Creating Soakaways to Manage Surface Water Runoff

When heavy rain hits a UK home or commercial site, the ground can only absorb so much water. Driveways, patios, roofs and compacted ground send surface water running towards drains, boundaries and low spots. A well-sized soakaway lets rainwater drain naturally, reducing pooling and pressure on local drainage systems during heavy storms. For anyone working in excavation and groundworks, getting soakaways right is a practical skill that protects properties and keeps sites compliant.

Why soakaways matter in UK groundworks

Soakaways are underground chambers or rubble-filled pits that collect surface water and allow it to infiltrate surrounding soil. They are especially useful where connecting to a mains sewer is impractical, or where local drainage is already under strain. In England and Wales, Approved Document H of the Building Regulations sets out expectations for rainwater drainage, and a soakaway must be sited and sized so it does not cause flooding, pollution or instability. Scotland and Northern Ireland have their own equivalents, so always check local rules before breaking ground.

A soakaway is not simply a hole filled with rubble. It needs to sit above the water table, away from foundations, and in ground that actually drains. Put one in heavy clay without proper testing, and you have created a sump that holds water rather than releasing it.

Check the ground before you dig

Before any excavation, dig a trial pit to the depth you intend to install the soakaway. Look at the soil profile: sandy gravels and chalk drain quickly; clay and silt usually do not. The pit should be at least 1m away from any boundary or building for testing, though final siting distances are greater.

A percolation test tells you how quickly water leaves the pit. Fill the trial pit with water, let it drain, then refill and time how long the water level drops. Repeat this to get a reliable infiltration rate. If water sits for hours, a traditional soakaway may not be suitable. You may need a shallow infiltration trench, a crate system with a larger base area, or an alternative drainage solution.

Also check for:

• Water table depth and seasonal high groundwater
• Existing services, drains, cables and pipes
• Contaminated ground or made-up ground
• Slope and run-off from surrounding land
• Distance from foundations, boundaries and highways

Keep soakaways at least 5m from buildings and 2.5m from boundaries unless a qualified engineer advises otherwise. Never put one where vehicle loading will crush it.

Sizing the soakaway correctly

Size matters. Too small, and the soakaway overflows during a storm. Too large, and you waste time, money and materials. The correct size depends on the roof or surface area draining to it, the percolation rate, and the storm return period you are designing for. A common approach is to use BRE 365 or the methods in Approved Document H, which combine rainfall intensity, area and infiltration rate.

As a rough starting point for free-draining ground, a rubble-filled pit around 1.5m deep and 1.5m to 2m square can serve a typical house roof. But this is not a substitute for calculation. For a 100m² roof, or for a driveway, you may need a much larger pit or a shallow crate system with a wider footprint. If the ground is slow to drain, increase the base area rather than just digging deeper.

Excavation and installation step by step

Mark out the soakaway, check for services again, and dig with a tidy, stable edge. Keep the bottom flat and level. If the sides are loose, batter them back or support them safely. Never let anyone work in an unsupported trench deeper than 1.2m without proper shoring.

For a traditional rubble soakaway:

• Line the pit with a permeable geotextile membrane to stop soil washing in.
• Add a 100mm layer of clean, single-size aggregate, such as 20mm to 40mm stone.
• Place the perforated distribution pipe on the aggregate, with a gentle fall.
• Fill around the pipe with more clean aggregate, leaving the top 300mm for topsoil.
• Wrap the geotextile over the aggregate before backfilling.
• Finish with topsoil and turf, or a permeable surface.

For a crate soakaway, assemble the crates on a prepared aggregate base, wrap them in geotextile, and backfill carefully. Crates give more void space than rubble, so they can be more compact for the same storage volume.

Pipework, silt control and connections

Surface water should reach the soakaway through a silt trap or catchpit. This catches leaves, grit and debris before they clog the infiltration surface. Fit a rodding eye at the head of the run and keep pipe falls steady, usually between 1:60 and 1:100. A sudden flat section will silt up.

Connect downpipes and gullies with sealed joints where they pass near buildings. Use perforated pipe only within the soakaway; solid pipe elsewhere. If the soakaway is downhill from the property, a shallow inspection chamber makes future maintenance much easier. Never connect foul water, kitchen waste or chemicals to a soakaway.

Maintenance and common pitfalls

A soakaway is a long-term asset, but it needs occasional care. Check the silt trap twice a year, especially after autumn leaf fall. Clear debris from gullies and downpipe outlets. Watch for soft ground, damp patches or slow drainage after storms, which can signal clogging.

Common mistakes include:

• Digging too close to foundations or boundaries
• Ignoring the water table or installing in clay without testing
• Using dirty aggregate or filling with soil
• Omitting geotextile, so fines block the stone
• Undersizing for the roof area or storm intensity
• Routing foul water or contaminated run-off into the soakaway

When in doubt, bring in a groundworks specialist or drainage engineer. A properly tested, correctly sized and cleanly installed soakaway will manage surface water quietly for years, keeping gardens drier and local drains under less pressure. For UK groundworks, it is one of the simplest ways to build resilience into a site.