Waterlogged Garden? French Drains, Soakaways and Land Drainage Explained

Why is my garden waterlogged?

• Clay soil: much of Surrey sits on Weald Clay or London Clay, which drains very slowly, so water sits on the surface through the winter.
• Compaction: building work, machinery and heavy foot traffic squash the soil and stop water getting through.
• Run-off: water from roofs, patios and driveways often ends up on the lawn.
• Springs and high water tables: some low-lying gardens and fields are simply wet for much of the year.
• Blocked or broken old land drains, which are common on older properties and farmland.

Drainage options

• French drain: a gravel-filled trench wrapped in geotextile with a perforated pipe at the bottom. It collects water along its length and carries it away. Ideal along the bottom of a slope or the edge of a lawn.
• Land drainage (herringbone drains): a network of perforated pipes laid across a lawn, paddock or field, feeding into a main drain. Used for larger areas.
• Soakaway: an underground pit or crate system that lets collected water soak gradually into the ground. It only works where the soil drains well enough, which a percolation test checks.
• Discharge to a ditch or watercourse: often the best outlet on clay. You may need consent from the Lead Local Flood Authority (in Surrey, Surrey County Council) to alter or connect to an ordinary watercourse.
• Soil improvement: aerating, adding grit and organic matter, or raising beds helps where the problem is mild.

Rules to know

• Building regulations (Part H) say soakaways should normally be at least 5 metres from a building and sized from a percolation test.
• You can't connect surface water to the foul sewer, and connecting to a public surface water sewer needs consent from the water company.
• You mustn't divert water onto a neighbour's land in a way that causes them flooding.

How we install a French drain

1. 01

   Find the source

   We look at levels, soil and where the water comes from, and dig a test hole if a soakaway is being considered.

2. 02

   Plan the route and outlet

   We set out the drain line with a steady fall, typically at least 1 in 100, to a soakaway, ditch or approved outlet.

3. 03

   Excavate the trench

   The trench is dug to the planned depth and fall, usually with a mini digger to keep the line accurate.

4. 04

   Lay membrane and pipe

   Geotextile membrane lines the trench, a bed of clean stone goes in, then the perforated pipe is laid to fall.

5. 05

   Backfill with stone

   The trench is filled with clean washed stone and the membrane folded over the top to stop soil clogging it.

6. 06

   Reinstate

   We finish with gravel, topsoil and turf, or a surface to suit, so the drain blends in.

Frequently asked questions