French drain

[Duchess]

Oh my lord!!! Today has been firstly quite scary...then unbelievable! So I have been in my ill cottage for about four weeks and we have had some unbelievable rain this weekend, before we moved in the surveyor recommended the whole front to be dug out and dropped by about a foot, a second surveyor said this is nonsense you just need to make the French drain at the front of the property wider and the damp problem would be resolved, opened the door this morning to two inches of sitting water outside the front door! Hubby has dug out the French drain to find the problem, the down pipe from the guttering has a hole cut into it to try and drain the water from the French drain!!this has prob been like this since 2008 when the previous owner had the front paved....my poor house

 

[MatthewC]

It is quite important that a French drain does actually drain somewhere! Does the rainwater and/or the French drain go to a soakaway? Personally, I think that if the lie of the ground permits, then you're much better off digging out the front and solving it once and for all. Either way, it sounds as though you will have a real digging job to do somewhen.

 

[Moo]

I'm curious about soakaways.
Our builder has suggested one as a possible solution to our very waterlogged side return and back of the house (a classic case of ground level over 12" too high + a very high water table).
However, I'd have thought that every soakaway must have a finite capacity, and wonder where the rest of the water goes during a period of prolonged heavy rain. Surely it's safer for it to be channelled into a proper drain?

 

[AMc]

Modern soakaways using plastic storm crates wrapped in geotextile have large capacity to deal with a flash downpour in a comparatively small volume
Detailed description here ...
http://www.pavingexpert.com/drain13.htm

The problem with directing rainwater into proper drains is fresh water is dumped into the foul water processing chain, this is increasingly problematic in urban areas where lots of gardens have been paved over for parking and the sewage system is overwhelmed during heavy rain. It reduces the rainwater replenishing the natural water table. In most cases it's not permitted to divert water into the sewage system if you make changes to your drainage, and driveways are now generally required to be permeable rather than drain into the main sewers.

I ended up with 8 storm crates which the builder organised on recommendation from building control as they decided together my rainwater was going into the drains. When I showed them the downpipes going to a soakaway under the back garden they agreed I didn't need them after all. The 8 crates are now redundant, I'd love to use them under the drive but they're not vehicle rated and the builders used them as saw horses and platforms so they can't be returned. I keep meaning to eBay them but it's such a faff

 

[Moo]

Hmm. Im not sure it would be feasible in our case. For a start we'd need to dig out a skip's worth of soil, and it would seem that craggy London clay and a very high water table mean that it probably wouldn't work anyway.
Thanks for the link, though; a site like that helps us to make what I believe is called 'an informed decision'.
However, the PavingExpert site has got lots of useful stuff about linear drainage, which I think will end up being our preferred course of action.

 

[Flyfisher]

That Paving Expert site has a lot of good information.

Soakaways are more of a buffer really. They have a finite capacity to store a downpour but an infinite capacity to allow the contents to, er, soak away over a longer period of time, though as you rightly note the ground conditions will have a big effect on the ultimate drainage rate.

AMc is right about problems with diverting all surface water into the drains and also about the modern solution of 'milk crates' and geo-textiles to construct modern soakaways. I believe there are various regulations for new areas of hardstanding in built-up areas these days and discharge of surface water into the mains sewers is no longer allowed.

I think the basic problem with soakaways is their tendency to silt up after many years, which is certainly what has seemed to happen to all of ours and is why I've laid a lot of new surface water drains. But we're fortunate to be able to send all this water into a large pond which acts a a soakaway buffer before gradually releasing the increased pond level into the river. I'm sure that modern geotextiles would reduce silting up so that modern soakaways would last a lot longer than they used to.

When I fixed the guttering to one face of our outbuilding roof I decided to divert the water into a 40 gallon water butt. During one downpour I was amazed that the empty water butt was overflowing within a matter of minutes - it really is amazing how much water a roof can collect - so I added one of those downpipe diverter gizmos for the water butt and let the surplus go into the new surface water drains.

