French drain – professional design

A French drain is essentially a trench which is designed and constructed with free draining granular material.

French drains have often not been designed or constructed correctly in the past, which has resulted in the UK having a considerable legacy of flooding claims.

The Association of British Insurers claim records show that a failure in such localised soakaway measures, such as French drains – is the most common flooding related claim in the UK!french drain design

The following equation provides a useful starting point for french drain design consideration:

Runoff or UKWIR

Do I need a french drain to be professionally designed by a civil engineering consultant? Can I do it DIY?

 Soakaway / french drain arrangements can appear simple and they are certainly easy to do wrong.

In the UK, french drains are required to be designed by civil engineering consultants or other hydraulic/engineering professional under the Code for Sustainable Homes or other similar codes. They are typically installed as drainage works support the discharge from buildings, as such they need to be designed and approved under the Building Regulations part H. Building Control check the design against the standards, their role is not to produce the civil design. Typically such work will also be covered by a planning condition. Similar to Building Control, UK Local Authorities typically require evidence of a professional design including tests and calculations fully to the national and local standards. Typically the LA planning officer will forward the design to their inhouse or external civil engineering consultancy team for agreement prior to discharging planning conditions.

If drainage works are installed by builders or others without competent design and approval, then not only is a client open to possible legal action, but they effectively invalidate their building insurance policy for flood damage.

The standard process in the UK is for a professional design and specification with supporting hydrological commentary and calculations to support a proposal. Most Local Authorities and Building Control Officers will require evidence that there design and calculations have been undertaken, prior to approval of conditions being discharged.

A french drain is used for the:

  • intercepting and collection of water – such as to reduce saturation near foundations
  • for the discharge of water. – such as a soakaway to discharge groundwater

If you get it wrong you could flood your property, or even worse someone else’s property. It’s likely your insurers will not pay for any  resulting flood damage. Or the legal costs if you flood others which can be considerable. The Building Regulations require designs  and calculations to be approved.

 In conclusion – Soakaway design is not a DIY job. You are liable for your actions. It is not like laying a patio or wall  papering.

How can a French drain be constructed?

A french drain is usually formed by excavating through the ground and backfilling with a single size coarse stone. Beach gravels or concrete rejects are common in the UK. It is usually a long trench, but not wide (approximately an excavator bucket width of 0.6m). The depth, depends on numerous factors, but is usually between 0.5m and 3m. The length all depends on the water to be collected or discharged and the design standard.

It is common for a porous drain / pipe to be installed within the french drain. Its location depends on if you are collecting, or discharging water. Such a drain is usually UPVC (plastic) or clay.

A key element in french drains is the voids naturally created within the granular material. The void provide water storage and easy transfer of flows. However, over time, silt (or fine sediment) will fill up these voids. This will reduce performance and ultimately lead to failure/flooding. One then needs to excavate the french drain and rebuild it all again.

To reduce such a major cost and disruption other techniques should be used. These include:

  • rodding points,
  • silt traps etc
  • concrete chambers

These provide a means of jetting the drain pipe.

Geotextiles are also used to avoid silts from the surrounding ground getting into the trench. A non-woven geotextile should always be used. Often a contractor will substitute a woven geotextile for a non-woven. This is because most either genuinely don’t know much, if anything, about geotextiles, because they may be cheaper, or because that’s what they used on the last job and no-one said anything.

French drain and concrete ring soakaway


The ground-worker / builder says they install french drains and soakaways “all the time”, is that OK?

You employ a civil engineering consultant to design, specify and advise / obtain the necessary consents. Their role is to provide professional advice to help you achieve your objectives, be they low costs, sustainability, minimal maintenance etc.

A ground-workers job is to build a french drain or soakaway from these designs. A contractor and a designer have completely different legal liability.

Ask them these questions:

  • What professional design liability insurance do you have? (Not public liability). Most ground-workers will have none.
  • Are you qualified to design french drains / soakaways? Can you produce the calculations to comply with the regulations?
  • Are you going to secure written permission from building control, the Environment Agency or the Council?

From next year all shared soakaways should be professionally designed such and adopted by new Suds Adoption Authorities – usually your Unitary or Highway Authority. It is hoped that this will finally stamp out the cowboy builders doing such work.

You also have to consider motives. A consultant can concentrate on designing for you the most appropriate solution. Ground-workers often see a soakaway as a way of getting rid of material. Soon it’s all buried and out of sight. By the time you find you have long terms flooding and settlement problems they are off the hook and they will say you never provided a design. Your not saving money at all.

There are inherent dangers in the use of french drains and dry out areas, in that they change the flow of water, not always to the best advantage. The ground may dry out excessively and although french drains may perhaps relieve any rising damp problems they may create problems of settlement of the building. Does your builder really understand any of these risks or take responsibility for them if they occur?

Can you provide professional advice or design of french drain or soakaway systems for free?

No. Many people call or email saying they “just want some free advice” and then say it is unfair that we can’t.

The main reasons are:

  • It is a strict requirement that all professional advice is covered by agreed written terms and condition to comply with Institution of Civil Engineers member’s rules.
  • It is a compliance term of our Lloyds of London specialist insurance policy for professional advice.
  • We are also a small private family company, without any public funding or charity support.
  • It would be unfair to existing clients who agree to pay.

How much does it cost to have a french drain or soakaway system designed?

Every site or development is unique so there are no set fees. It may be possible to provide a fixed fee, where the scope is reasonably certain and an accurate understanding of time can be conservatively evaluated. Given the considerable time necessary, the smallest single house arrangement will have fees exceeding £950 +VAT. The cost per house comes down considerably with larger developments.

Based on time charge our rates are:

1) A charge of £500+VAT which covers the financial, administrative, legal and insurance elements of setting up all new clients for their proposed project.

2) Professional advice (travel, meetings, design etc) associated with a specific project is charged at £150+VAT per hour.

The costs of design is typically between £1200 and £2500 for a single house site, which is a minimal compared to the construction costs.

By employing a professional, they can work with you to optimise the design and minimise costs while meeting the standards.

If you understand the above and wish to proceed, please forward a pack of information including your full details, proposals, any geotechnical reviews or permeability results and any other relevant background information.