Land Drainage and French Drains

Land drainage deals with water in the ground rather than water in a pipe from the house. If a lawn turns to a swamp after rain, borders never dry out or water collects against a wall, ring 07427 195142. We look at where the water is coming from and where it can go, and advise on land drains and French drains that carry it away.

What land drainage is

Your foul drains carry waste from toilets, sinks and showers. Your surface water drains carry rain from roofs and hard standing. Land drainage is a third thing: perforated pipe laid in the ground to collect water that is already in the soil and lead it somewhere it can be released safely.

A French drain is the most familiar version. It is a trench, usually lined with a permeable membrane, filled with clean stone and with a perforated pipe running along the bottom. Water in the surrounding soil finds the easiest path, which is into the gravel, and the pipe carries it away on a gentle fall.

Signs your ground needs draining

Some sites are simply wet: heavy clay that holds water, a garden at the bottom of a slope, or a high water table. Others become wet because something has changed, such as new paving next door, a blocked old field drain or a soakaway that has stopped working.

• Standing water on the lawn long after rain has stopped
• Moss and rushes taking over grass
• Damp patches at the base of a wall that faces rising ground
• Paths and patios that stay slippery and green
• Water running across the garden from a neighbouring higher plot

Finding out why before digging

Not every wet garden needs a new land drain. Sometimes the cause is a broken surface water pipe, a downpipe discharging into the ground, or an old clay field drain that has silted up or been crushed. Putting a camera into any existing pipework is a sensible first step, and a CCTV drain survey will show whether a drain that should be taking the water is damaged or blocked.

If the problem is an existing pipe, clearing it with high pressure water jetting or repairing a damaged section may solve it without any new trench at all. If the ground itself is the problem, a land drain is the more likely answer.

Where the water goes

The hardest part of land drainage is often not collecting the water but deciding where it can legally and practically go. Groundwater should not be connected into a foul sewer. Options can include a suitably sized soakaway, an existing surface water drain where that is allowed, or a ditch or watercourse, which may need permission. Our guide to soakaways and surface water drainage and our explainer on drainage regulations in Scotland cover the rules in plain terms.

A soakaway only works if the ground around it can take water. On heavy clay a soakaway can simply fill up and sit there, so the outlet has to be thought about at the start, not after the trench is dug.

How a French drain is laid out

The route follows the problem. A drain at the foot of a slope intercepts water running down towards the house. A herringbone pattern of smaller branches feeding a main run suits a lawn that is wet all over. A drain alongside a wall takes water away from the foundations.

The pipe needs a steady fall to its outlet, the stone needs to be clean so it does not clog, and the membrane keeps fine soil from washing in and silting the pipe. Inspection points along the run make it possible to flush the drain later. Where the drain passes close to existing services or foul pipework, tracing those first avoids accidental damage.

Looking after a land drain

A well-built land drain needs little attention, but silt does gradually find its way in. Flushing through the inspection points every so often keeps it running, and it is worth checking the outlet after a wet winter to make sure it is not blocked by leaves or debris. If water starts to collect again in the same places, a camera can show whether the pipe has silted or been disturbed.

Land drainage often sits alongside other surface water work. For water pooling on a drive or patio rather than in soil, see our page on drainage channel installation. For a broken pipe rather than wet ground, our drain repairs page explains the options.

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