Soakaways and land drains: where garden rainwater goes

When rain falls on a roof, a drive or a lawn, it has to go somewhere. In many homes the downpipes run to an underground drain. In others, the rainwater goes into a soakaway in the garden, and wet ground may be drained by a land drain. Both are about rainwater, also called surface water, and both are easy to confuse with other kinds of drainage.

This guide explains what soakaways and land drains are, how they differ from the systems around them, and the signs that one is not working. It gives no sizing or design method. That depends on the ground and the site, and it is a question for the council or a drainage designer.

Rainwater is not foul water

Drainage at home is split into two broad kinds:

• Foul water from toilets, sinks, baths, showers and appliances. It goes to the public sewer or, off mains, to a private system such as a septic tank.
• Surface water, mostly rain from roofs and hard surfaces. It may go to a surface water sewer, a combined sewer, a ditch or watercourse, or into the ground through a soakaway.

The two should not be mixed up. Foul water must never go into a soakaway, and rainwater should not be connected into a foul drain without the right advice, because it can overload the sewer or a private treatment system. If you are not sure which drain is which, outside drains and inspection chambers and manholes explain how to tell them apart.

What a soakaway is

A soakaway is a buried space in the ground that collects rainwater and lets it soak away slowly into the surrounding soil. Older soakaways are often a pit filled with rubble or stone. Newer ones are often made of plastic crates wrapped in a membrane. A pipe from the downpipes or a drive drain feeds in, and the water seeps out over time.

A soakaway only works if the ground can take the water. Free-draining soil can work well. Heavy clay, rock or a high water table may mean a soakaway barely drains at all. Whether one is suitable, how big it should be and where it can go are all design questions for someone who has looked at the site, often after a test of how fast the ground drains.

What a land drain is

A land drain is different. It is usually a perforated pipe laid in a gravel-filled trench to take water out of waterlogged ground, such as a soggy lawn or the area behind a retaining wall. The water it collects has to go somewhere: often a soakaway, a ditch, a watercourse or, with the right advice, a surface water drain.

So a soakaway lets water into the ground, while a land drain takes water out of the ground. They are often used together.

How they differ from a water butt and a drainage field

• A water butt stores rainwater from a downpipe for use in the garden. When it is full, the overflow still has to go somewhere. See garden irrigation and water butts.
• A rainwater harvesting system stores larger amounts for use indoors or outdoors. See rainwater harvesting.
• A drainage field belongs to a septic tank or treatment plant and handles treated sewage, not rain. It follows different rules entirely.

Storing rainwater can reduce how much reaches a soakaway, but it does not remove the need for somewhere for overflow to go.

Signs a soakaway or land drain is struggling

• Water ponds on the surface or around the downpipe after rain and is slow to clear.
• A gully or a grid by the house backs up in heavy rain.
• A damp patch spreads near the house wall or the soakaway.
• A land drain outlet runs dirty or not at all when the ground is wet.

Soakaways and land drains can silt up, fill with roots, or be crushed. A drainage firm can check them; a CCTV drain survey sometimes shows what has happened to the pipe leading in. Do not dig near the house foundations to find one without advice.

Rules and who to ask

Drainage work around homes can involve the Building Regulations and sometimes planning. In England, the Building Regulations' guidance on drainage is published by GOV.UK as Approved Document H. We have not summarised its requirements here.

Paving a front garden is one case where rainwater and planning meet. GOV.UK's guidance on permeable surfacing of front gardens explains that whether hard surfacing counts as permitted development depends on whether it is permeable, and describes ways of controlling run-off that include "permeable surfaces or soakaways and rain gardens." Read it before you pave.

Before you build a soakaway or a land drain, change where your downpipes go, or connect anything to a public sewer:

• Ask your council's building control team whether the work is controlled and what they need.
• Ask the council's planning team if the work changes the outside of the property or involves paving.
• Ask your water company before connecting anything to a public sewer.
• Use a drainage designer or drainage firm for sizing and siting.

Rules differ in Scotland, Wales and Northern Ireland, so check with the bodies where you live. This is not legal advice.

Can I connect my downpipe to a soakaway myself?

Ask before you dig. Whether a soakaway is suitable, where it can go and whether building control needs to be involved depend on the site. Speak to the council's building control team or a drainage designer first.

Why does my soakaway flood in heavy rain?

It may be too small for the area that drains into it, silted up, crushed, or in ground that does not drain well. A drainage firm can investigate.

Can a land drain go into the sewer?

Do not connect one to a sewer without advice. Ask your water company and a drainage designer what is acceptable where you live.