French Drains

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Using a French Drain to resolve a Damp Problem

We are finding where we are asked to look at damp problems in general (i.e. damp walls and floors) that commonly it is due to the external ground level being higher than the internal ground level. It could also be that air bricks have been blocked, or simply paving slabs, decking or briquettes have been used to form a patio area which then discharges any rainwater against the building. Quite often the solution is to add a French drain. Whilst French drains are quite simple and are basically nothing more than trenches filled with gravel (although there is a little bit more to them as we will explain), they are almost a DIY job for most people and they are relatively easy to install and are for the most part low cost. You do however need some care and attention when installing them. You could install what we have heard referred to as the “French pond”.

What use is a French Drain?

A French drain is a trench of approximately 6” or 150mm wide (or the width of your spade), approximately twice the depth (i.e. 12” or 300mm). In most cases this will suffice however where there is a large amount of ground water, you may wish to make the trench wider and deeper. A French drain acts as an area where water soaks away quickly. We often recommend them close to the building and not next to the building as this helps to reduce the ground level and it will take any water that is directed at that area away (for example as mentioned where a patio has been placed which aims any rainwater to part of the wall). As mentioned, whilst a French drain is a DIY job it does need some understand of how it works.

French Drains must be on a slope

The pipe that is at the base of a French drain should be perforated or as we did years ago for land drains, there should be gaps between each pipe which should be set onto a bed of firm ground and the pipes should be on a fall to the drain. Whilst you should be able to ensure that there is enough fall by site, we always like the idea of rolling a marble from one end to the other! You will then need to place the pipes down and fill the trench with 0.5” (7.5mm) to 1” (15mm) sized gravel. You can leave it at that, or in addition you can cover this with sand and then turf over it. This is how a basic French drain is carried out.

The French Drain System which we would recommend

The French drain system which we would recommend would be as described although we would add to the base an inch or two of gravel onto which the perforated drainage pipe would rest (the drainage pipe should be 4” (100mm) to 6” (150mm). We would then wrap around that drainage pipe a filter fabric. This is to stop the holes in the perforated pipe from blocking up! We would then add gravel around this and further fill with gravel. In addition to this, we would add a silt trap. This is added in the run of the pipe and is very similar to a road gully (not that this is of much use if you don't understand how a road gully works!). The silt trap is a rectangular box with a pipe opening at each end. The drain water passes into this. Any particles sink to the bottom of the box and then the water travels on to the other side of the box, enabling it to feed into a drain. These are usually made of glass reinforced polyester (it being available in this form since the mid 1980's) and then normally reinforced with a steel frame for additional strength and bedded in concrete.

The French Pond!

French drains will over time clog up, which is why we recommend using a filter fabric however even with this, they will eventually clog up. Unfortunately there is no Dyno Rod equivalent and it is normally fine sand organic matter or clay which clogs up the French drain. In this case it will have to be dug up and the pipe work will require cleaning (or it may be quicker to just replace it) adding a filter fabric and refilling the gravel.

You may also be interested in these other articles about dampness issues:

Condensation

Resolving Dampness in your Basement

Dampness in Buildings - Basics Article

Dampness in Buildings - Technical Article

Shared Freehold and Problems with Dampness

Dampness Defects Report

Dampness Problems

Condensation Problems

Damp Walls

Shared Freehold and Problems with Damp

Damp proof courses defined

Damp Proof Courses - What are these for?

Damp Proof Specialists - Talk to us First

How do Older Properties keep dry without a damp proof course?

Rising Damp and Independent Reports

Please see our section on:

Specific Defects Reports

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If you have a commercial property, be it leasehold or freehold, then you may wish to look at our Dilapidations Website at www.DilapsHelp.com and for Disputes go to our Disputes Help site www.DisputesHelp.com .

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