A French drain usually consists of a trench filled with gravel and is intended to redirect water. If water runs down a slope on your property and collects somewhere undesirable then a French drain is a perfect solution.
You need to find somewhere appropriate to direct the water to, this should NOT be someone else’s property. Often there is no desirable place in your garden to flood so it is likely that you will have to choose the ‘lesser of two evils’ when picking a location. Remember that the drain will need a slight slope to carry the water away effectively. A gradient of 1% is often used for French drains.
When you have found a location for your drain where it will be able to direct water to a safe location you will need to dig the trench. To ensure that you are digging to the correct depth you will need create a reference point. Place two wooden posts at either end of the trench and string some mason’s line between the two posts. The line needs to be taut and level. Use a spirit level to check that the line is even.
Dig a trench between the two posts. A typical French drain would be around 15 cm wide although you can make it wider than this if you feel that you need a greater amount of drainage. The drain should be around 15cm deep as well and should be sloped. Use the mason’s line to check that you are digging to the right depth remembering that you should be adding extra depth as you progress along the trench.
When the trench is finished, line it with landscape fabric and then fill with gravel. Fold the top of the landscape sheet over the gravel, effectively making a tube of gravel along the length of the trench. Cover this with a layer of coarse sand and then a sheet of landscape fabric. Finally cover this with a layer of topsoil and grass if necessary.
Your drain will not be visible but water should drain through it. If done correctly you will have stopped large amounts of water from rolling down onto your property.