French Drains appear to have become quite the trend again these days, but we're seeing so many installed incorrectly and in the wrong places. Here we discuss the advantages and disadvantages of French Drains and how they can often cause problems to your property, if installed incorrectly.
What is a French Drain?
The earliest forms of French drains were simple ditches or trenches that were pitched from a high area to a lower area and filled with gravel.
They were Predominantly used on farms.
These may have been invented in France, but Henry Flagg French (1813–1885) of Concord, Massachusetts described and popularized them in his book titled 'Farm Drainage' published in, 1859, so whether their name derived from Mr French's book is still debated.
Sometimes also referred to as Trench Drains because a French Drain consists of a perforated pipe sitting in a trench filled with gravel or shingle that redirects ground water away from your property, so it should always be installed away from your property because it's basically a perforated pipe sitting in a trench full of material that allows the free-flow of water/moisture.
Today we see more and more of them installed along the base perimeter of external walls in buildings by people thinking that they will prevent water from accumulating near the base of those external walls;
Unfortunately this is wrong thinking.
The design of a French Drain allows water to run into the pipe, through perforations in that pipe, and flow through the pipe itself to a discharge point, effectively managing excess water, but given that the pipe has perforations around it's entire diameter, water collected from one side can also very easily 'weep' through the opposite side and then become trapped tight against the base of the very walling it's supposedly protecting due to the gravel/shingle filled trench that the pipe sits in, aiding the escape of that water from the pipe, and reducing the chances of natural evaporation.
In order for that collected water to discharge again, the pipe needs to be installed with a constant 'fall' over it's length, but we're seeing more and more of them having been installed almost level.
Very often this causes damp and salts issues in walls, where there never were any issues before!
Benefits of a French Drain
Installing a French Drain correctly can help to protect your property from water damage by redirecting ground water away from your property whilst also reducing hydrostatic pressure a long time before it can reach your property. It can also help prevent soil erosion caused by excess water. Additionally, a French Drain can be installed directly behind the foundations of an exterior retaining wall, before backfilling, to reduce hydrostatic and ground pressure on that wall by providing additional water management.
French Drain vs Land Drain
We think this is where the confusion starts because a French Drain (derived from farming) is designed to divert gound (land) water away through a perforated pipe, reducing hydrostatic pressure in the process, whereas a Land Drain (often incorrectly referred to as an Aco drain (Aco is just one manufacturer of land drains, in the same way that a vacuum cleaner is not a hoover)) is designed to take water away to a safe discharge point though a non-perforated channel; very different.
When considering drainage solutions for your property, it's essential to understand the differences between a French Drain and a Land Drain. While both systems are designed to manage water runoff, a French Drain is typically used for larger areas of land and should be installed a good distance away from the building itself.
A Land Drain, on the other hand, is a more discreet option with a non-perforated (non weepy) channel and can be installed, bedded in an appropriate lime based mix, directly against the walls of a building itself, even comfortably taking rainwater downpipe discharge without the risk of causing damp problems to the walling of the property.
Both of these systems must ultimately discharge into a mains drainage system, a septic tank, or a soakaway again dug a good distance away from the building.
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