French Drain: What It Is, How to Build One and When It Fixes a Waterlogged Garden

A French drain is a trench lined with geotextile membrane, filled with clean gravel and usually fitted with a perforated pipe, that collects excess water from the soil and carries it away to an outlet. It can fix a waterlogged lawn or stop water running off a slope towards a house, but only when the water can reach the trench through the soil and has somewhere lawful to go. Our groundworks team installs French drains from £500 + VAT, and below we explain how they work and how to build one yourself.

What is a French drain?

Approved Document H of the Building Regulations defines them directly: "Filter drains or french drains consist of the trench, lined with a geotextile membrane and filled with gravel. Much of the flow infiltrates into the ground. A perforated pipe is often laid through the gravel to assist drainage." (Approved Document H, paragraph 3.33).

Water in the surrounding soil moves into the gravel because it is the easiest path, enters the pipe through its perforations, and runs along the pipe to the outlet.

The parts that matter are:

• a steady fall, so water keeps moving rather than sitting in the pipe;
• a geotextile wrap, which stops fine soil washing into the stone and clogging it;
• clean, free-draining stone around and over the pipe;
• an outlet that can accept the water even in a long wet spell;
• a rodding or inspection point, so the pipe can be checked and jetted later.

When will a French drain fix a waterlogged garden?

A French drain works best where water is moving through the upper layers of soil and simply needs a route away. Good candidates are:

• a lawn at the bottom of a slope that stays soggy after rain;
• water running off higher ground towards a house, patio or outbuilding;
• ground behind a retaining wall that stays saturated;
• the edge of a path or patio where water collects.

The RHS suggests a simple check first: dig a hole 30–60 cm deep, fill it with water and cover it. "If water remains for hours or even days the soil may benefit from installing drainage" (RHS).

When won't a French drain work?

A French drain will disappoint where water cannot reach it or cannot leave it:

• heavy clay, where water moves so slowly that little reaches the trench;
• no outlet, so the trench fills and stays full;
• a high water table, where the outlet is as wet as the garden for much of the winter;
• surface compaction, where aerating and improving the soil may do more good;
• a leak, such as a broken drain, a failed soakaway or a downpipe discharging onto the lawn.

A strip of greener or wetter grass along a straight line often follows a leaking pipe. A CCTV drain survey will show whether the drain is broken before you pay for garden drainage.

How do you build a French drain?

To make a French drain, plan the route and the outlet first, then dig. The figures below are the RHS's guidance for garden drainage; deeper drains next to buildings or slopes need a proper design.

1. Find the outlet. Confirm where the water will go, and that it is allowed to go there (see the next section). Everything else is set out from this point.
2. Check for buried services. Look at any plans you hold and use the free LinesearchbeforeUdig service. HSE warns that many serious accidents happen when buried services are hit during digging.
3. Mark the route. Run the trench across the slope above the wet area to intercept water, or through the wettest ground. For larger areas, the RHS describes a herringbone layout with side drains meeting a main line at 45 degrees.
4. Dig the trench. The RHS suggests pipes 40–60 cm deep, with runs 3–6 m apart for larger areas. Keep the spoil on a sheet so you can reuse the topsoil.
5. Set the fall. The RHS gives a fall of 1 in 40 towards the outlet, which is 2.5 cm for every metre. Check it with a spirit level and a straight board, or a string line.
6. Line the trench with geotextile membrane, leaving enough at the sides to fold over the top later.
7. Lay a gravel bed. The RHS suggests a firmed 5 cm bed of coarse gravel under the pipe.
8. Lay the perforated pipe on the gravel, holes as the manufacturer directs, and connect it to the outlet. Add a rodding point at the top end if you can.
9. Cover the pipe with at least 10 cm of gravel, then fold the membrane over the stone.
10. Backfill with topsoil and turf, or finish with decorative gravel if you want surface water to enter from the top as well.
11. Test it by running a hose into the top end and checking water arrives at the outlet.

What materials do you need?

Where can the water from a French drain go?

The outlet is the most important design decision, and the one most often overlooked.

The RHS also notes that local byelaws usually prohibit adding garden drainage water to sewers or storm drains, so check before connecting anything. Around Nottinghamshire and most of Derbyshire and Leicestershire, the sewerage company is Severn Trent; most of Lincolnshire is Anglian Water. Our guide to soakaways explains infiltration testing, and the foul vs surface water guide explains the two systems.

Wherever the water goes, do not simply move the problem onto a neighbour's land.

French drain or soakaway: which do you need?

They do different jobs and are often used together.

Can you install a French drain yourself?

A short, shallow run in a garden is feasible if you are fit and careful, but the details decide whether it works. The pipe must keep a consistent fall, the membrane must wrap the stone fully, and you will have a lot of spoil to move.

Safety matters more than people expect. HSE notes that "a cubic metre of soil can weigh in excess of 1.5 tonnes" (HSE). Never get into a deep trench with unsupported sides, keep trenches well away from foundations, and stop if you find an unmarked pipe or cable.

What else causes a waterlogged garden?

It is worth ruling out other causes before paying for a French drain:

• compacted soil, often after building work, which stops rain soaking in;
• downpipes discharging onto the ground, or a soakaway that has silted up;
• a leaking or broken underground drain;
• changed ground levels, including a neighbour's new patio or driveway;
• more hard surfacing, which sends more water onto the remaining soft ground.

Hard surfacing matters at street scale too. Nottinghamshire County Council's report into flooding in Mansfield on 10 June 2016, when 64 mm of rain fell in 24 hours and 17 properties flooded internally, pointed to paved-over front gardens as a factor. Our guide to preventing drain flooding in heavy rain covers the wider picture.

How much does a French drain cost?

Costs depend on length, depth, access for a digger, spoil removal and the outlet. Our drainage groundworks start from £500 + VAT. A new soakaway, a sewer connection or reinstating paving over the trench will add to the total. We give a fixed written price after seeing the site, and it is agreed before work starts.

National price guides publish their own ranges. Treat any national figure as a guide only, because depth, access and ground conditions change the job a lot.

When to call a professional

Get professional help if the waterlogging is close to the house, the trench would be deeper than about 60 cm, you are unsure where the water can go, or you suspect a broken drain. Our groundworks team installs French drains and land drainage across Nottinghamshire, Derbyshire, Leicestershire and Lincolnshire, including Nottingham and Southwell, and where the ground will take the water we can design and build a soakaway to go with it.

There is no call-out fee and the price is agreed before we start. Get a quote or call 07300 600 950.

Sources

• Approved Document H: Drainage and waste disposal (gov.uk)
• RHS: How to install garden drainage
• HSE: Excavations
• gov.uk: Flood risk activities and environmental permits
• Nottinghamshire County Council: Mansfield Section 19 flood report (2016)
• MyBuilder: French drain cost guide