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 to control ground water in farmers fields, yet now we are seeing them installed around the base perimeter of houses.

 

These may well have originally 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 must always be installed away from the property because it's basically a circular perforated pipe sitting in a trench full of loose material which allows for the free-flow of water/moisture.

 

Today we're seeing more and more of them being 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 completely wrong thinking, and in fact is more damaging than doing nothing at all.

 

French drains should always be viewed as preventative measures other than corrective measures; in fact when installed incorrectly they can turn your house into a moated house riddled with continual interal damp issues. Whilst living in a moated castle will of course be a 'want' for many these days, a moated castle was built to be a moated castle; houses weren't.

 

The design of a modern French Drain allows water to run into the inner pipe.

 

This may be where the confusion lies because the inner perforated pipe alone is called a land drain which allows groundwater to enter through the perforations in that pipe, and then travel through the pipe itself to an appropriate discharge point, effectively diverting excess water away, long before it can get to the property itself, and create damp issues. 

 

Don't forget that the internal pipe has perforations around it's entire diameter, which obviously means that ground water collected from one side will also easily 'weep' back out through the opposite side and become trapped and held  against the base of the very walling it's supposed to be protecting, greatly reducing the chances of natural evaporation.

 

In order for that collected water to discharge again, the pipe must be installed with a constant 'fall' over it's length to an appropriate discharge point; for example a mains drain or soakaway but we're seeing more and more of them being installed almost level!

 

This results in constant damp and damaging hygroscopic salts in walls and floors, and even creating issues with the buildings structure and foundations, potentially leading to subsidence and cracking through the building where people have gotten carried away with their trench excavation depths.

 

The foundations of a building are designed to take and distribute the weight loadings, both dead and imposed, to ground.

 

People really do need to start understanding and accepting that buildings are in fact a huge mix of engineering and sciences, they are not 'assets' and they really are not '60 minute makeover' or  'DIY SOS' projects as portrayed by TV personalities on TV programs, which clearly, looking at the huge amount of complete nonsense on TV these days, are no more than profit making shows delivered by people with no knowledge of construction sciences and engineering.

 

Interestingly today, visiting a L3 qualified 'beauty technician' to have your nails clipped or eyebrows shaved is a 'must', whereas carrying out deeply scientific and structural engineering works, with no knowledge of either, on a home is simply something that anyone can do at the weekend, because they've watched the TV!