoryID=4 as breathability and insulation are popular green topics. Some of the discussion can be 'robust' but there are a lot of experienced posters (like here really. Have a lurk around and see what you think. I have the same username but rarely post these days.

 

[Zebra]

We're currently digging a french drain around the perimeter of our Cottage.

The internal FFL was approx. 100mm below the external FFL, so we dug a trench, by hand, about 200-250mm deep. By digging by hand you get a better "feel" with regards to ground conditions and as mentioned above, you are less likely to damage anything.

We've already noticed an improvement in the moisture content of the external walls too.

Woody, what have you put inside your French drain?

Feltwell, I had thought of using those, but hadn't thought that you could then do away with the paving slabs! Good idea! Especially if the geotextile was only on the soil side so that wall would still have air circulating next to it. However, I wouldn't be able to have an open gully at the bottom because I wouldn't be able to get to it to clean it out. I could go back to the perforated drain pipe idea, with those on top perhaps. Yessss......

AMc, thanks, I will look on the Green Building Forum, I'm on there too with the same username, but I rarely go on there because they're all scary and not as funny as here.

 

[MdB]

AMc, thanks, I will look on the Green Building Forum, I'm on there too with the same username, but I rarely go on there because they're all scary and not as funny as here.

But they are green where as you are black and white. It is bound to create tension... Try posting as Kermit.

 

[Penners]

Try posting as Kermit.

Or (for our more mature readers) The Mekon.

 

[Feltwell]

Feltwell, I had thought of using those, but hadn't thought that you could then do away with the paving slabs! Good idea! Especially if the geotextile was only on the soil side so that wall would still have air circulating next to it. However, I wouldn't be able to have an open gully at the bottom because I wouldn't be able to get to it to clean it out. I could go back to the perforated drain pipe idea, with those on top perhaps. Yessss......

I don't quite get the perforated pipe idea, but you could lay that through the crates - I *think* they're designed with knockouts to take standard 110mm pipe.

For cleaning out, I reckon a leaf blower would do the job most of the time - just blow the leaves and light debris out to one end, no need to take anything apart. Cheap electric domestic "garden vacuums" are still quite powerful as leaf blowers.

 

[Zebra]

By perforated pipe I mean this stuff,
http://www.drainageonline.co.uk/Soakaway-%26-Geotextiles/100mm-x-25m-Perforated-Coiled-Drainage-Pipe.htm
or this stuff
http://www.drainageonline.co.uk/Soakaway-%26-Geotextiles/PVC-Perforated-Drainage-Pipe-110mm.htm.

Yes, perhaps it would be possible to clean it out without taking it apart. Perhaps even the 110mm pipe knockouts could be adapted into arm holes! I'll try to stop boring you all now with the intricate details of my French drain arrangement!

Actually, the green building forum is very helpful on this topic, so thanks AMc. As for being green though, look what happened to this chap!

 

[Feltwell]

By perforated pipe I mean this stuff

Slight misunderstanding - I know what perforated pipe is, I just can't quite see what the point of it is here. I presume you are having guttering on the house - unlike Paddy's thatched house - so is much water likely to get into this trench?

If the church land slopes significantly towards the trench then fair enough, some pipe in the bottom taking the surface water run-off to a soakaway may be a good idea. But if it's just whatever rain lands on the trench that needs to be disposed of, then surely it can just soak away to the base of the trench? I thought the main idea was to let air get to your wall rather than to actually act as a drain.

 

[Zebra]

Ah, that's interesting. (Sorry for misunderstanding, of course you know what perforated pipe is!) Much of what I've read so far about French drains says it's best if you can have a pipe in the bottom that takes any water that gathers there away. See for example the very first reply to my original post on page 1 of this thread!

Yes there is guttering, and you're right, not a great quantity of water is likely to get into this trench. The church land is pretty level, and it is only the ground water that seeps in sideways from the adjacent land. The recent snowmelt for example gave me a nice wet patch indoors.

What you say is interesting because I have myself wondered whether the pipe is doing any good in my existing arrangement. I can't tell from seeing what comes out of the other end of the pipe, because the gutter downpipe joins with it before the outlet. I have no idea whether any of the water coming through is from the ground, or whether it's all from the gutters.

Having said that however, I have read somewhere that one of the main risks of having a French drain installed right against the perimeter, is that it introduces water - not just moisture, but actual liquid - straight against the foundations. I'm on quite well-draining land, and it may be that any water that seeps in from the surrounding land is going straight past the pipe and into the soil anyway and not doing any harm to my foundations at all. But if it is sitting at the bottom of the trench, it would be best to get rid of it.

Just to complicate things, I have a small section of roof which never used to have any guttering on it, it just used to run any water into the brick wall with no overhang. I've had the roof redone, put an overhang on it, and a bit of guttering. It's at the far end of the French drain, and to avoid a messy, long diagonal downpipe run, I've sent that water through the French drain. To stop it leaking out into the bottom of the trench, I made a specially modified perforated pipe out of plastic terracotta underground pipe, that doesn't have any holes on the very bottom. I've tested it by pouring a measured amount of water into the guttering and measuring what comes out at the other end. All but a dribble of it does.

Is anyone still reading, or have you all gone to find more interesting threads? Would it help to show a picture of the back of the house?
The blue tarp is the roof that's now been done, and the mini-guttering on there goes through the drain.

