French Drain

[IanR]

38 minutes ago, Onoff said:

 

Even the geotextile? What's the answer then on clay....line the whole trench first?

 

My trenches were fully lined with a geotextile membrane, and I'm on clay.

 

[Marvin]

On my renovation I installed 80mm pipe in a pea shingle trench wrapped in geotextile. My narrow trench was against the external wall all around the building. I didn't put any soil on the top, just wrapped the shingle completely round and just added 2 inches of shingle on top for show. The shingle sits between the external wall and the block paving. 

We have timber joist floors and the top of the pipe lies just below the top of the floor slab. As there is no soil between the external wall and the geotextile I feel it won't easily clog whilst I feel the geotextile will keep the clay fines out of the shingle on the paving side.

 

Good luck

 

M

[joe90]

We have a very high water table and we back filled the foundation trench against the block wall of both the house and garage with 50mm drainage stone and lain a pipe out from it into a road drainage ditch, never had a problem since. I consulted a field drainage guy as our field needs drainage and he says fir my location (heavy clay, used to be a moor), perforated pipe and membranes all clog with silt/clay so they only used drainage stone in a trench with a plastic sheet above to stop top soil washing down into the trench.(it’s on my “to do” list).

[Adsibob]

The minimum recommended depth on pavingexpert.com for a french drain is 550mm (450mm to the bed, then a 100mm bed):

 

We are trying to fit the drain along the outer perimeter of the house, to keep the base of the walls dry. We realised today that we hit the first corbel at about 250mm below ground level. So the options are to either have a much shallower french drain (which I think will really be too shallow given it will be less than half the recommended minimum of 550mm) or to locate the pipe about 180mm or possibly 250mm further away from the house, which will allow us to go deeper (but probably still not as deep as 550mm) and for the bottom of the trench on the side of the house to be stepped to accommodate the corbels. Any ideas on which option will work best? Or should I give up on a french drain altogether and fit something more shallow, like a linear channel drain shown here?

 

[markc]

If you have shallow brick strip foundations, why do you want or feel you need to add a french drain? - the drain excavations can/will compromise the foundation and removing water will affect the soil underneath

[Adsibob]

34 minutes ago, markc said:

If you have shallow brick strip foundations, why do you want or feel you need to add a french drain? - the drain excavations can/will compromise the foundation and removing water will affect the soil underneath

it's okay, we have just spend a small fortune underpinning those foundations. The wall is damp and the damp surveyor (who is a RICS surveyor) recommended that part of the damp mitigation strategy was to improve the drainage of the walls. For the last 20 years or so, it's effectively had no drainage because the previous owners messed about so much).

But maybe a shallow trench filled with pea shingle is sufficient for these purposes. 

[joe90]

26 minutes ago, Adsibob said:

But maybe a shallow trench filled with pea shingle is sufficient for these purposes. 

As long as you can get any water away from the stone/drain.