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French drains explained: How they work (and why you need a professional)

How french drains work

French Drains are often recommended for heritage properties as a way to help control damp. They work by redirecting subsoil and surface water away from your property, with the help of gravity. French drains are installed near the building edge, generally externally but sometimes internally.

One of the most common reasons for installing a French Drain is damp within the body of an external wall, which often appears as rising damp. This can lead to damp and damaged plaster, stonework or brickwork and rotten wood.

Taking their name from 19th century judge and farmer, Henry French, rather than the country, a French drain is a type of land drainage, comprising a perforated pipe within a back-filled shingle trench, around 600mm wide. This is generally 100-150mm diameter, and installed at a 1:80-1:200 fall (slope).

To minimise clogging, the gravel size varies from coarse at the centre to finer at the outside. It is also advisable to include rodding points, for easy clearance of any blockages. When you backfill the trench, the aggregate should be compacted approximately every 150mm to minimise settlement of the reinstatement, and consequent ground movement near your foundations.

Line the Drain With Membrane to Minimise Blockage

Nowadays, people often use a geotextile filter membrane to line the trench, to help prevent sediment build up. French drains are used for intercepting and collecting water to reduce saturation near foundations, and for discharging water (such as a soakaway for discharging groundwater).

If there’s a lot of water to deal with, you may want to widen your French drain or use multiple pipes for extra water removal, and to provide an element of redundancy.

Once your French drain is installed, maintenance is essential. Don’t let it get blocked or you could end up with a moat around your building: great if you’re looking to protect it from invaders – and not so great if you’re trying to fight damp.

Is a French Drain Right For You?

You’ll need expert advice when it comes to French drain design for your home. It’s important that you avoid any cabling, pipes and other installations when fitting a French drain. In the UK, French drains must be designed and specified by civil engineering consultants or other engineering professionals, under the Code for Sustainable Homes and approved under Building Regulations Part H.

UK local authorities generally require evidence of a professional design, including tests and calculations to show that it conforms to local and national standards. If you install drainage works without competent design and approval, you may be liable for legal action, and invalidate your building insurance policy for flood damage.

The Structural Impact of French Drains

In addition, it’s worth considering the archaeological implications of disturbing the land close to your building’s foundations, particularly if it’s a heritage building. In older buildings, particularly medieval ones, the walls are only founded about 150 mm below ground level. Carrying out an initial trial excavation can help you ascertain the depth of the foundations, and what you are dealing with. If your home is listed, you may also need consent prior to starting excavation, in addition to permission for discharging water collected by the drain.

A French drain can cause structural damage if you’re not careful, particularly if you live in an old building with shallow foundations. To minimise this risk, a French drain should be positioned at least a metre away from walls, rather than directly next to them (though this may minimise its benefits for treating rising damp). You should also be aware that changing the flow of water may have knock-on effects elsewhere, and if the ground is already free draining, you may not gain much from installing a French drain.

When installing a French drain, it should be part of a complete waterproofing system. However, a French drain isn’t always the most effective way to treat damp. Open drains may be preferable if ground levels have risen around a building and moisture needs to evaporate from the bottom of walls. If your damp problem is severe, there may be other more effective solutions. You may want to consider installing a damp proof course instead. As such, you need to consult an expert to identify the source of damp problems before installation.

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