So perhaps if I didn't have a pipe in the bottom at all, I could just take the gravel out, stick the crates in, and divert the small gutter downpipe to join up with the main gutters, perhaps just below ground level inside the trench.

Just to complicate things again, to avoid the water from the main gutter backwashing up the French drain where they join, I ran a smaller flexible hose attached to the main downpipe, inside the underground pipe that takes the water away. It seems to work, but it is all a bit weird and of my own invention! I could replace this with a proper trap/gully perhaps, where the French drain and gutter downpipes join.

Or perhaps I could leave the drainage situation as it is, and simply take the gravel out and put crates in.

Perhaps I should just do something and stop thinking so much about it!!!!

 

[Feltwell]

Perhaps I should just do something and stop thinking so much about it!!!!

That tends to be my affliction as well

If you've already got the perforated pipe in then I can't see that it does any harm, even if it doesn't do any good. Snow melt is an interesting point though - the water will always try and find the easiest route, so melting snow trying to all drain away via your trench - when the ground everywhere else is frozen and not absorbing much - is a real danger, so perhaps better to have the pipe.

How to do it I guess in part it depends on the levels. If you used the crates, how would the bottom of the crate relate to your internal floor level? That could be the decider on if you put the pipe under the crates, surrounded by gravel and below your interior floor level, or inside the crates. The "pipe in a pipe" arrangement is unusual, the only downside I can see is that you are introducing an obstruction within the main pipe which debris could eventually back up behind. I guess the "belt and braces" approach would be the crates wrapped in geotextile on the churchyard side only, going down below your interior floor level, sitting on a bed of gravel with your perforated pipe in.

In reality there are probably 101 different ways of doing it, each with their advantages and disadvantages!

Anyone still awake?

 

[Penners]

 

[Zebra]

Gosh, I've just been over to the Green Building Forum. There they've got even MORE complicated ideas! Wing insulation! XPS vs EPS! Backfilling with leca.......
Yes I see what you mean by introducing a blockage. Perhaps not the best idea. Perhaps I'll put in a gully instead. Look at that, the afternoon's gone now!!

 

[Penners]

Look at that, the afternoon's gone now!!

Which means you don't have to do any real work. Well engineered!

 

[Zebra]

The water won't get through the wall now HA HA HA HA HA HA HA !!!!!!!!



Sudden change of plan. No backfill, no gravel or stones, no paving slabs to hold the ground away, and Hertfordshire milk crates are safe! Thank you all for your advice and ideas. Feltwell I may still use your grating plan - that's as in plan for grating, not annoying idea - as I will need to cover the trench with something.

But for now the wall will be drying, joy of joys!

Incidentally, the builder started today on the side extension, and digging out the ground for the foundation, he's found that I'm built on a kind of solid compressed sand, extremely hard, and very well draining. All of yesterday's torrential rain is no longer in the soil. So he doesn't reckon I need any way of draining the water from the bottom of the trench at all, and I can just leave it open at the bottom like it is now, it is effectively lowering the ground levels to below internal level.

 

[MdB]

I love how you have clay (bricks) on top of sandstone where as I have sandstone on top of clay. I know which I would prefer :-(

It looks like you are doing the right thing anyway.

 

[Feltwell]

Looks like a good job. That's effectively what we've got between our driveway and next door's house wall, except ours is wider. The bottom of it is mostly concrete as the neighbour's drains run under it - we just left the bit we broke out to build the new retaining wall. That very narrow strip, no more than 2" wide, has been enough to keep it dry. There is definitely no surface water run off into it though - just whatever rain lands in it or runs down next door's wall. It's actually coped fine when next doors gully has blocked a couple of times, and a bathfull of water has flowed into it - and our soil is not that free-draining.

Since doing the job the neighbour's wall has dried out quite nicely, despite being cement rendered.

A grating on top would help support the new retaining wall if the "downstand" idea is done, but it might not be necessary from that point of view, although it will of course stop anyone falling into it and keep some debris out. Is there a way that wildlife can get out of your trench? We deliberately left an "escape" gap at one end that was about hedgehog sized.

Yes - I am a big softie.

 

[Penners]

That looks like an excellent job, Zebra. Well done!

Have you made any provision for draining away water that falls into the channel? If I'd had that trench around my house over the past 36 hours, given the amount of water that's fallen on Suffolk it would now be a moat!

 

[Lime]

........
the builder started today on the side extension, and digging out the ground for the foundation, he's found that I'm built on a kind of solid compressed sand, extremely hard, and very well draining.

That sounds like silt.

 

[Zebra]

Thank you all for your kind words.

Have you made any provision for draining away water that falls into the channel? If I'd had that trench around my house over the past 36 hours, given the amount of water that's fallen on Suffolk it would now be a moat!

I had originally planned to do so - a gully in the bottom to join up with a chamber for the gutter downpipes. It may not be necessary though, the latest thinking is that it will drain away all by itself, hopefully I will get a chance to assess this next time it rains. Similar to Feltwell's situation, there will be no surface water running into the trench, just what falls there and what drips off the drip bead of the lime render above. And I'm on quite high ground, with well-draining soil. We shall see!

Is there a way that wildlife can get out of your trench? We deliberately left an "escape" gap at one end that was about hedgehog sized.
Yes - I am a big softie.

Oh gosh no, I didn't think of that. A ramp at the upper end perhaps? I'm a big softie too, I was rescuing worms whilst I was digging!