I mention that by way of stressing the importance of correctly sizing a soakaway so that it doesn't fill up and back up. We had one downpipe gulley that regularly overflowed, dumping loads of water at the base of the house wall. I cleared out the gully but it still regularly overflowed so I assumed the soakaway was all silted up. During some subsequent digging, I discovered the soakaway was only a couple of metres from the house:



From the brick and concrete construction I guess it is relatively modern - well decades rather than centuries old. There was no base to the hole and, presumably, the collected water simply filtered away through the chalk on which we sit. You can see from the photo that it was most certainly not silted up so I can only guess that it was simply too small for the roof area it was serving and would quickly fill up during heavy downpours.

 

[Moo]

Your water butt story rings bells! Some previous owner had installed a huge butt at the bottom of a drainpipe, and during wet weather there was a constant cascade of water as it overflowed on to the concrete path (12+inches above the damp course), so the poor little house had its feet in a almost permanent puddle.
There was a slow leak at the tap, and it quickly emptied the butt. Then, after an overnight downpour, the tank was overflowing again! As you say, it's amazing how much water runs off a roof.
The builders have been busy removing the concrete and digging out the soil. They rapidly hit solid, saturated clay.
This is proving a fascinating project - lots of interesting problems to solve.

 

[Duchess]

We also have a problem with roof rain water...it currently runs into our septic tank.......all of it! God this poor house needs some common sense to sort it out, the previous owner must of been as thick as three planks! I was thinking of installing a butt, but I dont think I'll bother now after reading your story's, hubby has already dug the trench to run the water pipe into, but at the moment we are wondering to run it straight into a ditch that runs along the back of the garden or build a soakaway.......

 

[Duchess]

And Matthew I have no idea where the pipe runs to the front has been paved and then there is a tarmac'd road with no sign of where it drains, on the opposite side of the road is another ditch so I am blooming hoping that is where it drains to :?:

 

[Keith Bowman]

And Matthew I have no idea where the pipe runs to the front has been paved and then there is a tarmac'd road with no sign of where it drains, on the opposite side of the road is another ditch so I am blooming hoping that is where it drains to :?:

If you really, really want to know where it goes, you need to put some tracer colouring stuff in it and watch to see where it appears. You might prefer not to know!

 

[Keith Bowman]

And as a PS, our house septic tank used to take all the effluent and yard run-off from a 50 sow pig unit (that's 50 big pigs and all their many, many little pigs!) Surprisingly enough, it coped.

 

[worms]

And as a PS, our house septic tank used to take all the effluent and yard run-off from a 50 sow pig unit (that's 50 big pigs and all their many, many little pigs!) Surprisingly enough, it coped.

:shock: You should have run it as a biogas plant!

 

[Keith Bowman]

It was before we bought the house, and that was 35 years ago. The septic tank is still working well though - it is a very big tank!

 

[Keith Bowman]

Actually, I was trying to reassure the Duchess that if her septic tank is working OK with the roof rainwater, she needn't get too stressed about it. Just something to put on the long list of things to do in due course.

 

[AMc]

Thinking about rainfall I decided to see just how much water falls on my roof.

Apparently 1mm of recorded rainfall equates to 1 litre of rain over 1 square metre.
The footprint of the house is approximately 230m2
The UK average rainfall for 2011 was 1172.5mm (Metoffice 2011 annual averages)

230m2 x 1172.5mm = 269,675 litres of water in 2011.

BUT I live in East Anglia which has lower than average rainfall, 453.7mm in 2011.

230m2 x 453.7mm = 104,351 litres of water per year.
Or about 104 cubic metres.
So a tank of 10m by 10m by 1.5m deep would be enough to store all my rainfall for a year.

According to the Telegraph there are 3,750,000 litres of water in each 50m pool at the Olympic park (the non SI standard way to account for lots of liquid).
That means it would take almost 36 years to fill and Olympic swimming pool from my roof, probably not worth bothering .

(I wouldn't be surprised if there are holes in my maths or research but it was an interesting exercise)

 

[Duchess]

I quite agree Keith, maybe I don't want to know! as long as its not underneath the house that will do me......and the tank has coped with all this rainfall, I just think hubby wanted to dig a hole :|