Are french drains a good thing?

[Karen]

Hi

The ground floor of my listed property was lowered many years ago. Outside, along the length of the building at the back is a patio. At the front is a pavement. On both sides the ground level is higher than the inside and is approx half way up the plinth. There is some damp, although it's not serious. I am not sure if I have the right to dig out at the front as I think the boundary of my property stops exactly where the pavement starts, however I can deal with the back wall.

Most of the people who've given me advice have suggested tanking - which I instinctively feel isn't the right solution. My inclination is either to lower the ground level outside and create a new patio or simply to get a channel dug and fill it with shingle - which I believe is known as a french drain? The latter solution is by far the cheaper but someone has told me that English Heritage now believes that these can cause problems. I haven't been able to speak to them yet, but I am not inclined to spend several thousand pounds if I don't need to and wondered if anyone could shed any light on what problems may be caused by french drains?

Thanks
Karen

 

[biffvernon]

Your ideas sound very sound. I'd try a French drain before attempting to lower a whole patio. Perhaps the patio could be made to stop just a foot short of the house and then dig out a little dry moat - a French drain but the top half filled with air so that the wall thinks the ground level is where it ought to be.
At the front is there any way to stop water running off the paving onto the wall and to stop rain splashing the wall? Sometimes you see the bottom foot of a wall of an old house next to a pavement has been painted with bitumen to prevent water getting in. Because the bitumen only goes up a little way it does not affect the breathability of the wall.
It would be interesting to know what negative comments English Heritage have made about French drains.

 

[PC]

I would'nt tank the wall internally if I were you, but would suggest the French Drain, however they can cause problems if not installed correctly.

So may people think that by digging a trench across the wall will solve the problem, dry or filled with shingle. The important thing (and Im sure this is obvious to most) is to ensure that the water can drain away either naturally or to a drainage system.

A lot of period buildings have no or very little foundation and excess water can sometimes create structural problems depending on the substrate.

I think that you could probably resolve the problem without having to lower the whole patio!

The front wall may not be so easy to resolve and you may need to speak to the local authority (who I assume owns the pavement) to see what they can suggest. Ive dealt with similar problems and in the past and if a major problem they will sometimes provide external vertical tanking, where they can't physically lower the ground levels.

Take further advice form a surveyor who knows the area.

PC

 

[Karen]

Many thanks for your helpful responses. I'll contact English Heritage about French drains, but my guess is that if they have any concerns they'll relate to the problems PC indicated.

Karen

 

[Gervase]

A proper French dran is invariably the answer to damp walls when the dmp isn;t coming from above.
By proper, as PC says, it needs to have a suitable 'fall' and somewhere for the water to end up - a sump, soakaway or a connection to an existing drain.
Ideally you'd put a length of perforated land-drain in the bottom, with a geotex covering and then shingle (nothing smaller than pea gravel) in top of that.
French drains are remarkable effective ways of getting rid of water.

 

[middi]

The only thing againt french drains is that unless you are extemely fit they give you a back problem.

 

[Ushy M]

We will need to dig a French drain on the gable end of our house where the external ground level is high due to it being a graveyard. I've seen several different construction methods and wondered whether others out there had views on the best approach.
The methods I've seen are:

a) dig trench, lay a perforated land drain wrapped in weedstop fabric in the bottom, then layers of cobbles graduating from large at the bottom to small at the top

b) dig trench, line whole thing with weedstop fabric, lay perforated land drain and cobbles/shingle as above, then wrap the weedstop over the top.

Any views or alternative approaches gratefully received.

P.S. This is my first post on the new site so sorry if it turns up in the wrong place or out of sequence!

 

[biffvernon]

I would not have thought the weedstop fabric was all that important. The drainage pipe will be below the level of roots of most plants and presumably you wont be allowing big things like elders and sycamores to establish themselves next to your house.

 

[Ushy M]

Biff, my understanding was that the weedstop (or equivalent) was there to stop the holes in the perforated drain pipe from clogging with soil/other debris, rather than actually stop weeds.

 

[Gervase]

That's right, the geotex is there to stop the fines from clogging up the land drain pipe.

 

[biffvernon]

Ah, yes, that makes sense.

 

[RobT]

Karen & umumford,

take a look at this document. It gives a decent account on how to build a french drain.

As stated in the document, ensure the trench walls are dug at 45 degrees to ensure the footings of your property don't subside. See below:

http://www.ihbc.org.uk/Technical Papers/French Drains.pdf

regards

Rob