Water always flows downhill and by the easiest possible route, is a well known scientific fact!   A slightly sloped trench filled with round gravel and perforated pipe used to divert underground water away from your house is the basic concept of a French drain.  This is a common drainage system primarily used to prevent ground water from penetrating or damaging building foundations by providing an easy channel allowing water to flow.

A French Drain has nothing to do with the country France!   It was designed and promoted by Henry French, an American judge and farmer from Concord, Massachusetts in his book in 1859 promoting farm drainage.  French´s drains were made with clay tiles, but modern drains use 4-inch diameter plastic pipes.   If you live on a slope and have a persistently wet basement or soggy lawn, a French drain could be the solution.

The spaces between the round gravel and perforated pipe at the bottom of the trench, which should be sloped, are available for the surface and subsurface water to run through.   Water can then travel freely through the pipe, emptying a safe distance from the house.

If there is a problem with surface water – a soggy lawn for example, or a driveway that continually has water rushing across it – then a shallow French drain or curtain drain is useful.   This type of drain need not be very deep, just sufficient to divert water.

You may need a deep French drain if the shallow drain doesn´t keep water out of your basement.  This is also called a footing drain and runs round the perimeter of the house at footing level.  This is difficult and expensive should it be necessary to add later but can easily be installed during the house construction.

A French drain is dependent on gravity.  

A property constructed on level ground may need to have collected water piped to a basin in the basement where a sump pump can lift it and send it to the storm drain system.

If you can´t keep water out of your basement, you can also build an interior French drain to intercept water where it comes in.   Install a perforated pipe all the way round the perimeter of the basement floor after cutting a channel and chipping out the concrete. A collection tank sunk into the floor receives the water via a solid pipe and a pump sends it out to the yard or storm drain.

Incorporate a French drain behind the first course of stones or blocks if you are building a retaining wall on a hillside.   The wall could be undermined of even tipped by water moving down the hill or building up behind the wall.  Drape landscape cloth across the base or footing and up the slope to avoid clogging with silt, before adding the pipe and drain gravel.  Nearing the top of the wall, fold the cloth over the top of the gravel and cover with several inches of soil.

French drains in Spain are a common occurrance due to the dry, hard ground and flash flooding makes them a necessity.

One French drain specialist in Spain is M3BT who are an establised construction company working with the major insurance companies throughout Spain.