If you live on a road comprised mainly of period houses, next time you are walking down your street, have a look at where the ground meets the walls of the buildings. You will probably notice that some of the properties have a gravel filled trench at the base of the walls.
This is a French Drain and its purpose is to disperse water that would otherwise soak into (and up) the wall. It also depressurizes moisture bearing against the base of the wall and will enhance drainage especially if the channel is linked to a suitable discharge point.
Contrary to what most damp proofing companies will tell you, moisture by-passing damp proof courses is normally due to building defects – most commonly high external ground levels which have been raised over time until they are less than 150mm (two brick courses) lower than the level of your DPC. This depth is suggested as a good building criteria in order to prevent bridging and to avert rainwater from splashing back over the damp proof course. Installing a French Drain to the correct specification is an effective method used to relieve the DPC where ground levels may be compromising it thus allowing it to function as originally intended.
How to Install a French Drain
Firstly, locate the damp proof course. You should be able to identify this as a material (most likely slate, bitumen felt or plastic membrane) sandwiched in between one of the lower brick courses.
When located a channel should be cut circa 150mm back from the peripheral walls and excavated downwardly circa 150mm below the damp proof course. Do not disturb the footings and watch out for pipes and cables.
The channel should then be back-filled using pea-shingle (but not above the damp proof course) to depressurize moisture bearing against the base of the wall and enhance drainage. Using pea-shingle removes the effects of rainwater splash-back as it is not a hard dense material.
Stop Earth ‘Bleeding’ Into Channel
Lining the channel with a geotextile membrane may be preferential so as to prevent soil from bleeding back into the gravel.
If the depth between the newly installed pea-shingle and retained ground levels is so great that it creates a trip hazard, then large stones sourced from a garden center can be loosely laid over the pea-shingle.
In my experience, raised external ground levels bridging the damp proof course are one of the most common causes of dampness in houses. Unfortunately, it is frequently misdiagnosed as rising damp, resulting in a prescription for treatment that is costly, invasive and unnecessary.