Tips for French Drains

Drying out your home starts at the foundations

Moisture is always on the move in solid walled buildings. We are constantly asking our houses to cope with water from cooking, breathing, rainfall, soil etc. Using nature to help us cope with this is the best way of managing these pressures.

One of the ways of reducing the pressure on walls, especially around the ground, is to give the walls an opportunity to release the water before it starts to cause internal damp issues. Ensuring that you have at least 150mm (6 inches) between the internal floor level and external ground level is really important. If your house doesn’t have this then you would be well advised to provide a well drained area around the wall and foundations.

Putting large stones / gravels around the base of the wall (especially when combined with a perforated drainage pipe) has two effects: 

1. It allows rainwater to drain away more easily into the surrounding soil / drains

2. It allows air to circulate next to the wall and hence help dry it

So it is a good idea to have a drain around a solid walled building (as long as it doesn’t affect the foundations). However, digging this out is a bit of a job and the last thing you want is to see it get compromised by an influx of small particles (dust, soil etc). This will just slowly clog up the system and eventually reduce the effectiveness of the system. So is there a way of stopping this from happening?

Thankfully the use of commonly available geo-membranes can help to stop dirt from getting into the system. So after you have dug out the trench (and potentially installed a perforated drainpipe) line it with some garden geomembrane (we would recommend a heavy guage system) so that the channel is lined along the bottom and up the earth side of the channel (allowing enough over to cover the channel and up the wall 10cm once filled). Then fill up the channel with large stones / old bricks etc almost to the level that you want. Once done then fold the geomembrane over the stones. The membrane is then held in place with smaller gravel that is spread over the top.

By having a slight lip on the geomembrane next to the wall means that if you need to do something to the drain then you can pull this back and not risk dropping the chippings into the channel itself.

Happy digging!